Engineering and Construction

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Certain themes are woven throughout this blog and surface again and again in different, often widely varying contexts. These represent a set of concerns that fall into two broad categories: building performance
and building construction. 

The performance concerns relate to the inescapable problems that must be confronted in every building: fire; building movement of every kind, including foundation settlement, structural defl ections, and expansion and contraction due to changes in temperature and humidity; the fl ow of heat and air through building assemblies; water vapor migration and condensation; water leakage; acoustical per-formance; aging and deterioration of materials; cleanliness; building maintenance; and others.

The construction concerns are associated with the practical problems of getting a building built safely, on time, within budget, and to the required standard of quality: division of work between the shop and the building site; optimum use of the various building trades; sequencing of construction operations for maximum productivity; convenient and safe worker access to construction operations; dealing with inclement weather; making building components fit together; and quality assurance in construction materials and components through grading, testing, and inspection.

To the novice, these matters may seem of minor consequence when compared to the larger and often more interesting themes of building form and function. To the experienced building professional, who has seen buildings fail both aesthetically and physically for want of attention to one or more of these concerns, they are issues that must be resolved as a matter of course before the work of a building project can be allowed to proceed.

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Improving Collaboration Among Team Members
The design and construction industry continues to evolve, testing innovative organizational structures and project delivery methods in which designers, builders, and owners assume less adversarial and less compartmentalized roles. Such approaches share characteristics such as:

•  Contractual relationships and working arrangements that foster collaboration between project members
•  Participation of the construction contractor during the design phases of a project
•  Overlapping of design and con-struction activities to reduce the “time to market”
• Expanded definitions of project services to encompass the full life cycle of a project—from its original conception, through planning, design and construction, to postconstruction occupancy—to better serve the
needs of the building owner

The growth of design/build in the construction marketplace is one example of this trend: Between 1980
and 2005 the share of private, nonresidential construction work per-formed as design/build construction increased from roughly 5 percent of the total market to an estimated 30 to 40 percent. Alternatives to traditional design/bid/build project delivery have gained increased acceptance in the public construction sector as well. Other new practice models, with such names as teaming, concurrent design, integrated practice, or alliancing, combine efficient project delivery methods with innovations in team member relationships in a variety of ways, with the aim of aligning all parties’ efforts with the shared goal of a finished product of  the highest possible quality and value to the owner.

Improving Effi ciency in Production
Other efforts within the construction industry focus on improvements in the efficiency of construction methods themselves. Unlike factory production, much building construction takes place outdoors, is performed
within constrained and often physically challenging work areas, and is executed by a highly fragmented workforce. Despite the differences in these production environments, the construction industry is looking
to lessons learned in factory production for approaches to improving the quality and effi ciency of its own processes. Such so-called lean construction methods attempt to:

•  Eliminate wasteful activities
•  Structure the methods of production and the supply chain of materials and products to achieve the quickest and most reliable workfl  ow
•  Decentralize information and decision making so as to put control of construction processes into the
hands of those most familiar with the work and most capable of improving it

Current estimates of labor in efficiency in building construction run as high as 35 to 40 percent, and estimates of materials wastage are 20 percent or more. The challenge of lean construction is to restructure the way in which construction materials and building components are manufactured, delivered, and assembled so as to reduce these inefficiencies and improve the quality of the delivered product.

Improving Information Management
Developments in information technology also are infl uencing the way buildings are designed and con-
structed. Most notable is  building information modeling (BIM), the computerized, three-dimensional modeling of building systems. Unlike the two-dimensional representation of building systems characteristic of
conventional  computer-aided design (CAD), BIM involves an intelligent model. Components are not only represented geometrically, but are also linked to data describing their intrinsic properties and their relationships to other components. Originally developed for use in highly capital-intensive industries such as aerospace and automobile manufacturing, this modeling technology is now fi  nding increased application in the design and construction of buildings.

BIM has the potential to impact many aspects of the building life cycle. It can aid the design team with the visualization and realization of complex geometries. It can improve coordination between design
disciplines––for example, searching out “collisions” between mechanical system ductwork and structural framing or other such physical interferences between systems––and it can facilitate the modeling of building energy use and the performance of other building systems. For the builder,  BIM can be used to improve coordination of trades, to drive the automated fabrication or preassembly  of building components, and to integrate cost and schedule data more closely with building design. For the building owner, information accumulated in the model during design and construction can be carried forward for use with postconstruction building operations and facilities planning.

As the implementation of BIM matures, it is expected to have a profound impact as a communication tool used to improve the coordination and sharing of information among all of the parties to a project.

As the integrated building model is shared across the traditional boundaries of disciplines and project phases, the boundaries of responsibility between the designers, constructors, and owners will also blur, and
new, more integrated relationships between these parties will likely be required to fully enable the potential of this technology.

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Once a construction project is underway, the general contractor or construction manager assumes responsibility for day-to-day oversight  of the construction site, management of trades and suppliers, and communications between the construction team and other major parties, such as the owner and the designer.

On projects of any signifi cant size, this may include responsibility for filing  construction permits, securing the project site, providing temporary power and water, setting up office  trailers and other support facilities, providing insurance coverage for the work in progress, managing personnel on site, maintaining a safe work environment, stockpiling materials, performing testing and quality control, providing site surveying and engineering, arranging for cranes and other construction machinery, providing temporary structures and weather protection, disposing or recycling of construction waste, soliciting the work of subtrades and coordinating their efforts, submitting product samples and technical information to the design team for review, maintaining accurate records of the construction as it proceeds, monitoring costs and schedules, managing changes to the work, protecting completed work, and more.

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Constructing a building of any signifi  cant size is a complex and costly endeavor, requiring the combined  efforts of countless participants and the coordination of myriad tasks.

Managing this process requires an in-depth understanding of the work required, of the ways in which differ-
ent aspects of the work depend upon each other, and of the constraints on the sequence in which the work must be performed.

Figure 1.11 captures one moment in the erection of a tall building. The process is led by the construction
of the building’s central, stabilizing core structures (in the photograph, the pair of concrete towerlike structures extending above the highest fl  oor levels). This work is followed by the construction of the surrounding fl  oors, which rely, in part, on the pre-viously completed cores for support.

Figure 1.11 In this photo, the construction sequence of a tall building is readily apparent: A pair of concrete core structures lead the construction, followed by concrete columns and fl oor plates and, fi nally, the enclosing curtain wall.

Attachment of the exterior skin can follow only after the fl oor plates are in place and structurally secure. And as the building skin is installed and fl  oor areas become enclosed and protected from the weather, further operations, such as the roughing in of mechanical and electrical systems, and eventually, the installation of fi  nishes and other elements, can proceed in turn. This simple example illustrates considerations that apply to virtually every aspect of building construction and at every scale from a building’s largest systems to its smallest details: Successful construction requires a detailed understanding of the tasks re-
quired and their interdependencies.

The construction project schedule is used to analyze and represent construction tasks, their relation-
ships, and the sequence in which they must be performed. Development of the schedule is a fundamental part of construction project planning, and regular updating of the schedule throughout the life of the project is essential to its successful management.

In a Gantt chart, a series of horizontal bars represent the duration of various tasks or groups of tasks that make up the project. Gantt charts provide an easy-to-understand representation of construction tasks and their relationships in time. They can be used to provide an overall picture of a project schedule, with only a project’s major phases represented (Figure 1.10), or they can be expanded to represent a larger number of more narrowly de-fi  ned tasks at greater levels of project detail (Figure 1.12).

Figure 1.10 In sequential construction, construction does not begin until design is complete. In phased construction, design and construction activities overlap, with the goal of reducing the overall time required to complete a project.

Figure 1.12 In a Gantt chart, varying levels of detail can be represented. In this example, roughly the top three-quarters of the chart is devoted to a breakdown of preconstruction and procurement activities such as bidding portions of the work to subtrades, preparing cost estimates, and making submittals to the architect (a). Construction activities, represented more broadly, appear in thebottom  portion (b).

The critical path of a project is the sequence of tasks that determines the least amount of time in which a project can be completed. For example, the construction of a building’s primary structural system is commonly on the critical path of a project schedule. If any of the tasks on which the completion of this system depends—such as design, shop drawing production and review, component fabrication, materials de-livery, or erection on site—are delayed, then the fi nal completion date of the project will be extended.


On the other hand, other systems not on the criti-cal path have more fl exibility in their scheduling, and delays (within limits) in their execution will not necessarily impact the overall project schedule.

The  critical path method is a technique for analyzing collections of tasks and optimizing the project schedule to minimize the duration and cost of a project. This requires a detailed breakdown of the work involved in a project and the identifi cation of dependencies between the parts (Figure 1.13). This information is combined with considerations of cost and resources available to perform the work, and then analyzed, usually with the assistance of computer software, to identify optimal scheduling scenarios. Once the critical path of a project has been established, the elements on this path are likely to receive a high degree of scrutiny during the life of the project, since delays in any of these steps will directly impact the overall project schedule.

Figure 1.13 The critical path method depends on the detailed analysis of work tasks and their relationships to generate an optimal construction schedule. Shown here is a schematic network diagram representing task dependencies. For example, task 6 cannot begin until tasks 1, 4, and 5 are completed, and tasks 7 and 9 cannot begin until task 6 is fi nished. The dashed lines on the diagram trace two of many possible paths from the start to the end of the diagram. To determine the critical path for this collection of tasks, all such paths must be identifi ed and the time required to complete each one calculated. The path requiring the most time to complete is the critical path, that is, the sequence of activities that determines the least time in which the collection of tasks as a whole can be completed.

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In sequential construction (Figure 1.10), each major phase in the design and construction of a building is completed before the next phase begins.

Sequential construction can take place under any of the project delivery methods described previously. It
is frequently associated with design/bid/build construction, where the separation of design and construction
phases fi ts naturally with the contractual separation between design and construction service providers.

Phased construction, also called fast track construction, aims to reduce the time required to complete a project by overlapping the design and construction of various project parts (Figure 1.10). By allowing construction to start sooner and by overlapping the work of design and construction, phased construction can reduce the time required to complete a project.

Figure 1.10 In sequential construction, construction does not begin until design is complete. In phased construction, design and construction activities overlap, with the goal of reducing the overall time required to complete a project.

However, phased construction also introduces its own risks. Since construction on some parts of the project
begins before all design is complete, an overall cost for the project cannot be established until a signifi  cant portion of construction is underway.

Phased construction also introduces more complexity into the design process and increases the potential for costly design errors (for example, if foundation design does not ad-equately anticipate the requirements
of the not yet fully engineered structure above). Phased construction can be applied to any construction
delivery method discussed above. It is frequently associated with design/build and construction management project delivery methods, where the early participation of the construction entity provides resources that are
helpful in managing the complex coordination of overlapping design and construction activities.

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With  fixed  fee or  lump sum compensation, the contractor or other construction entity is paid a fixed dollar amount to complete the construction of a project regardless of that entity’s actual costs. With this compensation method the owner begins construction with a known, fixed construction cost and assumes minimal risk for unanticipated increases in cost. On the other hand, the construction contractor assumes most of the risk of unforeseen costs but also stands to gain from potential savings. Fixed fee compensation is most suitable to projects where the scope of the construction work is well defi ned at the time that the construction fee is set, as is the case, for example, with conventional design/bid/build construction.

As an alternative, compensation may be set on a  cost plus a fee basis, where the owner agrees to pay the  construction entity for the actual costs of construction—whatever they may turn out to be—plus an
additional fee. In this case, the construction contractor is shielded from most cost uncertainty, and it is the
owner who assumes most of the risk of added costs and stands to gain the most from potential savings. Cost
plus a fee compensation is most often used with projects where the scope of construction work is not fully known at the time that compen-sation is established, a circumstance most frequently associated with con-struction management or design/build contracts.

With fi xed fee compensation, the builder assumes most of the risk related to unanticipated construction costs; with cost plus a fee compensation, the owner assumes most of this risk. Between these two extremes, many other fee-structuring arrangements can be used to allocate varying degrees of risk between the two parties.

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In conventional design/bid/build project delivery (Figure 1.8), the owner first hires a team of architects and
engineers to perform design services, leading to the creation of drawings and technical specifi cations, referred to collectively as the construction documents, that comprehensively describe the facility to be built. Next, construction firms are invited to bid on the project. Each bidding firm  reviews the construction documents and proposes a cost to construct the facility.

The owner evaluates the submitted proposals and awards the construction contract to the bidder deemed most suitable. This selection may be based on bid price alone, or other factors related to bidders’ qualifications may also be considered. The construction documents then become part of the construction contract, and the selected firm proceeds with the work. On all but small projects, this firm acts as the  general contractor, coordinating and overseeing the overall construction process but frequently relying on smaller, more specialized subcontractors to perform significant portions or even all of the construction work.

Figure 1.8 In design/bid/build project delivery,  the owner contracts separately with the architect/engineer (A/E) design team  and the construction general contrac- tor (GC). In a design/build project, the  owner contracts with a single organiza- tional entity that provides both design  and construction services.

During construction, the design team continues to provide services to the owner, helping to ensure that the facility is built according to the requirements of the documents as well as answering questions related to the design, changes to the work, payments to the contractor, and similar matters. Among the advantages of design/bid/build project delivery are its easy-to-understand organizational structure, well-established legal prec-edents, and ease of management. The direct relationship between the owner and the design team ensures that the owner retains control over the design and provides a healthy set of checks and balances during the construction process. Also, with design work completed before the project is bid, the owner starts construction with a fixed construction cost and a high degree of confidence regarding the final costs of the project.

In design/bid/build project delivery, the owner contracts with two entities, and design and construc-
tion responsibilities remain divided between these two throughout the project. In  design/build project de-
livery, one entity ultimately assumes responsibility for both design and construction (Figure 1.8). A design/
build project begins with the owner developing a conceptual design or program that describes the functional or performance requirements of the proposed facility but does not detail its form or how it is to beconstructed.

Next, using this conceptual information, a design/build organization is selected to complete all remaining as-
pects of the project. Selection of the designer/builder may be based on a competitive bid process similar to that described above for design/bid/build projects, on negotiation and evaluation of an organization’s qualifi  cations for the proposed work, or on some combination of both. Design/build organizations themselves can
take a variety of forms: a single fi rm encompassing both design and construction expertise, a construction management fi rm that subcontracts with a separate design fi rm to provide those services, or a joint venture between two fi rms, one specializing in construction and the other in design.

Regardless of the internal structure of the design/build organization, the owner contracts with this single entity throughout the remainder of the project, which assumes responsibility for all remaining design and construction services. Design/build project delivery gives the owner a single source of accountability for all aspects of the project. It also places the designers and constructors in a collaborative relationship, introducing construction expertise into the design phases of a project and allowing the earliest possible consideration of constructability, cost control, construction scheduling, and similar matters. This delivery method also readily accommodates fast track construction, a scheduling technique for reducing construction time that is described below.

Other delivery methods are possible: An owner may contract separately with a design team and a construction manager. As in design/build construction, the construction manager participates in the project prior to the onset of construction, introducing construction expertise during the design stage. Construction management project delivery can take a variety of forms and is frequently associated with especially large or complex projects (Figure 1.9). In  turnkey construction, an owner contracts with a single entity that provides not only design and construction services, but financing for the project as well. Or design and construction can be undertaken by a  single-purpose entity, of which the owner, architect, and con-tractor are all joint members. Aspects of these and other project delivery methods can also be intermixed, allowing many possible organizational schemes for the delivery of design and construction services that are suitable to a variety of owner requirements and project circumstances.

Figure 1.9  In its traditional role, a construction manager (CM) at fee provides project  management services to the owner and assists the owner in contracting directly  for construction services with one or more construction entities. A CM at fee  is not directly responsible for the con- struction work itself. A CM at risk acts  more like a general contractor and takes on greater responsibility for construction  quality, schedule, and costs. In either case, the A/E design team also contracts  separately with the owner.

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An owner wishing to construct a building hopes to achieve a fi nished project that meets its functional requirements and its expectations for design and quality, at the lowest possible cost, and on a predictable schedule. A contractor offering its construction services hopes to produce quality work, earn a profi t, and complete the project in a timely fashion. Yet, the process of building itself is fraught with uncertainty: It is subject to the vagaries of the labor market, commodity prices, and the weather; despite the best planning efforts unanticipated conditions arise, delays occur, and mistakes are made; and the pressures of schedule and cost inevitably mini-mize the margin for miscalculation.

In this high-stakes environment, the relationship between the owner and contractor must be structured to share reasonably between them the potential rewards and risks.

⇒ Construction Project Delivery Methods   In conventional design/bid/build project delivery (Figure 1.8), the owner first hires a team of architects and engineers to perform design services, leading to the creation of drawings and...

⇒ Paying for Construction Services   With  fixed  fee or  lump sum compensation, the contractor or other construction entity is paid a fixed dollar amount to complete the construction of a project regardless of that entity’s actual costs. With this compensation method the owner begins construction with a known, fixed construction cost...

⇒ Sequential versus Fast Track Construction

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A building begins as an idea in some-one’s mind, a desire for new and ample accommodations for a family,
many families, an organization, or an enterprise. For any but the smallest buildings, the next step for the owner of the prospective building is to engage, either directly or through a hired construction manager, the services of building design professionals. An architect helps to organize the owner’s ideas about the new building, develops the form of the building, and assembles a group of engineering specialists to help work out concepts and details of foundations, structural support, and mechanical, electrical, and communications services.

This team of designers, working with the owner, then develops the scheme for the building in progres-sively fi ner degrees of detail. Drawings and written specifi  cations  are produced by the architect–engineer
team to document how the building is to be made and of what. The drawings and specifi cations are submitted to the local government building authorities, where they are checked for conformance with zoning ordinances and building codes before a permit is issued to build. A general contractor is selected, either by negotiation or by competitive bidding, who then hires subcontractors to carry out many specialized portions of the work. Once construction begins, the general contractor oversees the construction process while the building inspector, architect, and engineering consultants observe the work at frequent intervals to be sure that it is carried out according to plan. Finally, construction is finished, the building is made ready
for occupancy, and that original idea, often initiated years earlier, is realized.

Although a building begins as an abstraction, it is built in a world of material realities. The designers of a building—the architects and engineers—work constantly from a knowl-edge of what is possible and what is not. They are able, on the one hand, to employ a seemingly limitless palette of building materials and any of a number of structural systems to produce a building of almost any desired form and texture. On the other hand, they are inescapably bound by certain physical limitations: how much land there is with which to work; how heavy a building the soil can support; how long a structural span is feasible; what sorts of materials will perform well in the given environment. They are also constrained by a construction budget and by a complex web of legal restrictions.

Those who work in the building professions need a broad understanding of many things, including people
and culture, the environment, the physical principles by which buildings work, the technologies available for utilization in buildings, the legal restrictions on building design and use, the economics of building,
and the contractual and practical arrangements under which buildings are constructed.

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In the United States, the most widely adopted method for rating the environmental sustainability of a building’s design and construction is the  U.S. Green Building Council’s Leadership in Energy and Environmental Design, or  LEED™,  rating system. LEED for New Construction and Major Renovation projects, termed LEED-NC, groups sustainability goals into categories including site selection and development, effi  ciency  in water use, reductions in energy consumption and in the production of atmospheric ozone-depleting gases, minimizing construction waste and the depletion of nonrenewable resources, improving the quality of the indoor environment, and encouraging innovation in sustainable design and construction practices (Figure 1.1).

Within each category are specifi  c credits that contribute points toward a  building’s overall assessment of sustainability. Depending on the total  number of points accumulated, four  levels of sustainable design are recognized, including, in order of increasing performance, Certifi  ed,  Silver, Gold, and Platinum.

Figure 1.1 The LEED-NC 2009 Project Scorecard.  The document shown here was in draft status at the time of this publication.  See the U.S. Green Building Council web site for the most current version of this document.

The process of achieving LEED certifi  cation for a proposed new building begins at the earliest stages of project conception, continues throughout the design and construction of the project, and involves the combined ef-forts of the owner, designer, and builder. During this process, the successful achievement of individual credits is documented and submitted to the Green Building Council, which then  makes the fi  nal  certifi  cation of the  project’s LEED compliance.

The U.S. Green Building Council continues to refi ne and improve upon LEED-NC and is expanding its
family of rating systems to include existing buildings (LEED-EB), commercial interiors (LEED-CI), build-
ing core and shell construction (LEED-CS), homes (LEED-H), and other categories of construction and
development. Through international sister organizations, LEED is being implemented in Canada and other countries. Other green building programs, such as the Green Building Initiative’s  Green Globes, the Na-
tional Association of Home Builders’ Green Home Building Guidelines, and the International Code Council and National Association of Home Builders’ jointly developed National Green Building Standard, offer alternative as-sessment schemes.

Some green building efforts focus more narrowly on reducing building energy consumption, a measure of building performance that frequently correlates closely with the generation of greenhouse gas emissions and global warming trends. The American Society of  Heating, Refrigerating and Air-Con-ditioning Engineers’ Advanced Energy Design Guides and the U.S. Environmental Protection Agency’s  Energy Star program both set goals for reductions in energy consumption in new buildings that exceed current national standards.

These standards can be applied either as stand-alone programs or as part of a more comprehensive effort to achieve certification through LEED or some other green building assessment program.

Buildings can also be designed with the goal of zero energy use or carbon neutrality. A  net zero energy
building is one that consumes no more energy than it produces, usually when measured on an annual basis to account for seasonal differences in building energy consumption and on-site energy production. Net zero
energy use can be achieved using current technology combining on-site renewable energy generation (such as wind or solar power), passive heating and cooling strategies, a thermally efficient building enclosure, and highly efficient mechanical systems and appliances.

A  carbon-neutral  building is one that causes no net increase in the emission of carbon dioxide, the most
prevalent atmospheric greenhouse gas. If emissions due only to building operation are considered, the calcu-
lation is similar to that for a net zero energy building. If, however, the embodied carbon in the building’s full
life cycle—from materials extraction and manufacturing, through building construction and operations, to demolition, disposal, and recycling—is considered, the calculation becomes more complex. Carbon-neutral calculations may also consider the site on which the building resides. For example, what is the carbon footprint of a fully developed building site, including both its buildings and unbuilt areas, in comparison to that of the site prior to construction or in comparison to its natural state prior to human development of any kind?

Another possible consideration is, what role, if any,  should carbon offsetting (funding of offsite activities that reduce global carbon emissions, such as planting of trees), play in such calculations? Questions
such as these and the concepts of sustainability and how they relate to building construction will continue to evolve for the foreseeable future.

In addition, a sidebar in nearly every chapter describes the major issues of sustainability related to the materials and methods discussed in that chapter. These will be helpful in weighing the environmental costs of one material against those of another, and in learning how to build in such a way that we preserve for future generations the ability to meet their building needs in a reasonable and economical manner.

For more information on organizations whose mission is to raise our awareness and provide the knowl-
edge that we need to build sustainably, see the references listed at the end of this chapter.

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Sustainability must be addressed on a life-cycle basis, from the origins of the materials for a building, through the manufacture and installation of these materials and their useful lifetime in the building, to their eventual disposal when the building’s life is ended. Each step in this so-called  cradle-to-grave cycle raises questions of sustainability.

Origin and Manufacture of  Materials for a Building
Are the raw materials for a building plentiful or rare? Are they renewable or nonrenewable? How much of
the content of a material is recycled from other uses? How much embodied energy is expended in obtaining and manufacturing the material, and how much water? What pollutants are discharged into air, water, and soil as a result of these acts? What wastes are created? Can these wastes be converted to useful products?

Construction of the Building
How much energy is expended in transporting a material from its origins to the building site, and what pollutants are generated? How much energy and water are consumed on the building site to put the material
in place? What pollutants are associ-ated with the installation of this material in the building? What wastes are generated, and how much of them can be recycled?

Use and Maintenance of the Building
How much energy and water does the building use over its lifetime as a consequence of the materials used in its construction and fi nishes? What problems of indoor air quality are caused by these materials? How much maintenance do these materials require, and how long will they last? Can they be recycled? How much energy and time are consumed in maintaining these materials? Does this maintenance involve use of toxic chemicals?

Demolition of the Building
What planning and design strategies can be used to extend the useful life of buildings, thereby forestalling resource-intensive demolition and construction of new buildings? When demolition is inevitable, how will the building be demolished and disposed of, and will any part of this process cause pollution of air, water, or soil? Can demolished materials be recycled into new construction or diverted for other uses rather than disposed of as wastes?

One model for sustainable design is nature itself. Nature works in cyclical processes that are self-sustaining
and waste nothing. More and more building professionals are learning to create buildings that work more nearly as nature does, helping to leave to our descendants a stock of healthful buildings, a sustainable supply of natural resources, and a clean environment that will enable them to live comfortably and responsibly and to pass these riches on to their descendants in a never-ending succession.

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In constructing and occupying buildings, we expend vast quantities of  the earth’s resources and generate
a signifi cant portion of the earth’s  environmental pollution: The U.S.

Green Building Council reported in  2008 that buildings account for 30 to 40 percent of the world’s energy  use and associated greenhouse gas emissions. Construction and operation of buildings in the United States  accounted for more than one-third of this country’s total energy use and the consumption of more than two-
thirds of its electricity, 30 percent of its raw materials, a quarter of its harvested wood, and 12 percent of
its fresh water. Building construction and operation is responsible for nearly half of this country’s total greenhouse gas emissions and close to a third of its solid waste stream. Buildings are also
signifi  cant emitters of particulates and other air pollutants. In short, building construction and operation cause many forms of environmental degradation that place an increasing burden on the earth’s resources and jeopardize the future of the building industry and societal health and welfare.

Sustainability may be defined  as meeting the needs of the present generation without compromising the ability of future generations to meet their needs. By consuming irreplaceable fossil fuels and other nonrenewable resources, by building in sprawling urban patterns that cover extensive areas of prime agricultural land, by using destructive forestry practices that degrade natural ecosystems, by allowing topsoil to be eroded by wind and water, and by generating substances that pollute water, soil, and air, we have been building in a manner that will make it increasingly diffi cult for our children and grandchildren to meet their needs for communities, buildings, and healthy lives.

On the other hand, if we reduce building energy usage and utilize sunlight and wind as energy sources for our buildings, we reduce depletion of fossil fuels. If we reuse existing buildings imaginatively and arrange our new buildings in compact patterns on land of marginal value, we minimize the waste of valuable, productive land. If we harvest wood from forests that are managed in such a way that they can supply wood at a sustained level for the foreseeable future, we maintain wood construction as a viable option for centuries to come and protect the ecosystems that these forests support. If we protect soil and water through sound design and construction practices, we retain these resources for our successors. If we systematically reduce the various forms of pollution emitted in the processes of constructing and operating buildings, we keep the future environment cleaner. And as the industry becomes more experienced and committed to designing and building sustainably, it becomes increasingly possible to do these things with little or no increase in construction cost while creating buildings that are less expensive to operate and more healthful for their
occupants for decades to come.

Realization of these goals is dependent on our awareness of the environmental problems created by
building activities, knowledge of how to overcome these problems, and skill in designing and constructing buildings that harness this knowledge.

While the practice of sustainable design and construction, also called  green building, remains a relatively recent development in the design and construction industry, its acceptance and support continue to broaden among public agencies, private developers, building operators and users, architectural and engineering fi  rms, contractors, and materials producers.

With each passing year, green building techniques are becoming less a design specialty and more a part of mainstream practice.

The Building Life Cycle   Sustainability must be addressed on a life-cycle basis, from the origins of the materials for a building, through the manufacture and installation of these materials and their useful lifetime in the building, to their eventual disposal when the building’s life is...

Assessing Green Buildings   In the United States, the most widely adopted method for rating the environmental sustainability of a building’s design and construction is the  U.S. Green Building Council’s Leadership in Energy and Environmental Design, or  LEED™...

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Various types of localized corrosion can also occur:

(1) Pitting corrosion. In some circumstances the attack on the original anodic area is not stifled and continues deep into the metal, forming a corrosion pit. Pitting more often occurs with mild steels immersed in water or buried  in soil rather than those exposed in air.

(2) Crevice corrosion. Crevices can be formed by design-detailing, welding, surface debris, etc. Available oxygen in the crevice is quickly used by the corrosion process and, because of limited access, cannot be replaced. The entrance to the crevice becomes catholic, since it can satisfy the oxygen-demanding cathode reaction. The tip of the crevice becomes a localized anode, and high corrosion rates occur at this point.

(3) Bimetallic corrosion. When two dissimilar metals are joined together in an  electrolyte an electrical current passes between them and corrosion occurs on the anodic metal. Some metals (e.g. nickel and copper) cause steel to corrode preferentially whereas other metals corrode preferentially themselves, thereby protecting the steel. The tendency of dissimilar metals to bimetallic corrosion is partly dependent upon their respective positions in the galvanic series  (Table 35.1): the further apart the two metals are in the series the greater the tendency. Other aspects which influence bimetallic corrosion are the nature of the electrolyte and the respective surface areas of the anodic  and cathodic metals. Bimetallic corrosion is most serious for immersed or buried structures but in less aggressive environments, e.g. stainless steel brick support angles attached to mild steel structural sections, the effect on the mild steel sections is practically minimal and no special precautions are required.

Bimetallic corrosion and structural steelwork

Further guidance for the avoidance of bimetallic corrosion can be found in BS PD 6484, Commentary on corrosion at bimetallic contacts and its alleviation.

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In the mid 1990s, a series of realistic fire tests were carried out on a full-sized eight-storey steel-framed building at the Building Research Establishment’s large-scale test facility at Cardington, Bedfordshire (see Fig. 34.8). Full analysis of the test results has shown that the behaviour of steel members in a whole building frame with all of the restraint, continuity and interaction that can occur is very different from the behaviour of single members in unrestrained standard fire tests. Columns will need protection because deformation of columns can cause damage beyond the compartment of fire origin, but unprotected composite beams were able to survive a temperature of 1100°C without collapse (see Fig. 34.9). It became clear that mem- brane action in the composite slab was responsible for the observed high performance and that it would be possible to design steel-framed buildings in such a way as to allow the beams to remain unprotected.

Fig. 34.8 Realistic fire tests in a modern steel-framed office building showed that stability requirements can be maintained without beam protection

A further floor test was conducted at BRE’s Garston laboratory to quantify the effect of tensile membrane action, and Bailey and Moore  showed that the strength and location of the reinforcement in a composite slab and the aspect ratio of the slab panel itself largely govern its load capacity.

This analysis has been incorporated into simple design guidance by the UK Steel Construction Institute (Newman et al.), which allows secondary beams to remain unprotected in composite slab panels up to 9 metre span for fire ratings up to 60 minutes. These limitations may be extended by use of the Bailey and Moore calculation method from first principles or by application of a specialized finite element program such as VULCAN, which is adapted to deal with fire conditions.

Fig. 34.9 Unprotected beams were heated to 1100°C without collapse

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1 Erection sequence
Columns are placed in grids to suit the grid/zone areas as previously agreed. The columns are lifted with a Dawson Ratchet (see Fig. 33.2), nylon slings or chains.
Care must be taken to minimize damage to painted surfaces as repairs can often be difficult. Once placed into position the column is roughly positioned and plumbed. In extreme cases the columns may need guying off to ensure stability during the erection sequence. This procedure is repeated until a grid is formed.

Beams are then placed in location and secured by two bolts at each end connection throughout the designated area. Once all of the beams are located, the remaining bolts are placed. Since many structures employ composite slab construction, an intricate part of the site erection is the placing and fixing of the decking bundles.A grid system should have been agreed for the placing of bundles which the erection crew situates in convenient locations to ensure that connections are not obstructed, nor the structure eccentrically loaded. Heavy wires are then placed and left in position.

When this is achieved the erection crew can move on to the next area.The engineer with the line and levelling crew then follows on behind, pulling the structure to its correct alignment. Once this task is completed, the bolting-up crew can tighten all of the bolts and complete.The decking is then placed, followed by the stud welding.

Fig. 33.2 Dawson Ratchet

A typical flow chart of the process is shown in Fig. 33.3.

Fig. 33.3 Flow chart of the erection process

2 Lining, levelling and plumbing
If, in spite of having taken care over the initial levelling of packers and carefully positioning the column bases to the pre-set lines on the foundations, the structure still needs adjustment during lining, levelling and plumbing, then something may be wrong. The first thing to check is that no error has been made in the erection. If nothing is out of place then the drawings and the fabrication need to be checked.

If there is an error it will be necessary to take careful note of all the circumstances and advise all concerned immediately.

If the steelwork has just been thrown together with no great regard for accurate positioning it will be necessary to provide equipment and manpower to carry out the lining, levelling and plumbing.The equipment will include jacks, tirfor-type wire pullers with wires, straining screws, wedges, piano wires, and heavy plumb bobs with damping arrangements, all in addition to the level and simple theodolite which are needed for the basic check.

It is necessary to do the lining and levelling check before final bolting up is done.
In practice this means that it must be done immediately following erection since it is inefficient to slacken off bolts already tightened in order move the steelwork about.There is no way that erection can go ahead until the braced bay is bolted up, and that cannot be done until it is level and plumb.

A supply of steel landing wedges and slip plates is needed to adjust the levels.
They must be positioned in pairs opposite to each other on two sides of the base plate of the columns to be levelled. If the wedge is placed on one side only the column will be supported eccentrically and can in fact be brought down, especially if more than one column has been lifted on a series of wedges all driven in from the same direction. Alternatively toe jacks (jacks that can be used in tight areas) can be used in pairs to lift the columns. A temporary bench-mark should be established, and agreed with the client’s representative, in the vicinity of the columns in a position where it cannot be disturbed. The level will then be used to check the final setting with the seating packers inserted, and the column landed back on them and bolted down with the holding-down bolts. The column  can then be moved about in plan on these packers to bring it into line and bay length from its neighbours.

The position of a line, offset from the column centreline in order to clear the columns, should be marked and agreed. This line is then used either to string and strain a piano wire or to set and sight a theodolite telescope. The readings from it will then give the amounts by which movements must be made to keep the structure in tolerance.

Running dimensions from the previous column will give required  longitudinal movement. However, care should be taken to watch the plumb of the columns in this direction in view of the tendency of steelwork to ‘grow’ due to cumulative tolerances. Regard should be paid to column-to-column dimensions rather than to running dimensions from the end of the building.

Having levelled the bases of the columns in their correct positions, it is possible to check the plumb. Attention to temperature effects is necessary as it is futile to check the plumb of a building which is not all at the same temperature and, in the case of a long building, if the temperature is not standard.

The fact that the column bases are level does not mean that a level check is not
needed on the various floors in a tall building. It is important that the levels of any one floor are checked for variations from a plane rather than on the basis of running vertical measurements from the base, since these are affected by temperature and the variable shortening effect on the lower columns as weight is added to them.

Plumb can be most readily checked with a theodolite using its vertical axis and reading against a rule held to zero on the column centreline. This eliminates the effect of rolling errors and is a check against the same centreline used in the fabrication shops. If a theodolite cannot be used, a heavy plumb bob hung on a piano wire and provided with a simple damping arrangement, such as a bucket of water into which the bob is submerged, is a second-best alternative. Measurements are made from the wire to the centrelines in the same  way as before. The disadvantage of using a plumb wire is that all the operatives have to climb on the steel to take and then to check the readings. Optical or laser plumbing units are available; these are particularly useful for checking multi-storey frames.

Adjustments, and in extreme cases provision for holding the framework in its correct position, are normally necessary only if the frame is not self-stable. For instance, if structural integrity depends on concrete diaphragm panels for stability, consideration should have been given at the design stage to one of two alternatives: either the concrete panels should be erected with the steel frame, or temporary bracing should have been provided as part of the original planning.To ensure proper function, any bracing must be positioned so that it can be left  in place until after the concrete panel has been placed and fixed.

If diagonal wires have to be used they should be tied off to the frame at node points rather than in mid-beam, in order to avoid bending members.Also, their ends should be fixed using timber packers in the same way as is done when the pieces are slung for lifting. Turnbuckles or tirfor-type pullers provide the effort necessary to tension the wires. The sequence of placing, tensioning and ultimately removing these temporary arrangements should form part of the method statement prepared for each particular situation.Once the components of the building have been pulled into position the final bolting-up can be completed.

Portal frame construction in its temporary state often has the legs pre-set out of vertical, and becomes vertical only after the final loading has taken place. It is the responsibility of the designer to calculate the extent of pre-set. This often proves a difficult process to assess. Further, allowance must be made if the final position is needed for aesthetic purposes.

It is essential to ensure that the lining and levelling keep pace with the erection, since when additional floors are added it may become increasingly difficult to adjust, pull, or move the structure. As a rule of thumb, the lining and levelling of heavy sections should be within two to three bays of the erection face, whereas the lining and levelling of lighter sections should be within four to six bays of the erection face.

33.4.3 Tolerances
Prior to determining tolerances it is important to understand why they are needed. Rolling tolerances of the steel and manufacturing tolerances need to be accommodated. Tolerances must accommodate preceding work and subsequent building components.Tolerances must meet design requirements and architectural/ aesthetic requirements, and maximize buildability.

Tolerances can be categorized as structural (necessary for the  integrity of the intended structure), architectural (necessary for the aesthetics of the structure), and buildability (necessary to construct all of the components in a building). For a detailed coverage of tolerances in steel construction.

It is important to inform the designer of any deviation from specified tolerances, as failure to comply can often adversely affect both the structure and subsequent operations.

33.4.4 Holding-down bolts
To facilitate the erection of the structure, holding-down bolts  are normally positioned by the civils contractor. Movement tolerances must be accommodated in the foundation to ensure that correct alignment can take place (see Fig. 33.4).

Fig. 33.4 Holding-down bolts

Conical sleeves maximize movement without reducing the area of anchorage. It is important to have the bolts set both vertical and loose in the sleeve. The key benefit of using sleeved cast-in bolts is to allow movement for the alignment of the structure to meet tolerances.After checking and confirming that the holding-down bolts are acceptable, typical 100mm x 100mm steel packs (1–20mm thick) or similar are placed on the base to the required level.

33.4.5 Site bolting
Wherever possible, site splices should be designed to be bolted. This process is less affected by adverse weather than welding, uses simple equipment, and presents much less complex work access and subsequent inspection problems.

Bolts should be chosen to minimize size variation, and ease of repetition and selection. Design for Construction makes the following recommendations when selecting bolts:

(1) Preloaded bolts should only be used where relative movement  of connected
parts (slip) is unacceptable, or where there is a possibility of dynamic loading.
(2) The use of different-grade bolts of the same diameter on the same project should be avoided.
(3) Where appropriate, bolts, nuts and washers should be supplied with a corrosion
protection coating that does not require further on-site protection.
(4) Bolt lengths should be rationalized.
(5) Bolts should be threaded full length where possible.
(6) Connections should be standardized where possible.
(7) ‘Just-in-time’ delivery should be used, based on simple, easy-to-follow bolt lists.
(8) Approximately 90% of simple connections could be made using M20 60mm
long bolts.With a choice of three lengths, 95% of connections are possible.

Impact wrenches and powered nut runners speed the tightening of bolts.When high strength friction-grip bolts (HSFG) are necessary the Construction Industry Research and Information Association technical note for friction grip bolted connection should be used. If small quantities of HSFG bolts are to be used, manual torque wrenches are available.Where large quantities of friction-grip bolts have to be tightened, an impact wrench is an essential tool. Where access is  difficult, wrenches are made which run the nut up and are then used as manual ratchet wrenches to finally tighten the bolt.

Friction-grip bolts must be tightened first to bring the plates together and there-after be brought up to the required preload either by applying a further part turn of the nut or by using a wrench calibrated to indicate that the required torque has been reached. The manual wrench has a break action which gives the indication by means of a spring without endangering the erector by a sudden complete collapse.

Alternatively, load-indicating washers may be used to demonstrate that the bolt has been tightened to the required tension.

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1 Design information
Construction planning should start at the design phase since decisions at this stage will dictate the performance of the fabrication and erection teams. Site erection performance depends on many external and internal factors, and it is essential that these areas are properly managed.The site operation process is a complex interrelationship of other disciplines which influence performance, as shown in Fig. 33.1.

The use of value engineering has produced spectacular improvements in on-site performance by identifying the cost of alternative methods and  highlighting the need to minimize change in the site erection programme. The precise methodology it offers provides a unique vehicle for the incorporation of a wide range of construction expertise into the design process, with the ultimate objective of achieving building designs and construction which offer clients better value for money.

Fig. 33.1 Chart showing the dependence of the erection process on other disciplines

2 Programming
Resources can be best utilized when the delivery and erection operations are well planned to follow a logical sequence.Typically, civil and other trades need to be sufficiently ahead of the steel erection (i.e. one week minimum) to allow for a reasonable flow of uninterrupted work. This will allow the steelwork contractor time to survey the foundation bolts prior to accepting them. It is not unusual for the civil contractor to have to carry out minor remedial work due to poor alignment of the holding-down bolts, and to this end it is advisable to allow the maximum time possible for handovers. Accurate positioning of foundations is essential, and tolerances defined in the specification and the National Structural Steelwork Specification (NSSS) should be strictly adhered to.

Access restrictions or phasing of the works often govern the sequence of  erection, which normally follows a grid pattern, split into zones. The siting of the erection crane will also dictate the construction pattern. Once  the sequence and phasing have been agreed, the steelwork contractor can determine the resources required to meet the programme.

3 Delivery and off-loading of steelwork
A construction sequence programme is agreed prior to delivery of any materials to site, from which a delivery schedule may be produced. A more detailed piece-by-piece schedule for the steel erection method will need to be carried out to ensure smooth working on site. Particular attention should be paid to the logistics of this exercise to prevent members being missed out, double handling of materials on site, lorries being under-utilized, and the positioning of steel in the wrong location.

Delivery of the steelwork is normally in 20-tonne lorry loads, often sub-divided into four bundles of 5 tonnes each to suit tower crane capacity, on a ‘just in time’ delivery sequence in order to alleviate site congestion, and minimize double handling and on-site damage. Material needs to be stacked in the lorry for easy off- loading, with items that are needed first, readily available. Often the loading of thelorry can be dictated by crane off-loading capacity or stability of the load as columns are usually required first, yet they need to be at the bottom of the load because of their weight.

Shakedown areas (areas for separating and selecting steel bundles) are required on site to allow the sorting of the steelwork.The area should be firm and level with a plentiful supply of wooden sleepers or other suitable material.The steel members will have been allocated a unique code related to section, level and piece number: this is usually stamped on the top surface to identify its orientation. In multi-storey high-rise buildings it is usual to create temporary shakeout platforms at various stages of the construction to minimize crane hook time.

Cold-formed roof members and steel decking packs also need to be placed in location as the work proceeds since crane access is often restricted later. Each bundle should be numbered and placed in convenient locations as subsequent  relocation may prove difficult and costly.

4 Sub-assemblies on site
The designer may have been limited by the size and weight of components for transportation: thus it may prove advantageous to erect canopies, roof trusses, lattice girders and the like ‘piece small’ and then lift them as completed components.

If the decision to sub-assemble has been made there will remain the further question of deciding whether an area in the stockyard is to be dedicated for this purpose or whether the work will be done behind the cranes on the erection front in order that the assembly can be lifted straight off the ground and into position as the crane is moved back.

The most common components to be assembled behind the crane are roof trusses or lattice girders, which, because of their size, are almost always delivered to site ‘piece small’. However, there is not always space for this to be done and other locations, such as the stockyard, have to be made available.

Where the potential sub-assembly area, and even the stockyard itself, is remote from the erection site, very careful investigations are needed before a decision to sub-assemble can be agreed. Local transportation size restrictions on the route to the erection cranes may rule this option out.

There are three factors which affect the practicability and economy of subassembling a unit on the ground:

(1) The weight of the eventual assembly, including any lifting beams required
(2) The degree to which the unit is capable of being temporarily stiffened without unduly increasing its weight
(3) The bulk of the unit, i.e. will it be possible to lift it to the height needed without fouling the crane jib?

Sub-assembly is only worthwhile if the unit can be lifted and bolted into place almost as easily as a single beam. In other words, the object of  the exercise  is to avoid carrying out operations at height which can easily be done at ground  level.

The speed of construction is dependent on the number and type of crane lifts and connection.The rate of erection will also vary according to the weight, size and location of the piece being erected. Erection speed is often dictated by the distance between the crane and hook, thereby emphasizing the need to consider lifting bundles closer to the erection face and providing shake-out platforms.

It is impossible to provide a chart showing piece counts per hour or day.However, as a broad rule of thumb, an experienced erection crew working in ideal conditions  at low level with average weight, standard-sized components should be able to erect 40 to 60 pieces per day per hook.

5 Interface management
In order to minimize on-site disruption, the specialist/trade interface must be managed successfully. Failure to understand the requirements of  other trades through unclear specifications and uncertain allocation of responsibility may result in conflict and disagreement. Thus the development of open communication between all parties and the establishment of clear objectives during regular site meetings between overlapping trades are necessary measures to ensure proper component, plant, and work area management. Unfortunately, these measures  constitute some of the most neglected areas in the construction process. Failure  to adequately address these issues can result in cladding fixings being left off or located in the wrong place, holding-down bolts being incorrectly positioned, lift shaft tolerances proving incompatible with the main frame, and so on.

Thus co-operation between all parties is necessary to ensure enhanced efficiency.

This may be achieved via:

(1) An improved communication process
(2) A clear contractual definition
(3) The use of single-source suppliers.

6 Surveying and aligning the structure
Surveying should be carried out in accordance with BS 59644 (1990) Parts 1 to  3  Building setting out and measurement, and adjusted for temperature outside  the range of 5°C to 15°C. Accurate setting-out is crucial for maintaining control  of tolerances and achieving a structure which is acceptable to  all the subsequent  trades.     

The site should be provided with a dumpy or quick-set level and with a simple theodolite. On larger sites, it is common practice to use an electronic digital  measurement (EDM) instrument. The advantage of a theodolite is that the transit of the instrument can later be used for checking the plumb of the columns. The
telescope can be employed for the reading of offsets to check their alignment.Time spent on laying down and marking the cross-centre lines of the  column bases on individual boards at each base and subsequently on to the concrete of the base is time very well spent. The base can then be quickly and accurately set in both  directions and to the correct level on the pre-set pads.The value of this work is that it enables errors in the concrete work to be identified at a time when corrective measures can be undertaken.The accurate setting of grouted packer plates, on which to land the column, serves the same purpose, i.e. it will facilitate the identification of concrete which is high and holding-down bolts which are low.

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It is now customary for the steelwork specialist to provide an erection method statement for the site work. The purpose of this is to set out a safe system of work for the delivery, erection and completion of the intended structure, thereby allowing the design team the opportunity to appraise this plan and make any appropriate observations or changes.

The erection method statement will set out procedures for delivery and  assembly including the phasing of the erection, bolting, welding, and methods of erection and safety of the steelwork package. Guidance note GS28 Parts 1 & 2, produced by the Health and Safety Executive (HSE), contains full details of the contents of a method statement. The main items described include:

(1) Arrangements for scheme management, including co-ordination and  responsibility allocation of supervisory personnel at all levels
(2) Delivery and off-loading of the steelwork and other materials, including lifting methods and weights of each load.
(3) Erection sequences, notification of the scheduled starting position or positions if phased construction is required.
(4) Methods of ensuring continual stability of individual members (including columns) and sub-assemblies, as well as of the partially erected structure.
(5) The detailed method of erecting the structure or erection scheme devised to ensure that activities such as lifting, un-slinging, initial connecting, alignment and final connecting can be carried out safely.
(6) Provisions to prevent falls from height, including safe means of access and  safe places of work, special platforms and walkways, and arrangements for the early completion of permanent walkways, mobile towers, aerial platforms, slung, suspended or other scaffolds, secured ladders, safety harnesses and
safety nets.
(7) Protection from falls of materials, tools and debris by the provision of  barriers such as screens, fans and nets.
(8) The provision of suitable plant (including cranes), tools and equipment of  sufficient strength and quantity.
(9) Contingency arrangements to guard against the unexpected, such as a breakdown of essential plant, or the delivery of components out  of  sequence.
(10) Arrangements for delivery, stacking, storing, on-site movement, fabrication or pre-assembly and the siting of offices and mess rooms.
(11) Details of site features, layout and access, with notes on how they may affect proposed arrangements and methods of working.
(12) A detailed risk assessment to ensure all safety precautions have been taken.
(13) Handover procedures, including a method of checking the alignment of the structure and methods of determining bolting and welding completion.

These items enable the requirements of the Health and Safety at Work Act 1974 to be satisfied.
A complex job may require a series of complex method statements, each  covering a particular aspect of the work.

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Introduction

Planning for the erection of any structure should commence at the design phase.
The incorporation of buildability into the design phase may allow significant additional benefits. If a structure is to be erected quickly, to programme, and to the lowest possible cost, consideration must be given to the planning of the site works.

Work at a height and on site should be minimized, especially if this work could be carried out at the factory in ideal conditions. Where possible, frames and  components should be designed to be built in the factory or assembled at low level for subsequent incorporation into the works. This will save time and cost as the  work is not so weather dependent and expensive temporary works are not required.

Planners should:

• plan for repetition and standardization
• plan for simplicity of assembly
• plan for ease of erection – keep it simple
• agree information handover dates and design sign-off dates
• make allowance for trade interfaces
• allow realistic programmes for manufacture and erection
• recognize the complexity of the design process
• hold regular co-ordination meetings
• identify and fairly allocate risk
• consider long-term stable relations with teamwork.

Attention to site construction methods are now considered an essential part of any design. So too is observance of the Construction (Design and Management) (CDM) Regulations aimed at enforcing the safe construction and use of structures.

The designer of the structure should take into account site access, materials handling, the construction sequence, and any limitations these may impose on the construction project.

One of the key factors to improvement in construction is the specialist  trade contractors (STCs) as they carry out as much as 90% of the contract value.

Utilizing the expertise of specialist contractors at an early stage will maximize the speed of construction, reduce conflict, and give the opportunity of adding real value to the building.

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Open-grid proprietary systems are often the economical solution for industrial flooring and walkways, particularly where ventilation or light must be available through the flooring.They are generally available in spans up to 2.0 metres, but can be supplied to span 4.0 metres. The design allows spanning in one direction only.

Stair treads are also available. (See Fig. 30.2.)

Open-grid systems are usually manufactured in grade S275 steel, with painted or galvanized finish. Aluminium and stainless steel types are also available. Load-carrying capacities and fixing details are readily available from suppliers’ catalogues.

Fig. 30.2 Open grid flooring. (a) Open bar; (b) diamond pattern; (c) expanded metal; (d) Q-grating

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Steel plate used for floors, walkways or for staircase treads normally has a raised pattern of the non-slip type such as Durbar plate supplied by Corus.Where a plated area will have only occasional use, such as walkways for maintenance access, then plain plate may be used.

If properly designed, both plain and non-slip plates have a dual function in carrying floor loads and in acting as a horizontal diaphragm in place of a separatebracing system. For example, a plate may be connected to the top flange of crane gantry girders to act as a surge girder and also as a means of crane or maintenance
access; the surge girder elements should be designed as a horizontal plate girder.

When steel plate floors are in situations where moisture or high humidity is present, then the plates should be continuously welded to the support members to avoid corrosion problems. In dry areas connections may be fastened by bolting with countersunk bolts, intermittent welds, or clips. When clips are used they must be
arranged so that they cannot move out of position.

Plain plate is commonly available in grades S275JR and S355JR to BS EN 10025.

Durbar plate
The pattern of Durbar plate is shown in Fig. 30.1. The pattern is raised 1.5mm to 2.2mm above the plate surface.When ordering, the thickness should be specified as ‘thickness on plain’.

Durbar-pattern plate is free draining and can be matched width to length without affecting appearance.
Durbar plate is commonly available in grade S275JR and S355JR to BS EN 10025, also ‘commercial quality’ with a UTS not less than 355N/mm2.

Fig. 30.1 Durbar plate