13
Bolts and Bolting
13.1 Introduction Information on the types of bolts used in structural connections, including their areas of application, strengths, the specifications to which they are manufactured and their installation procedures is given in the Steel Construction Handbook (Ref. 5). It will be noted that the most commonly used bolts are the Grade 4.8 ordinary bolt to SABS 135, the Grade 8.8 high-strength precision bolt to SABS 136 and the Grade 8.8S high-strength friction-grip (HSFG) bolt to SABS 1282. The strengths of these bolts are as follows: Bolt type (Grade)
Min tensile strength, MPa
Yield stress, MPa
0,2% perm set stress, MPa
4.8
420
340
–
8.8
800
–
640
8.8S
830
–
660
The Grade 8.8.S bolt, although having the same nominal strength grading as the Grade 8.8 bolt, is in fact slightly stronger. The dimensions of the head and nut are larger than those of the ordinary and precision bolts.
13.2 Force transfer in bolts Shear bearing The most common mode of force transfer is by shear and bearing. The bolt generally used because of its low cost and easy installation is the Grade 4.8 ordinary bolt. It is used in holes punched or drilled 2 mm larger than the nominal bolt diameter. This hole clearance allows easy assembly of the connected parts, but does result in a small amount of slip taking place when a force in the plane of the connection is applied. The force is then also transmitted in bearing between the bolt and the side of the hole. In the great majority of shear connections this is acceptable, and consequently, on the basis of cost, Grade 4.8 bolts are used in such connections.
13.1
Because Grade 4.8 bolts have rather long threads the shear plane of the connection usually intercepts the threaded part of the bolt, which has a cross-sectional area of about 0,75 times that of the unthreaded shank. (The tabulated shear resistances given in Table 6.22 of the Steel Construction Handbook are based on the reduced area.) If it is desired to mobilise the full shear resistance of the bolt it is necessary for the threaded part of the shank to be excluded from the shear plane. Since the number of bolts to be used is usually determined by the draughtsman, it is essential for him to specify that the threads be excluded. In practice this is done by using a longer bolt to ensure that the unthreaded part of the shank is in the shear plane, but this requires special supervision. In the case of Grade 4.8 bolts this procedure is not worth adopting since it is cheaper to use Grade 8.8 bolts with the threads in the shear plane. Where the threaded portion is in the shear plane this is referred to as the 'threads included' or 'threads in' case, and where it is not in the shear plane as the 'threads excluded' or 'threads out' case. The savings to be gained by using a higher bolt resistance, whether by adopting the 'threads out' procedure or by using a higher grade of bolt, extend far beyond the savings in direct bolt costs. Additional savings accrue from the reduced number of holes to be marked off and punched or drilled (at least two holes per bolt), smaller gusset plates and reduced installation time. However, these procedures should only be used where a large number of bolts is involved. Friction-grip Where friction-grip bolts are used to transfer a shear force in a connection, the force is transmitted by the shear resistance of the contact surfaces of the components and not by shear in the bolt shank. It is thus immaterial whether the bolt threads are in or out of the shear plane. HSFG bolts are more expensive than either Grade 4.8 ordinary or Grade 8.8 precision bolts and the cost of surface preparation, installation and inspection is very much higher. These bolts are only used in connections where slip under shear loading is not permitted. When detailing HSFG bolts it is necessary for the designer or detailer to specify the class of contact surface required for the connected parts. Table 2 of SABS 0162-Part 1 lists three classes, A, B and C, in ascending order of friction resistance. It is essential that the type of surface in the structure as built corresponds with the class specified and the class (or type) of surface should thus be clearly stated on the assembly or erection drawings. If a clean mill-scale or blast-cleaned finish is specified it is necessary for contact surfaces to be left unpainted. This, however, creates serious problems in practice since the painting is often done by persons not familiar with reading drawings. The identification and marking of friction contact surfaces needs extra supervision and is not popular with contractors. An alternative procedure is to paint the steelwork with a primer that provides the surface with an adequate frictional resistance, e.g. an inorganic zinc-rich primer. The whole of the steel surface is then painted and no precautions are necessary to exclude contact zones.
13.2
It is clear that this matter should be addressed seriously and in good time to enable a decision to be taken that will result in an economical solution. In certain connections HSFG bolts may be used to guard against slip occurring at working loads or to prevent the nuts from working loose. In such cases, provided the loading is unidirectional and slip at a slight overload is not detrimental to the connection, the postslip reserve resistance of the bolts may safely be mobilised. The ultimate design strength may then be taken as that of the bolts acting in shear and bearing. Shear – bolts in close tolerance holes Another mode of shear transfer when slip is not permissible is the use of precision bolts in close tolerance holes. This is a method not commonly employed in structural work, but is certainly worth considering when circumstances allow. When a precision bolt is installed in a close tolerance reamed hole, with the unthreaded shank in the shear plane, it acts as a fitted bolt and is thus a very efficient means of nonslip shear transfer. Pre-assembly of the joint in the workshop is required before reaming can be undertaken, but wherever a trial fit-up is a prerequisite for other reasons this procedure could result in significant savings. Tension When bolts are loaded in direct tension the applicable area is the 'stress area' across the threaded shank. Grade 4.8 bolts may be used, but in cases where the tensile force is at all significant Grade 8.8S bolts should be used, both because of their high tensile resistance and because of the larger heads and nuts. Although not acting in friction-grip, they should be fully torqued to prevent separation of the abutting surfaces and a working loose of the nuts. In this instance the contact surfaces do not require special treatment. Bolts used in this manner represent a highly efficient means of tensile load transfer. Particular applications would be the tension regions of beam-to-column moment connections, including the haunch connections of portal frames.
13.3 Sizes of bolts The most commonly used bolt sizes are the M12, M16 and M20, with the M24 being specified only for exceptionally heavily loaded connections. For the general run of main connections in a typical medium to heavy structure M20 bolts would be the standard. For light structures, including smaller buildings, latticed towers, platforms, canopies, etc, and also for cold-formed purlins and girts, M16 bolts would be used. M12 bolts would be suitable for stairways and for the smaller sizes of cold-formed purlins and girts. Different bolt sizes should not be mixed indiscriminately in a single structure. An assessment should be made of the most economical size, which should then be used throughout the structure. This makes for greater uniformity in the workshop and for greater efficiency on site, where the use of a single size of bolt is obviously an advantage.
13.3
On larger jobs with numerous connections of each type, a range of different bolt grades and sizes would be justified, for example the M20 HSFG for the main lattice girders, the M20 Grade 4.8 for the general run of connections and possibly M16 Grade 4.8 bolts for the purlins. Careful attention should be given to this matter by the designer (or possibly the detailer) to ensure an optimum balance of connection types.
13.4 Bolt usage A summary of the types of bolts, their areas of application and their installation procedures as discussed above is given in Table 13.1.
Table 13.1: Bolt Usage Type of bolt
Type of loading
Gr. 4.8 ordinary Slip permitted*
Gr. 8.8 precision Gr. 8.8S HSFG
SHEAR
Gr. 8.8 precision in close-tolerance holes ** Gr. 8.8S
Static loading
x or xx
Vibratory loading
Slip not permitted
xx
xxxx x or xx
[TENSION]
Static loading
xx or xxx
Variable loading
xxx
*
If threads are excluded from the shear plane, the shear resistance is 1,33 times the resistance with the threads in shear plane.
**
Threads excluded from shear plane.
x:
Normal hand-tightening.
xx:
Tightened by power wrench to ensure nuts will not work loose.
xxx:
Fully pre-tensioned; contact surfaces not treated for friction.
xxxx: Fully pre-tensioned; contact surfaces Clause 23.3.4 of SABS 0162 – Part 1.
treated
for
friction
as
per
13.5 Summary of bolting economics For any given structure or project the following basic rules should be borne in mind: •
Adopt simple details, as outlined elsewhere in this book, to reduce the labour content of connections.
13.4
•
Use standardised bolted connections, as given in the Steel Construction Handbook, wherever possible.
•
Standardise on bolt size and type wherever possible. Choose the most economical combination of diameter and strength and use this throughout the job.
•
On larger structures, where the quantity of bolts warrants it, use more than one size and or type, but do this on the basis of a careful preliminary study of the cost aspects.
•
Never mix bolt sizes within a particular connection. This adds considerably to the cost of making the connection components.
•
Where non-standard bolts or bolting procedures are used in any particular joint, ensure that this is emphasised on the erection drawing.
•
In designing connections, take advantage where appropriate of the greater resistance of double-shear bolts as compared with single-shear bolts.
13.5
