11
Bracing Systems
11.1 Introduction The function of vertical and horizontal bracing systems in providing overall stability to a building structure has been discussed in earlier chapters; see Sections 5.7, 7.3 and 7.7. In this chapter the more detailed aspects of economical bracing design will be dealt with.
11.2 Vertical bracing Vertical bracing to columns provides lateral stability to a structure and resistance to wind loading. The bracing is thus subject to horizontal loading acting in either the left-to-right or right-to-left direction. The most commonly used configurations are illustrated in Fig 11.1. Those shown in details (a) to (c) can be used in multi-storey buildings, with the floor beams being located at each panel height of the system. They could also be used, along with the configurations shown in details (d) and (e), for tall columns in single-storey buildings. In this case the beams indicated in details (a) to (c) would be replaced by horizontal struts. In type (a) the diagonals could be designed to act either in tension only or in combined tension compression; in the latter case the horizontal members would carry no load. The tension-only system is very efficient since the diagonals can be designed to minimum size and with a large slenderness ratio. It is especially applicable to bracing systems with large panel sizes, i.e. in height or width or both. In detail (b) the diagonals act in tension and compression and thus need to be stiffer; the horizontal beams do not carry any bracing load. Note that at ground level the full horizontal load is resisted by a single column foundation, which is a less favourable situation than when it is shared between two column bases. It is nevertheless an efficient system, provided the lengths of the diagonals are not excessive, since a minimum number of members and connections are involved. The inverted-V or chevron bracing in detail (c) is a tension compression system with shorter diagonal members and each horizontal member acting half in tension and half in compression. It is thus an efficient system, but if applied to a multi-storey building the bracings act as props at mid-length of each beam which would result in a lighter beam section, but a much heavier bracing section.
11.1
(a)
(b)
(c)
(f)
(g)
(d)
(e)
(h)
Fig 11.1: Vertical bracing The system shown in (e) is similar to the tension compression bracing shown in (a), but with the horizontals omitted. For single-storey buildings any of the layouts shown in details (a) to (e) can be used, in one or more panel heights. The bracing shown in detail (f) is equivalent to a single panel of the (c) type, but is used where the aim is to separate the overhead beam from the bracing itself, as in a crane gantry. In this case the bracing resists horizontal loading only and does not pick up any load from the beam. The configurations shown in (g) and (h) may be used for single-storey buildings where greater clearance between the columns is required. They are obviously less economical than any of the others and are only used when called for. The (g) type may also be used in multi-storey buildings in special cases where clearance is required. Sub-bracings, as shown dotted, may be added to reduce the effective length of the bracing members in the plane of the frame.
11.2
11.3 Horizontal bracing Horizontal bracing to floors, when required as temporary bracing prior to the casting of concrete slabs, should be kept as light and simple as possible. It is often left in place after completion of the floor, thus saving the expense of removing it. Eccentricities at node points are usually not serious and are permissible wherever they allow simpler end connections to be used. The simplest form of bracing would usually be single-angle cross-bracing acting in tension only. Temporary bracing should be clearly noted as such on the design drawings so that the detail draughtsman can treat it accordingly. Fig 11.2 shows a few possible floor bracing layouts and is presented for the purpose of discussing the pros and cons of various configurations. The comments below apply in principle to both permanent and temporary bracing. In all cases it is assumed that the plane of the bracing is some distance below the top flange level of the beams so as to avoid interference by the bracing with the floor slab or deck. If the beams are all of the same depth the bracing plane could be located at the underside of the beams, with the bracing gussets bolted directly to the bottom flanges; this would save the use of cleat for connecting the gussets to the beams.
(a)
(b)
(c)
(d)
(e)
(f)
Fig 11.2: Floor bracing
11.3
The layout shown in detail (a) appears simple and has the advantage that the members can be tension-only single angles. It has the drawback, however, that the nodes coincide with the column positions, while the gussets tend to be awkward in shape, as in detail (e) of the figure, and have to be checked for compressive force transfer. In the layout in (b) all the bracing ends are clear of the columns, resulting in much simpler beam connections, as shown in detail (f). The bracings must be designed for tension compression, but because of their short length do not require a heavy section. The bracing shown dotted is optional; it does not carry load. In the (c) layout the bracing-column interference is minimised, the number of gussets is reduced as compared with (b) and the members are again in tension compression, but have a greater length. Where there are no intermediate columns within the bracing system the layout shown in detail (d) can be used. Here there is no column interference and the diagonal in the centre panel can be designed as tension compression or, with the member shown as dotted added, in tension only.
11.4 Rafter bracing Rafter bracing used in industrial buildings to resist wind loading is located across the width of the roof, usually at the gables, but sometimes within intermediate bays. It may also be required along the length of the roof, adjacent to the eaves, to provide lateral support to intermediate side columns. Under wind uplift loading the purlins are often highly stressed in bending, with their bottom flanges having to resist lateral-torsional buckling caused by the negative moment. They therefore have little or no reserve of strength left to enable them to participate in the roof bracing system. For this reason the bracing is often designed as a self-contained system comprising tension and compression members, and not relying on any assistance from the purlins. An efficient section for the struts is a circular hollow section (CHS), whilst a single angle is suitable for the ties in an X-braced system. In a configuration where all of the members are subject to tension compression the CHS would be the best section. Fig 11.3 shows three layouts of rafter bracing. In detail (a) the X-braced system is used with angle diagonals and CHS 'verticals', the latter usually coinciding with alternate purlin positions for the sake of good appearance. In detail (b) the diagonals are tension compression CHS members, with the nodes again coinciding with the purlins positions. In this layout the diagonals pass underneath alternate purlins and the bracing plane has to be dropped below the top of the I-section rafter as shown in detail (d). Alternatively, and especially if the rafter is an angle or tee section (as in a truss), the purlins could be raised, as shown in detail (e), and the bracing connected to the top of the rafter. In smaller buildings, where the ratio of bay length to purlin spacing is lower, the arrangement shown in detail (c) of the figure will be found to be simple and efficient. Here the panel lengths are equal to the purlin spacings, so there is no interference with the purlins and the CHS diagonals can be gusseted to the top of the rafter, resulting in a cheaper connection.
11.4
(a)
Purlins
(b)
(c)
(d)
(e)
Fig 11.3: Rafter bracing layouts
11.5 Bracing sections As stated earlier, CHS sections are often used for strut bracings in buildings and single angles for ties. For large structures and especially industrial applications such as buildings for plants, towers, mine headgears, conveyor trestles, etc, the bracing may have to take a different form. Fig 11.4 shows a number of sections commonly used, ranging from light simple ties to heavy compound struts.
11.5
(a) )
(b)
(g)
(c)
(d)
(h)
(e)
(j)
(f
(k)
Fig 11.4: Bracing sections The double angles shown in details (b) to (d) are used for both ties and struts and are efficient as regards their end connections because the bolts are in double shear. They may be used in indoor locations in non-corrosive environments; if used in corrosive situations they should be galvanised or treated in some other form because of the difficulty during subsequent maintenance of painting between the angles. As mentioned in Section 9.4, the starred-angle strut shown in detail (e) is not as costeffective as it might appear because of the stringent code requirements, and also because of the wide gussets required at the ends. It is, however, popular section in heavy structures with large bracing lengths and forces. The CHS bracing shown in detail (f) is very efficient structurally when used as a single strut. It should preferably not be used in the X-configuration because of the difficulty in providing a suitable gusset at the intersection of the X. Detailed information on bolted-end connections for CHS members is given in Chapter 7 of the Structural Hollow Sections Handbook (Ref. 8). When compared with a starred-angle section as used in long or heavily loaded compression members the CHS shows up well. The higher cost per unit mass and the welded T-connections at the ends are offset by the much higher mass per metre and the battens of the starred angle. The twin-angle section shown in detail (g) is suitable as a strut, but according to clause 19.1.7 of the code the battens or lacings require welding or FG bolting. When used as a tie the battens or lacings could be omitted unless the slenderness ratio is very high. The I-section in detail (h), or alternatively an H-section, is efficient when used in systems where a member with a depth perpendicular to the bracing plane is required; doubleplane gussets are used, attached to the flanges.
11.6 End connections For information on end connections for bracings reference should be made to Section 10.6 of Structural Steelwork Connections – Limit States Design (Ref. 7) and Chapter 7 of
11.6
the Structural Hollow Sections Handbook, where a large number of typical connections are presented. Where bracings are connect to a beam or a column, as shown in Fig 11.5, it is usually not necessary for the member axes to meet at a common point as shown in detail (a). A more compact gusset can be achieved if the axes meet at the column-beam flange intersections, as shown in (b). Because the eccentricity is in the plane of the column and beam webs the eccentric moment produced can usually be absorbed easily by the bending strength of the column and or beam about their x-axes. The gusset welds and the bolts are in pure shear and would not need to be checked for shear tension as would be required for detail (a).
(a)
(b)
(c)
X
X Locating mark
(d)
(e)
(f )
Fig 11.5: Bracing end connections
Detail (c) of Fig 11.5 shows a bracing connected to the column only. This results in even greater simplicity since far fewer bolts are required. Furthermore, the beam requires no holing and can thus be detailed as a standard beam. The end plate on the gusset is, however, subject to prying action and would need to be of adequate thickness. The bolts would also need to have sufficient tensile strength and might have to be high-strength. For bracings with lesser load the detail shown in (d), in which a simple, compact gusset is used, represents an extremely economical solution. The plate is shop-welded to the column and needs to be located accurately. This can be done by marking the plate as shown and providing a corresponding mark on the column flange; these marks would be included on the detail drawing.
11.7
In cases where the beam forms part of the bracing system and acts as a tie or a strut the horizontal component of the force in the inclined bracing member needs to be transmitted into it. In details (a) and (b) this is done via the beam flange, but in details (c) and (d) the force is transmitted from the bracing to the column and then from the column into the beam end. Where the resulting force in the beam is tensile the end plate and its bolts need to be checked for this force, in addition to the end shear in the beam being tested. As has been stated, the effect of the eccentricity in connections (b), (c) and (d) is usually not significant, but should be checked nevertheless, especially in cases (c) and (d) where it is more pronounced. When a gusset as shown in detail (c) is used the line of action of the bracing force should pass through the mid-depth of the flange of the cleat as shown. Details (e) and (f) show connections where this is not so; although the bracings have been brought closer to the beam flange and the bracing-beam-column eccentricity has thus reduced, the eccentricity on the flange bolts and the welding will cause a serious overstress at points X because of the concentration of force at these points.
11.7 Gussets The setting-out by the detail draughtsman of gussets with welded plates that are bolted to beam and column flanges can in certain instances be simplified and their production speeded up by slightly displacing the setting-out points (SOP's) as shown in Fig 11.6. Detail (a) shows the conventional position of the SOP on the beam-column flange intersection and detail (b) the displaced position on the corner of the gusset plate itself. This applies also to floor bracing where the bracings are connected to the webs of the beams. The displacement is insignificant, but the setting out is made much easier. It is of course necessary for the draughtsman to allow for the true position of the SOP's when detailing the bracing members. The same principle can be applied to many other cases, e.g. the welded-T cleat shown in details (c) and (d) of the figure.
11.8 Bolting In the great majority of cases the type of bolt used in site connections of bracings would be the Grade 4.8 ordinary bolt because of its cheapness. This would include rafter, eaves and truss tie bracing, floor bracing in general and the vertical bracing to buildings of moderate height. Tall buildings would be subject to lateral displacement owing to bolt slip under wind load, which might be reversed under opposite wind conditions. In this case Grade 8.8S HSFG bolts should be used, as they should be in bracing in all situations where dynamic loading and or vibration are present.
11.8
Gusset S.O.P
Column S.O.P
Beam
(a)
(b) Welded T
S.O.P S.O.P
(c)
(d)
Fig 11.6: Location of bracing S.O.P's.
An exception to the general use of Grade 4.8 bolts might be in very heavily loaded bracing where the number of bolts could be reduced significantly by using Grade 8.8 bolts in shear bearing. This results in a reduction in bolt costs, drilling time and gusset size.
11.9 Summary •
Several configurations are available for vertical bracing to columns, viz. X-bracing acting in tension only or in tension compression, single-strut tension compression bracing, chevron bracing, zig-zag tension compression bracing, knee bracing, etc.
•
The tension-only X-braced configuration is the cheapest and most effective and is employed where other considerations do not preclude its use.
•
Knee-bracing is less stiff and more expensive, but is necessary when clearance is required between the columns.
•
Horizontal bracing to floors, when required as permanent bracing, should be properly triangulated, but if attachment to columns is avoided, simpler end connections will result. The lengths of compression members should be kept as short as is practicable.
11.9
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Temporary horizontal bracing should be as simple as possible, with end-connection eccentricities tolerated. It may be left in place after floor construction has been completed.
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Rafter bracing, when acting in tension only, should be of single-angle form, but when in compression it must be of a stiffer section such as a CHS.
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Information on the design of CHS bracing end connections is given in Chapter 7 of the Structural Hollow Sections Handbook and Section 10.6 of Structural Steelwork Connections - Limit States Design.
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Vertical bracings connecting to a beam or a column should have their axes meeting at the column-beam flange intersections rather than on the column-beam centreline point. More compact gussets are achieved in this way.
•
Alternatively, vertical bracings may be connected to the column flange only, dispensing with the need for cleat attachments to the beam. For tension bracings the bolts and T-cleat must be checked for prying action.
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The setting-out of gussets can be simplified by locating the SOP's of the bracing members on the edge or the corner of the gusset instead of on the beam-column flange intersection.
•
The site bolts used in bracing systems should generally be Grade 4.8, but HSFG bolts would be necessary where slip under alternating loads (or wind loads in tall buildings) might cause displacement of the structure.
11.10
