Engineering and Construction

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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

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It is important that the long-term performance of the structure is considered both in the choice of structural form and in the design of construction details. Failure to do so may result in maintenance problems requiring costly repair.

1 Corrosion allowances
The means for countering the effect of corrosion of steel piles are well developed.
Guidance is given in the British Steel Piling Handbook.

BS 80028 considers that the end of the effective life of a steel sheet pile occurs when the loss of section, due to corrosion, causes the stress to reach the specified minimum yield strength. A pile section chosen for the in-service condition has to be adequate at its end-of-design-life, at which time the effective pile section will have been reduced by corrosion.

As the corrosion loss allowance varies along the pile according to the corrosion environment, the designer needs to be aware that the maximum corrosion may occur at a different level to that of the maximum forces and moments, and should allow for this accordingly.

Also, since redistribution of earth pressures may occur as a result of increased flexure of a corroded section, the end-of-design-life condition may be a critical design load case in the selection of the sheet pile section.

2 Corrosion and protection of steel piles
The design life requirements for proposed buildings and individual components or assemblies are defined in BS 7543 Guide to durability of buildings and building elements, where a building design life can range from 10 years for a building with a ‘short’ life to 120 years for civic and other high quality buildings. A retaining wall which is part of a building structure, i.e. a basement, must therefore comply with these requirements and be designed with sacrificial thicknesses  applied to each surface, depending on the exposure conditions. The exposure conditions are based on the advice given in BS 8002 clause 4.4.4.4.3 and are shown in Table 29.4.
Another way of allowing for sacrificial thickness is to use a higher strength steel than would be required if no corrosion were assumed (i.e. use steel grade S355GP, to BS EN 10248, in a wall designed for steel grade S270GP).This permits a greater loss of metal before stresses become critical

It should be noted that the corrosion allowances apply to unprotected steel piles.
Although it is generally cost effective to provide the sacrificial steel thickness, consideration can alternatively be given to the following corrosion protection options:

• Protective coatings, particularly in the exposed section of the pile.
• Cathodic protection in soil below the water table or in a marine environment.

Details of these options are given in the British Steel Piling Handbook.


3 Corrosion in fill or industrial soils
Buildings can be constructed in areas of recent fill or industrial soils. Corrosion protection of the steel in contact with the fill material may be required, and this can be assessed by testing the material for pH and resistivity.

The nature of in situ fill soils can be variable, and a full soil analysis is required to assess the likely corrosion performance of steel in the environment. Soil tests to determine the pH of the soil should be in accordance with BS 1377-353 and as directed by the contract to determine resistivity. Other tests may be relevant, and
most of these are reviewed in CIRIA’ s series of reports on contaminated land (contact CIRIA for further details).

In a controlled fill, no special measures are required, and the same corrosion rates as in natural undisturbed soils can be assumed.

Corus (formerly British Steel) has undertaken significant research and development into corrosion of steel and corrosion protection. Further advice on corrosion assessment and protection can be obtained from Corus Construction Centre or from
The Steel Construction Institute.

4 Corrosion and structural forces
It is not immediately obvious whether the start of in-service life case or the end-of-design-life case will be the most critical for the structural design of retaining walls.

At the end-of-design-life, the reduced stiffness of the corroded steel pile will permit increased deflection, which will in turn reduce the soil pressures acting upon it (and therefore the induced moments and shears).

As the corrosion loss allowance varies along the pile according to the corrosion environment, the designer should consider that the maximum corrosion may not occur at the same level as the maximum section stresses.

The reduced (corroded) section properties can be obtained either by calculation or from the British Steel Piling Handbook or other Corus brochure.

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1 Steel pile installation tolerances
Information on tolerances that are achievable using commonly available pile driving
equipment and methods is quoted in the Institution of Civil Engineers publication


Specification for piling and specifications issued by the Federation of Piling
Specialists, the CEN Standard EN 12063 Execution of special geotechnical works – Sheet-pile walls, and the TESPA publication Installation of steel sheet piles.

Table 29.3 is included in the TESPA publication and represents tolerance levels for sheet piling which should not be too onerous to achieve but will give results which are visually acceptable – an important feature for permanent exposed sheet piling.

Steel pile driving tolerances (TESPA)

Accuracy of alignment will also be affected by pile stiffness, the driving equipment, and the experience of the workforce.Use of pile guide frames,which are often formed from universal beams aligned with their webs horizontal, will ensure that good alignment of the sheet piles is achieved.

2 Environmental factors: noise and vibration prediction

Increasing attention has been directed to environmental factors with regard to driven piles in recent years. Although the duration of the piling contract may be short in comparison with the whole contract period, noise and vibration perception may be more acute during the piling phase. Human perception is very intolerant of noise and vibration or shock transmitted through the ground, and tolerance requires careful prior education of the public. Efforts made to advise the public and to plan the precise times of driving carefully can reassure those likely to be affected in the vicinity of a pile installation and can result in the necessary cooperation.

In the UK, the Control of Pollution Act (1974) provides a legislative framework for, amongst other things, the control of construction site noise. The Act defines noise as including vibration and provides for the publication and approval of codes of practice, the approved code being BS 5228. Part 4 of the code deals specifically with piling noise. This code was revised in 1992 to include guidance on  vibration.

Two relevant documents include the TRRL Research Report RR53  Ground vibration caused by civil engineering works, and the British Steel publication Control of vibration and noise during piling.

The SCI publication Specifiers’ Guide to Steel Pilingalso contains useful advice. BS 6472 deals specifically with evaluation of human exposure to noise and vibration in buildings.



2.1 Noise from piling operations

Pile driving is perceived to be an inherently noisy operation because impact-based methods of installation have historically been used.Typical data on noise levels produced by piling operations have been published by CIRIA Report No. 64 Noise from construction and demolition sites – Measured levels and their prediction.

These are discussed and interpreted in CIRIA Report PG9 Noise and vibrations from piling operations. It is important to note that modern pile installation equipment such as the noise-free pile jacking machines will not be included as they post date these publications.

Impact driving of steel sheet piling is often noisy since the operation involves steel-to-steel contact. In areas where severe restrictions are placed on noise levels, pile vibratory or jacking equipment should be used.
Such machines emit a different frequency and lower level of noise which may be acceptable, and recent advances in noise reduction technology can ensure that the noise from the auxiliary power pack is also lowered.

2.2 Ground vibrations caused by piling

It is widely recognized that noise and vibration, although related, are not amenable to similar curative treatment. In the main, noise from a site is airborne and consequently the prediction of noise levels is relatively straightforward, given the noise characteristics and mode of use of the equipment. On the other hand, the transmission of vibration is determined largely by site soil conditions and the particular nature of the structures involved. General guidance can be derived from the study of case histories of similar situations. Useful references on the subject of ground vibrations are provided by CIRIA Technical Note 142 Ground-borne vibrations arising from piling, the publication  Dynamic ground movements – Man-made vibrations in ground movements and their effects on structures, and BRE Digest No. 403 Damage to structures from ground-borne vibration.

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Benefits of the use of steel sheet piling include:

• Construction is faster than that for reinforced concrete walls
• Are a narrow form of construction, which can be installed close up to the boundary of the site, maximizing usable building space
• Suitable for all soil types
• No requirement to excavate for wall foundations
• No disturbance of existing ground or arisings unlike that for bored concrete piling
• The steel components are factory quality as opposed to site quality
• High ductility that can reduce bending stresses and soil reactions
• Can easily be made aesthetically pleasing
• Can be placed in advance of other works
• Immediate load-carrying capacity
• Forms a ‘curtain walling’ to contain the working site
• They are a ‘sustainable’ product as they can be extracted easily and minimize waste

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There are various types of earth-retaining structures.Where a differential surface level is to be established with a vertical interface, a retaining wall is used. Steel sheet piling is one form of retaining wall construction. Sheet piles resist soil and water pressures by functioning as a beam spanning vertically between points of support, as shown in Fig. 29.10.

Steel sheet piles are used for temporary and permanent retaining walls. In tem- porary works and cofferdams the sheet piles enable deep excavations to be made to facilitate construction below ground and water level of other permanent works.

On completion of construction the sheet piles are usually extracted for reuse on other projects.

More recent applications have seen the use of sheet piles as permanent retaining walls. Structures include basements, underground carparks and abutments for bridges including integral bridges.

Fig. 29.10 Sheet pile retaining wall

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Steel piles can be installed by a variety of methods and equipment.There are three types of installation equipment, which operate by impact, vibration or by jacking.

Each has particular advantages and disadvantages, and the final choice is, in most cases, a balance between speed and economy of installation. A further deciding element is the increasing concern for noise and vibration control to which the industry has responded with the development of new installation equipment such as hydraulic hammers.

The installation of steel piles is a specialist activity, calling for considerable knowledge and experience of handling piles and hammers to achieve an acceptable placement within specified tolerances of position and level. Guidance on the practical limits that can be achieved in position and level for driven steel piles is available from the FPS (Federation of Piling Specialists) and in the TESPA (Technical

European Sheet Piling Association) publication  Installation of steel sheet piles.

Guidance is also included at the back of the new ICE Specification for Piling and Embedded Retaining Walls and in the Corus Piling Handbook.

The designer should refer to these documents before carrying out a design, because the advice given will often affect the details at connections to the pile cap in the structure.

There is much to gain from matching the stiffness of the pile to the hammer and to the anticipated soil resistance at the site to achieve satisfactory driveability and to ensure achievement of the required design penetration.

In addition, following research work over the last 30 years, there is a developing understanding of the benefits of measuring the soil resistance during driving as a check on the designer’s predicted compressive axial static capacity (see the ICE

Specification for Piling and Embedded Retaining Walls).Although there are some caveats to be applied to this practice, it is indisputable that  both designer and installer gain from using dynamic analysis of pile driving and that this is of ultimate benefit to their clients in many ways. For instance, it is a QA tool to permit the evaluation of piles that have an unexpectedly high resistance at a higher level; it can lead to fewer and shorter piles being acceptable; it can save construction time by avoiding delays; and it provides an equitable means of resolving disputes about specifications that turn out to be unrealistic due to inadequate site investigation or inaccuracy in design prediction methods.

The relationship between the static and dynamic capacity is now better under- stood, and in many soils and rocks there is very little difference between them.

However, it is still sound practice to carry out static load testing to check the design penetration because in some soils pile capacity has been known  to decrease after driving. Test piles should therefore have both static and dynamic measurements in order to relate the two, and then dynamic pile analysis can be used to check the project piles.

1 Determining bearing capacity
The safe load capacity of driven steel piles should be verified.The only certain way to do this is by static test loading. Unfortunately, to do this for every pile is neither practical nor economic for most projects.

Alternative ways of load testing piles are by stress wave analysis using the CAPWAP program21
and a dynamic pile analyser.

Pile head settlement is most accurately determined by static test load. Dynamic formulae give no indication of this but stress wave analysis will give an indication the technique is rapidly improving.

Static test loading
Two forms of test loading are in general use, but the maintained load method is considered the more accurate.Details of both test methods are given in BS 8004 clauses 7.5.5 and 7.5.6.

Excessive conservatism has been found in current practice and in the currently used specifications for load testing piles, which has been compounded by unrealistic design assumptions on the soil parameters that are used in pile resistance prediction methods to determine a ‘working load’. This has led to costly overdesign of
piling.

The amount of kentledge or tension resistance should always be in excess of the estimated ultimate bearing capacity of the pile.A factor of 1.5 should be a minimum, since loading to twice the ‘working’ or ‘serviceability’ load is the accepted requirement, and the working load is normally about half the ultimate capacity in both  LSD and ASD procedures.

In the maintained increment load test (MLT), kentledge or adjacent tension piles or soil anchors are used to provide a reaction for the test load applied by jack(s) placed over the pile being tested. Figure 29.8 shows a typical arrangement.The load is increased in defined steps, and is held at each level of loading until all settlement has either ceased or does not exceed a specified amount in a stated period of time.

Fig. 29.8 Test load arrangement using kentledge

A plot of load versus head settlement (Fig. 29.9) provides an understanding of pile head performance, which is an essential requirement for ensuring compatibility between superstructure and foundation, but it reveals only part of the picture of overall pile performance.

Fig. 29.9 Load–settlement plot

Elastic shortening can be significant in steel piles, especially those which are long and fully stressed in grade S355 quality. Such shortening is of no consequence provided it is anticipated and allowed for in design and evaluation.

2 Pile-driver calibration
To overcome the need to test load every pile, careful records taken during the driving of a subsequently satisfactorily test-loaded pile can be used to evaluate later piles. This system is reliable when the hammer, pile size, pile length, and geology remain unchanged for subsequent piles. However, as soon as any one of these elements is changed, further test loading and recalibration is required.

3 Dynamic formulae
These were an early attempt to relate the energy output of the impact hammer and the pile penetration per blow of the hammer to the static resistance of the pile.There are a number of such formulae available and they date from the 1920s but all have proved to be unreliable and are no longer used.

4 Dynamic stress wave analysis
Dynamic methods have gained wide acceptance and are embodied in the new ICE Specification for Piling and Embedded Retaining Walls. Direct on-site computer analysis of pile gauge readings provides information on pile toe resistance, pile shaft resistance, pile shaft integrity, and actual energy input to the pile head.

This form of evaluation provides valuable information as the pile is driven, which is an ideal arrangement. However, it still relies on relating dynamic resistance and ‘set’ to static resistance, and care must be taken to ensure the results are not mis-leading.Redrive checks at ‘set’ are essential, and a cross-check by static loading may be advisable at the start of a large contract.

Redrive checks must be carried out to ensure that the resistance of the pile does not change after driving is completed.This is most likely to occur if pore water pressures in the soil are altered by the action of driving the piles.

5 Design stresses during driving
A driven pile is usually driven to a resistance of twice the working load or to ultimate bearing capacity to demonstrate an adequate factor of safety. BS EN 12699 embodies offshore practice where it states that the maximum dynamic stress in the shaft during driving should be limited to be less than 90% of yield.

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1 Design basis
The basis of design of any bearing pile is its ultimate axial capacity in the particular soil conditions at the site where the structure is to be built.The design resistance is determined from the ultimate capacity, and the designer verifies that this is adequate to carry the required loads from the structure.

Until recently, design resistance of foundations has been evaluated on an allowable stress basis. However, structural design is now using a limit state design (LSD) basis, whereby partial factors are applied to various elements of the design according to the reliability of the method and of the soil properties. However, the application of limit state design philosophy to geotechnical design is causing difficulty in a discipline where the allowable stress approach and terms such as the ‘allowable bearing pressure’, ‘permissible steel stress’, and ‘allowable pile capacity’ are widely accepted and understood. Nevertheless, the LSD approach is being progressively adopted in the British Standards as they are revised, and is the basis for all the Eurocodes, including that for foundation design.

2 Design standards
The common design standard used for the design of bearing piles is the offshore industry’s recommended practice for steel tubular piles, based on US and UK North Sea experience, which is contained in the American Petroleum Institute Code RP2A (API RP2A) that will be adopted in the ISO Code 13819-2.

A draft of the new Eurocode 7: Part 16 has recently been produced after several years of effort by an eminent team, and the UK pre-standard DD-ENV 1997-17 with the National Application Document (NAD), was published in July 1995. It presents a more rigorous treatment of limit state design (LSD) than any of the British Standards relating to foundations so far.Allowable stress design (ASD) is still, however, permitted in BS 80028 and BS 6349, in order to be compatible with the approach taken in BS 449.

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Combi-piles are another form of part-fabricated piling used to form walls for deep excavations. The wall is formed from alternating tubular and sheet piles as shown in Fig. 29.4. The number of sheets between the tubes varies dependent on the required section modulus. The tubular piles take vertical loads and stiffen the wall.

Tubular piles are available in a wide variety of sizes, and both Larssen and Frodingham sheet piles are suitable as infill piles.Appropriate clutches are welded to the walls of the tubes, and these provide the designer with simple corner details for many different wall angles. It is apparent that the clutch locking bar strips from U-section sheet piles (or Larssen sections) are more stable to weld onto the tubulars than those from Frodingham section sheet piles.

Fig. 29.4 Combi-piles

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Buttressing a sheet pile wall with deep universal beams placed  in ‘soldier’ fashion creates a high modulus pile wall as shown in Fig. 29.3. This section provides additional moment capacity and bearing capability. The UBs are welded to the back of a pair of Frodingham sheet piles and all three are driven together clutched to the next panel.

Fig. 29.3 High modulus piles

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Hot-rolled steel sheet piling is more extensively used nowadays in permanent as well as temporary retaining walls. Applications are not only above-ground earth-retaining structures but also composite basement wall construction, urban railway and road cuttings, bridge abutments, river bridge pier caissons, as well as the traditional river and coastal protection works. Economic design is enabled by better knowledge of soil–structure interaction and highly developed design and analysis programmes.

Box piles are particularly suitable for marine structures, such  as jetties and dolphins, where part of the pile shaft is exposed above seabed level and the pile functions as a free-standing column or is connected at the head in clusters of columns.

Box piles are formed by welding two or more sheet pile sections together. Both Larssen and Frodingham steel sheet piles (see section 29.2.3) can be used.They can be introduced into a line of sheet piling at any point where local heavy loads are to be applied, for instance beneath bridge beams, or used separately.They are clutched together with adjacent sheet piles and can be positioned in a sheet pile abutment so that its appearance is unaffected.

Larssen box piles are formed by welding together two sheet pile sections with continuous welds, and Frodingham plated box piles are formed by  continuously welding a plate to a pair of interlocked and intermittently welded sheet piles (see Fig. 29.2).

Special box piles can be formed using other combinations of sheet piles. Further information can be obtained from steel manufacturers.

Fig. 29.2 Box piles

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Tubular piles were first used as foundations for offshore oil platforms in the oil fields of Lake Maracaibo in Venezuela in the 1920s. Initially, spare oil pipe was used out of convenience but, as the supporting structures became more sophisticated, the cold rolling of piles in structural plate to project-specific diameters and wall thicknesses became more common.
 
Purpose-rolled tubular piles can be used, but high quality steel line pipe is perfectly suitable for piling. Tubular piles are available in a large range of diameters and wall thicknesses. Typical sections used for piling purposes are produced as line pipe to API 5L grades X52 (yield stress of 52ksi, approximately equal to 355N/mm2) to X80 (yield stress of 80ksi, approximately equal to 555N/mm2).

Tubular sections can be hot- or cold-rolled. The cold-rolling process produces consistently higher yield strengths than those of hot-rolled steel products and this can have significant benefits for highly loaded bearing pile and structural column-pile applications, and can also permit harder driving.
 
Steel tubular piles have a high stiffness and are therefore also suitable for sites where it is necessary to transfer bearing loads into buried bedrock. They are particularly suitable for marine structures, especially where sited in deep water, e.g. berthing jetties for deep draught vessels. Increasing use is being made of steel tubulars in composite columns for buildings and bridges where the tube is first driven as a pile before filling the upstand above ground level with a reinforced concrete core for added strength and rigidity. This permits significant savings in cost due to faster construction.More applications will become possible as the results of research into the effects of pile driving, dynamic load testing, corrosion, and new coatings are analysed and reported to design engineers.

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Universal bearing piles are H-sections produced on a universal hot rolling mill–hence their name.They are essentially the same as universal column sections, except that they have uniform thickness throughout the section. See Fig. 29.1. Universal column H-piles are made in two basic qualities of steel, grade S275 (yield  = 275N/mm2) and grade S355 (yield = 355N/mm2) to BS EN 10025. Typical section sizes are presented in the Appendices.
 
Steel H-piles are very efficient in providing a large surface area for generating shaft friction resistance. In any given foundation plan area, a greater number of  steel H-piles can be used in a group with a standard spacing of 2B (or 2 x dia.) than concrete piles and either the load supported can be greater or the size of the group made smaller.The stiffness of H-piles is different about each orthogonal axis, allowing designers to select the orientation necessary to achieve the most efficient design.

Fig. 29.1 Universal bearing pile

H-piles are used principally for terrestrial structures where the full length of the pile is embedded, e.g. foundations for bridges and buildings, and therefore buckling is not a problem because there is good soil lateral support.They are used very effectively to transfer bearing loads into buried bedrock and to get around buried obstructions. It is sometimes advisable to use special cast steel pile shoes to strengthen the tip and prevent damage or buckling under hard driving conditions into bedrock where very high end bearing pressure can be achieved.

They are generally not the most suitable for conditions where long lengths of the pile shaft protrude above ground level and are unsupported by surrounding soil or through water, because buckling failure about the minor axis can occur at relatively low axial loads. In these cases consider the use of tubular or box piles that have more stiffness.

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Steel bearing piles are available in three basic profiles: universal column H-sections, tubular sections and box piles. In addition steel sheet piles, High Modulus Piles and Combi-piles can be used to support vertical load as well as a soil retaining function.

1 Universal bearing piles   Universal bearing piles are H-sections produced on a universal hot rolling mill–hence their name.They are essentially the same as universal column sections, except that they have...

2 Tubular piles   Tubular piles were first used as foundations for offshore oil platforms in the oil fields of Lake Maracaibo in Venezuela in the 1920s. Initially, spare oil pipe was used out of...

3 Box sheet piling

4 High modulus piles

5 Combi-piles