MPA Precast are committed to providing technical information on precast concrete elements and associated fixings and design considerations. As part of this commitment, MPA Precast is now the new home for Key2Concrete. Key2Concrete has been an industry driven reference site for information on the applications and techniques behind the use precast concrete. The aim of Key2Concrete was to provide information and guidance on good practice within the concrete sector.
This extensive body of knowledge has been reinvigorated through collaboration with Cliff Billington MBE, the original author of Key2Concrete. The MPA Precast technical library is now the home of up-to-date information on precast construction methods.
The use of Key2Concrete content should only be done in conjunction with relevant product standards and safe installation guidance found within the Code of Practice for Safe Installation.
The practices, products and techniques set out in Key2Concrete represent typical scenarios in the opinion of the author. However, for up-to-date project specific advise contact your precast supplier directly.
Precast concrete units are commonly supported on bearing shims. These provide a means of fine adjustment of levels. Shims are usually steel, stainless if corrosion might occur, although hard plastic shims are also available.
The size of shims is determined by the bearing stress of the materials involved. Although precast concrete is usually of high strength, the supporting material may be weaker concrete or masonry. It is the weaker material that will control the sizing. Using multiple shim points is not normally an option on simple units, since the self weight of the unit will only be taken by the two highest shim packs.
The table gives general guidance on the minimum bearing sizes required for various unit weights. It is based on a minimum material strength of 30 N/mm2 and assumes that 50% of the unit weight is on each shim. Shims are relatively inexpensive, and it is good practice to size shims on the large size rather than simply using the minimum. If units are stacked, then it is the total weight that must be considered.
Using shims of different sizes in the same stack should be avoided.
Typically a shimming allowance of between 25mm and 50mm will suffice. This gap should be agreed at an early stage and shown on drawings. It is bad practice to have excessive shim height, particularly if the shims are small, as this can result in instability. A check should also be made that the stack of shims is in line vertically as this can also cause stability problems.
For shims to function correctly they should be full contact with both bearing surfaces. If surfaces are excessively uneven, then a high strength grout bed may be required to provide a suitable surface.
Shims should not be positioned closer to a vertical face than the reinforcement cover on that face. The area around a shim is highly stressed, and if a high spot occurs, it can cause the concrete cover to crack and spall off. This is especially likely if supports are uneven or rotate (see diag A).
One way of reducing the chances of this happening, especially on a corbel, is to incorporate a chamfer, say 20 x 20 at the corner. Shims should then be set a further 30mm inside the chamfer, i.e. a minimum of 50mm from the vertical face. (see diag B).
Erection tolerances are recommended to achieve the intended appearance of the cladding. This is particularly important at joints and corners etc.
BS8297 clause 11.5 gives the following...
The offset in planes formed between vertical faces of one panel to another should not exceed 6 mm.
Bowed panels (within manufacturing tolerances) should be arranged so that offset between adjacent panels caused by bowing does not exceed offset tolerances.
The width of joints should be such as to ensure that joints perform as intended and conform to the recommendations of the joint sealant or gasket manufacturer.
In addition to these erection tolerances, it is vital that due allowance is made for inaccuracies in the structure the panels are fixed to. A column may be positioned ‘in the wrong place’ and it might also be out of plumb as it goes up. The cladding is however required to be ‘in the right place’. Therefore there must always be a clear gap between the frame and any cladding to allow for such problems. It is poor practice not to allow a significant tolerance gap between a structure and any cladding units.
It is obviously beneficial from the point of bracketry to keep the gap to a reasonable minimum, but each project requires careful consideration and the intended design tolerances must be stated by the Design Team at the onset. A figure of 50mm of ‘fresh air’ is likely to overcome all but the worst of errors on site. This gap is between the inner face of the cladding (including any pre-fixed insulation) and the outer face of the frame.
With a steel frame it is important to be aware of the position and type of any column splices. Flange plates are typically the same thickness as the flange, and if these are bolted on the outside of the flanges, the bolts could easily project as much as 75 mm or more beyond the flange. Similarly, flange plates may also extend beyond the face of the flange. Far preferable is to have the splice plates inside the flanges, or a welded splice with no flange plates.
Reference may also be made to BS 5606:1990 Guide to accuracy in building. This document addresses certain aspects such as position, and verticality that are not covered in BS 8297.
Untreated non-stainless steel is not commonly used in precast construction due to the long-term danger of corrosion. It may however be galvanised to give better performance. It is often used for beams, lifting devices, and temporary fixings where the risk of corrosion is not critical. In such instances, non-stainless is preferable since it is not only far cheaper but also stronger than stainless steel.
‘Normal’ steel members were for many years covered by BS 4360:1990 “Specification for weldable structural steels” but this is now withdrawn and superseded by a host of other standards too numerous to list.
Members such as beams and columns are generally available in two ‘standard’ grades of material. These were traditionally known as grade 43 and grade 50. Grade 43 is the basic steel grade commonly available. Grade 50 is higher yield steel, but if specified then care should be taken that the material is actually available within the required delivery period. There is also a grade 55, but this has a cost implication and is used less.
The current steel design standard BS EN 1993-1-1:2005 (commonly called EC3) uses a different terminology to specify steel grades. Grade 43 is now known as S275, grade 50 is S355, and grade 55 is S460. These are based on the design strength of the material.
Thus for example the yield strengths for 10 mm thick plate are:
S275 (grade 43) steel = 275 N/mm2
S355 (grade 50) steel = 355 N/mm2
In comparison, grade1.4301 stainless steel = 210 N/mm2
The most commonly used bolts in structural connections are grade 4.6 black bolts and 8.8 high strength bolts. Grade 4.6 is the basic steel grade commonly available and would be used unless specified otherwise for design reasons. As with steel members, the standards have changed recently and are numerous.
To give a similar indication of the relative strengths of steels, typical ultimate capacities for M20 bolts are:
Stainless (class 50) 42.7 kN
Stainless (class 70) 91.5 kN
Grade 4.6 steel 47.8 kN
Grade 8.8 steel 110.2 kN
A further grade of bolt is also available, known as ‘High Strength Friction Grip’. These are expensive and require special detailing.
Grade 4.6 bolts should not be used where there may be stress reversals except where due to wind only.
As the name suggests, a sandwich panel consists of a sandwich of concrete with a filling of insulation. It has the advantage of incorporating the outer skin, inner skin and insulation in a single element.
The outer skin (or ‘wythe’ as Americans call it) is usually quite thin, in the order of 50 – 80mm. The insulation is in the order of 80mm – 100mm thick depending on the required thermal performance and the inner layer may be anything from 100mm upward, depending on the size of the panel. In use, the inner layer is the loadbearing one, with the insulation and the outer layer hanging off it.
The make-up of the panel also depends on the system being used to fasten the two layers together. There are several systems available. It is possible to make a sandwich by using ‘traditional’ reinforcement passing between the layers, however in practice, commercially available systems are used, and there are several such systems available.
There is a drawback with steel systems in that they create relatively rigid connections between the two layers at ‘strong points’. This means that differential movement due to the sun expanding the outer layer can set up quite high stresses. For this reason most steel systems limit the maximum size of the outer layer to 4.5m - 5m. Beyond this, expansion joints must be provided.
By agreement with the precaster, it is possible to have the inner face suitable to be left exposed, or suitable for direct decoration, The outer skin can be treated in the same way as single skin panels, including facing with natural stone or brick. It must be remembered though that such panels can get quite heavy, and site cranage must be considered.
Stainless steel is ‘ordinary’ (carbon) steel to which various other things, particularly chromium, have been added to make it ‘stainless’. The chromium reacts with air to form a light, invisible oxide covering which protects from further corrosion. If the covering is damaged, further corrosion takes place thus maintaining the film.
In the same way that carbon steel has various strengths, so stainless steel does as well, and combining this with the amount of ‘stainlessness’ can result in a wide variety (over 200) of types. The position is further complicated by the fact that steel for different purposes has different standards and different ways of specifying. The following is a guide to the various standards and the situation generally with regard to the use of stainless steel.
The types fall into 3 main usage groups:
Groups 1 & 2 are treated in the same way, group 3 is totally different.
Flat plate and sockets etc:
The common grades used for fittings are listed below, together with the various ways of specifying them, which may be found in specifications and literature.
The common application of these is as follows:
303/1.4305 - generally used for sockets etc
304/1.4301 - suitable for rural, urban, light industrial. Not suitable for heavy industrial or coastal
316/1.4401 - suitable for industrial and coastal
the L grades signify low carbon and need only be used when welding plates thicker than 16mm
For most angles and plates therefore ‘standard’ grade is 104301 (was 304)
and ‘higher’ grade is 1.4401 (was 316)
For sockets, the ‘standard’ grade is 1.4305 (was 303)
and the ‘higher’ grade is 1.4401 (was 316)
The normally used British Standards for these in the past have been
BS970 - Wrought steels for mechanical and allied engineering purposes Part 1:1991 General inspection and testing procedures and specific requirements for carbon, carbon manganese, alloy and stainless steels
BS1449 - Steel plate, sheet and strip Part 2:1983 Specification for stainless and heat resisting steel plate, sheet and strip
A range of new British Standards have been issued which supersede both of the above. The terminology is ‘messy’ but the important ones relating to properties are
BS EN 10088-2:2005 – Technical delivery conditions for sheet/plate and strip for general purposes (replaces BS1449 Part 2)
BS EN 10088-3:2005 - Technical delivery conditions for semi finished products, bars, rods, and sections and bright products of corrosion resisting steels for general purposes (replaces most of BS970)
Being a ‘Eurocode’ the BS EN range does not use either the UK or USA naming methods. Instead, the numbering system similar to that used by Germany is used.
Thus grade 304 became 1.4301
1. = steel 43 = group of stainless steels 01 = grade identification
The system used for fasteners is totally different to that used for plates etc.
Basically the specification consists of two parts, which give the degree of stainlessness and the strength.
The steel grades are:
A1 - Chromium-nickel steel (sometimes called free-machining grade 303/1.4305). Corrosion resistance is reduced and it is not suitable for coastal or industrial environments. This grade should not normally be specified
A2 - Chromium-nickel steel (grade 304/1.4301) suitable for rural, urban and light industrial.
A4 - Chromium-nickel-molybdenum steel (grade 316/1.4401) suitable for industrial and coastal areas.
The grades given are not true equivalents as they are strengths whilst the letters are for stainlessness. They are however often quoted (wrongly) in specifications. The letter A in the above stands for austenitic. This is the common type of stainless steel and can be readily formed and welded.
Other types which may be mentioned are martensitic (stronger but cannot usually be welded), and ferritic (less strong).
The property classes are:
50 - Softened
70 - Cold worked
80 - High strength
In theory any grade can be mixed with any class. However in practice the most common combinations are A2 70, A2 80, A4 80. Many suppliers say that class 50 is not easy to obtain and can in fact carry a cost penalty if insisted upon.
For most fasteners therefore it is normal to specify grade A2 class 70.
The British Standard for these is BS EN ISO 3506-1: 2009 “Mechanical properties of corrosion-resistant stainless steel fasteners. Bolts, screws and studs”. This BS gives design capacities for fasteners.
There is however a snag. For grades 70 and 80, figures are not given for diameters greater than 24 mm. The background for this is that there are 2 main ways of making a circular section. The steel can be hot-rolled down to the required diameter, or cold-drawn. Hot rolling is gentler and leaves the cross section with approximately the same properties across the full width. Once sizes exceed 24 mm, very few manufacturers have the capacity to hot roll and thus they cold-draw.
This entails squeezing the steel between rollers until the correct diameter is reached. The effect of this is to work-harden the outer part of the section, which is fine if a smooth bar is required. But if a thread is then cut into it, the hard part is removed, leaving the softer core and a weaker section. This does not happen with hot rolling, thus the BS requires that the design capacity of these larger sizes should be agreed between the user and the manufacturer. Very few British manufacturers exist, and most supplies come from Sweden, Germany and the Far East.
It is difficult therefore to pre-agree strengths. What can be done is to get formal confirmation from suppliers that they will obtain and supply materials with an agreed capacity. In order to do this it is important to buy only from those suppliers who have agreed a quality, and to insist that the product can be identified as complying. Bolts and setscrews to BS EN ISO 3506 are stamped on the head with a code identifying the manufacturer, the steel grade and the property class.
Studding is more difficult as there is no way to identify the material. When ordering, it should always be specified as class 70. In the event that class 50 studding is supplied, this still has a capacity greater than that of standard sockets. The risk area is where studding is used without sockets and then torqued up. In this situation designs should be based on class 50. A major risk is if suppliers make up their own ‘bolts’ from studding and a nut as some have done. In this instance there is no guidance whatsoever as to its strength, hence the need for strict specification and identification.
It is not normal to use bolts (threaded for only part of the shank) because they will not thread fully into a cast-in socket.
Setscrews are similar to bolts but are threaded for the full length of the shank. The relevant British Standard is BS EN ISO 3506-1: 2009 “Mechanical properties of corrosion-resistant stainless steel fasteners. Bolts, screws and studs”
The designation of material consists of two blocks separated by a hyphen. The first block is a letter showing the type of steel (A = austenitic), and a digit showing the chemical composition. The second block is a value = 1/10th of the tensile strength.
The two main classifications of stainlessness/strength are:
A2-70 indicates austenitic steel, cold worked minimum 700 N/mm2 tensile strength
A4-80 indicates austenitic steel, high strength minimum 800 N/mm2 tensile strength (but see below)
A2 is the general grade suitable for most precast work. It should not however be specified for use in areas with high chlorine content such as swimming pools or seawater. In these cases A4 should be specified.
Most designs call for a classification of A2-70, and this is the ‘standard’, with the higher grade being A4-70. Whilst A2-80 and A4-80 are theoretically possible, they would be ‘special’ and would not normally be specified.
Marking of setscrews should be on the head as shown.
Nominal length does not include the hexagonal head.
The normal thread is an ISO metric coarse thread.
When determining lengths of setscrews, a check should always be made that the embedment into a cast-in socket meets the minimum depth as below, after allowing for shims etc.
As the name suggests, starter bars are reinforcing bars cast into a member to give a lapped connection to further reinforcement in another concrete element to be cast against it.
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