Engineering and Construction

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In the UK, two profiles, designated as U and Z, are available. U profiles are also commonly referred to as Larssen sections and Z profiles as Frodingham sections.

See Fig. 29.11. In addition, where the depth of excavation is small, trench sheet sections may be used as alternatives to sheet piles.

The essential difference between these sections and ordinary structural beams is their ability to interlock with each other to form a continuous membrane in the soil. (Sections and their properties are included in Appendices.) Steel sheet piling, including sections for box piles, is produced in accordance with BS EN 10248, the most typical grades being S270GP and S355GP with yield strengths of 270N/mm2
and 355N/mm2 respectively. In addition, higher-strength steel sheet piles can be obtained to grades S390GP and S430GP (yield strengths 390N/mm2 and 430N/mm2  respectively).

Fig. 29.11 Larssen and Frodingham sheet piles

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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

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Bearing piles are used mainly to support vertical loads for which the main design requirements are to restrict average settlement to a reasonable amount, eliminate differential settlement, and achieve an adequate factor of safety or load factor against overload.

Steel piling offers many advantages compared with other types including:

• Reduced foundation construction time and site occupation
• Reliable section properties without need for onsite pile integrity checking
• Inherently high strength to weight ratio
• Higher end bearing resistance in granular soils and rocks mobilized by pile driving as compared with boring
• Pile load capacity can be confirmed during driving by dynamic pile analysis (DPA) on each pile
• Low displacement of adjacent soils during driving and no arisings
• Easily removed for site reinstatement
• Reusable or recyclable following extraction
• Closer spacing possible.

The ability to be transported and handled without concern for overstressing due to self-weight is an important consideration where piles need to be lifted, pitched and driven in long lengths, e.g.maritime structures. Steel piles can withstand high driving stresses, thereby enabling them to be installed without significant damage through difficult strata such as brownfield sites and subsurface layers of boulders or weath- ered rock. Energy-absorbing structures such as jetties and dolphins can be particularly well constructed in steel by virtue of its ability to offer large elastic deformation

and bending strength without affecting its durability. On projects such as river or estuary crossings, steel piles offer clear advantages, as the soils are typically granular and waterlogged and unsuitable for satisfactory pile boring. New applications such as supports and abutments for integral bridges offer more  economic con- struction methods and the property of compliance under lateral loading that reduces bending stresses and therefore section requirements.

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The casting-in of the holding-down bolts with adequate provision for adjustment requires that they are positioned in the concrete surrounded by a tube, conical or cylindrical, or a polystyrene former. After removal of the tube or former, which should be delayed until the last possible time before the erection of the columns, the available lateral movement of the bolts should be between three and four times the bolt diameter. In cases where open tubes are used they should be provided with a cap or cover to prevent the ingress of water, rubbish and mud. After erection, lining, levelling and plumbing of the frame the grout voids around the bolts should be cleaned out by compressed air immediately prior to grouting. The bolt grouting and baseplate filling should be done as two separate operations to allow shrinkage to take place. During the levelling and plumbing operations wedges and packings are driven into the grout space. Before final grouting these should be removed, otherwise, after shrinkage of the grout filling material, they will become hard spots, preventing the even distribution of the compressive forces to the concrete base.

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The most generally used holding-down bolts are of grade 4.6 although 8.8 grade are also available (see Table 27.3). They are usually supplied : square head, square shoulder, round shank, hexagon nut. Each bolt must be provided with an anchor washer (square hole to match the shoulder) or an appropriate anchor frame to embed in the concrete in circumstances of high uplift forces. In such cases the anchor frame may be composed of angles or channels. Typical frames are shown in Fig. 27.15.When long anchors are required a rod threaded at both ends may be used, and in exceptional circumstances when prestressing is required a high tensile rod (usually Macalloy bar) is adopted. In both these cases provision should be made to prevent rotation of the rod during tightening which may result in the embedded nut being slackened.

Corrosion of holding-down bolts to a significant extent has been reported in some instances.This usually occurs between the level of the concrete block and the under- side of the steel baseplate, in aggressive chemical environments or at sites where moisture ingress to this level is recurrent. Fine concrete grout, well mixed, well placed and well compacted will provide the best protection against corrosion, but in cases where this is not adequate for the prevailing conditions, an allowance may be made in the sizing of the bolts or by specifying a higher grade bolt which  provides a larger factor of safety against tensile failure in the event that some corrosion does occur.

Fig. 27.15 Typical anchorages for holding-down bolts

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1 Bolt forces
The area required to transmit the compressive forces under the  baseplate is calculated at the appropriate bearing strength of the concrete.The stress block may be assumed to be rectangular with a maximum stress of 0.6 fcu where fcu is the characteristic cube strength of the concrete base or the bedding material, whichever is less. The lever arm for the design of the bolts is then from the centroid of this  stress block to the bolt position as shown in Fig. 27.12. The centroid of the stress block is often less than the edge distance from the compression edge of the plate to the holding-down bolt: it is therefore often assumed that the lever arm for  the bolts is equal to the centres of the bolts. It is also very  likely that the point of application of the compressive forces will be near to the holding-down bolts  due to the extra stiffening that is often included in the vicinity of the bolts. This is illustrated in the typical design given as the fourth worked example at the end of this part.

Fig. 27.12 Distribution of forces and equations of equilibrium

2 Bolt anchorage
Anchorage of the holding-down bolts into the concrete foundation should be sufficient to cater for any uplift forces and to provide for any shears applied to the bolts.

Attention is particularly directed to the last paragraph of clause 6.7 of BS 5950, which states that rag bolts and indented foundation bolts should not be used to resist uplift forces.The elastic elongation of indented screwed rods or bolts under tension causes the breakdown of the grout surrounding the bolt. This is even more critical in the case of resinous grouts.

The failure mode of bolts pulled from a concrete block is shown in Fig. 27.13(a); a reasonable approximation is shown in Fig. 27.13(b).The surface area of the conical pull-out (Fig. 27.14) is 4.44D2, where D  is the depth of embedment. The factored tensile capacity of an M20 (4.6) bolt is 245 x 195 x 10-3
= 47.7kN. For an M20 HD bolt 450mm long with an embedment of say 350mm


As holding-down bolts usually act in pairs the conical pull-outs often overlap depending on the depth of embedment.The BCSA have tabulated the surface areas including those which overlap (Table 27.2).

Fig. 27.13 Typical pull-out from concrete block

Fig. 27.14 Conical pull-out

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The function of a column baseplate is to distribute the column  forces to the concrete foundation. In general a plain or slab base is used for pinned conditions or when there is very little tension between the plate and the concrete.A gusseted base is used occasionally to spread very heavy loads but more generally for conditions of large moment in relation to the vertical applied loads, the principal function of the gusset being to allow the holding-down bolt lever arm to be increased to give maximum efficiency while keeping the baseplate thickness to an acceptable minimum. Gusseted or built-up bases give an ideal solution for compound or twin crane stanchions in industrial shed buildings.
 
Fixed bases are used primarily in low-rise construction either in portal buildings specifically designed as ‘fixed base’ or in industrial sheds in which the main columns cantilever from the foundations. They are also used, though less  frequently, in  multi-storey rigid-frame construction. In each of these cases it is assumed by definition that no angular rotation takes place, and although this is unlikely to be achieved it is generally accepted that sufficient rigidity can be obtained to justify the assumption.
 
Pinned bases are those in which it is assumed that there is no  restraint against angular rotation. Although this is also difficult to achieve it is accepted that sufficient flexibility can be introduced by minimizing the size of the foundation and  similarly reducing the anchorage system. Pinned bases are used in portal and in multi-storey construction.
 
Typical pinned and fixed bases are shown in Fig. 27.6.

Fig. 27.6 Typical column base connections

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In order to assess the distribution of pressure under a foundation it is necessary to make a reasonable estimate of the weight of the foundation. In addition to distrib- uting the forces to the ground the foundation block is also required to provide stability in cases where overturning moments are present.
 
Referring to Fig. 27.2, loads P, H and M are factored as appropriate while W, the foundation mass, is factored by 1.0, being a restoring moment. Moments about A give



From this a minimum value of W for stability is produced.

The minimum value for D  for a mass concrete foundation is established by 45° dispersal from the edge of the baseplate shown in Fig. 27.3. Shallower foundations can be used if they are suitably reinforced.

The distribution of pressure under the foundation is then assessed as follows.

Fig. 27.2 Stability of foundation

Fig. 27.3 Thickness of foundation

Case 1
See Fig. 27.4(a):

It is necessary for fgmax to be less than the stipulated ground bearing capacity for the foundation to be satisfactory.

For fgmin to be zero (Fig. 27.4(b)):


Replacing the forces by the resultant acting at eccentricity x:


This is the limiting condition for the application of Case 1.

Fig. 27.4 Ground pressure – Case 1

Case 2
This occurs when  fgmin is negative. As no tension can exist between the soil and  the underside of the concrete base a compressive stress wedge is formed at the  compression side of the foundation. The summation of the stress under the block must equal the resultant of the applied loads. When  x > L/6, the length of the  triangular stress wedge is three times the edge distance (L/2  - x) in order that  the resultant acts at the centroid of the wedge. The theory proposes that  3(L/2 - x) is the length of surface contact between the foundation and the ground (Fig. 27.5).


When  fgmax exceeds the stipulated ground bearing pressure, dimensions B or L or both may be increased within the limits of economy, after which piling or ground mprovement techniques can be investigated.

Fig. 27.5 Ground pressure – Case 2

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Pad foundations are used primarily to support the major structural elements in either sheds or multi-storey buildings. The pad foundations to major elements may be either mass concrete or reinforced concrete, the latter when either heavy loads or very poor ground conditions are present. They may be used in  the context of cladding to support intermediate posts carrying sheeting rails, in which case the load is almost all from wind forces and is horizontal.

Strip foundations are used in steel-framed buildings to support external masonry or brickwork cladding and masonry internal partitions. In some cases the ground floor is thickened at these locations to provide a foundation but care should be taken with respect to the appropriate depth for clay or frost heave and for compatibility between such foundations and those of the main frame.

Piled foundations, either driven, bored or cast in place, are used on sites where ground conditions are poor or for buildings or structures in which differential set- tlement is critical. They may also be required in circumstances where heavy con- centrations of load occur. In general when piled foundations are used the whole of  the construction should be supported on piles. The ground floor slab, ground floor cladding and internal partitions should be carried by ground beams between the pile cap locations. If it is necessary for reasons of economy to support the ground floor independently, provision should be made for differential settlement by the inclusion of suitable movement joints.

Ground improvement techniques are appropriate for some types of poor ground.
 
The most usual techniques are vibro-compaction or vibro-replacement but dynamic
compaction can also be useful for improvement of large isolated sites. Ground improvement specialists or specialist consultants should be approached as economy will be the most important factor in the decision.
 
Typical foundation layouts are shown in Fig. 27.1.

Fig. 27.1 Part plan of typical two-bay crane shed.

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Welding offers many advantages over other joining methods:

• Freedom of design, and the opportunity to develop innovative structures
• Easy introduction of stiffening elements
• Less weight than in bolted joints because fewer plates are required
• Welded joints allow increased usable space in a structure
• Protection against the effects of fire and corrosion are easier and more effective.

1 Aesthetics and freedom of design
Probably the main benefit of welded construction is the freedom  of design, compared with bolted joints.

Some important types of structure, such as orthotropic bridge decks, Vierendeel trusses, tubular frames, tapered beams, and even most T-joints, could not be made as easily by any other method. Welded joints allow  more freedom in the use of rolled shapes, high strength steels, and corrosion- resistant steels.

Provided that the joints are made using appropriate materials and practices, designers should feel free to develop structures that are aesthetic as  well as functional.

Hollow sections, both rectangular and circular, can sensibly be joined only by welding, even if the final connection is by bolting.

2 Stiffness
Welded construction allows a designer to introduce stiffness and strength where required, in the most discreet and/or the most structurally efficient manner. The transfer of load and the stiffness can, with welding, be introduced in a gradual continuous manner, instead of in step changes through bolted pieces.

3 Weight, volume and size
Bolting requires the use of lap joints, which add weight to a structure. For example, the use of site welds, instead of bolted joints, to join plate girders to crossheads saved about 50 tonnes of connection plates, bolts, and nuts in the construction of the new Thelwall Viaduct.

In addition to savings in weight, the use of welded joints provides more usable volume in a typical building. Bolted joints reduce usable volume significantly because current legislation and fire engineering can demand that the fireproofing coatings that must be applied give a minimum thickness over every protruding nut and bolt head.

4 Durability and corrosion resistance
Total life cycle costing, including the costs of maintenance, is becoming increasingly important in all structures. For one aspect of this, the durability, the corrosion of structural steelwork can be retarded by three means – controlling the surrounding environment, effective coating systems, and the use of steels (and weld metals) with improved inherent corrosion resistance.Coating systems, however effective they are on plate surfaces, can only with great difficulty prevent corrosion in all the crevices of bolted joints, but the clean lines of welded structures allow the full effectiveness of modern coatings to be demonstrated.The use of corrosion resistant (weathering) steels (from BS EN 10155: 19931) imparts long life, and although they can be used in bolted connections, they perform poorly in crevices and thus they work far better in welded solutions.

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The traditional method of designing cased columns, presented in BS 449: 1969, can be very conservative but is readily accepted as a method of simple design.

The minimum width of the concrete casing, according to BS 449, should be the flange width b of the steel section plus 100mm.At least four reinforcing bars are to be located in the concrete at a cover not exceeding 50mm, to which 5mm diameter steel stirrups are attached at spacings not exceeding 200mm.The normal aggregate size is 10mm and the minimum concrete grade is 21N/mm2.

To establish the axial capacity of the cased column (or strut), the radius of gyration of the solid section is taken as 0.2 (b + 100) mm, or alternatively that of the major axis of the steel section. The cross section excludes any concrete cover  in excess of 75mm from the overall dimension of the steel section. The net area of concrete is replaced by an equivalent area of steel by dividing by a modular ratio of 30.

Therefore, knowing the permissible axial stress, as a function of the effective slen- derness of the cased column, and the equivalent area of steel, the axial capacity of the section can be easily calculated. However, an onerous limit on the use of this method is that axial load on the cased column should not exceed twice that permitted on the uncased section.