Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 60 11.3.2 Safety in erection It is usual to provide at least four bolts in the base plate for stability during erection. The alternative is to provide temporary support immediately after the erection of the column, which on most sites would be impractical and is likely to create hazards. 11.3.3 Resistance to horizontal forces The highest horizontal forces acting at the base of the column are generally those that act outwards as a result of bending in the column caused by vertical loading on the roof. Horizontal reactions acting outwards can be resisted in a number of ways, by:  Passive earth pressure on the side of the foundation, as indicated in Figure 11.6(a)  A tie cast into the floor slab connected to the base of the column, as shown in Figure 11.6(b)  A tie across the full width of the frame connecting both columns beneath or within the floor slab as illustrated in Figure 11.6(c) and (d). By far the most popular method of resisting horizontal forces is to use passive earth pressure. This has economic advantages in that the foundation size required to resist uplift is usually adequate to provide adequate passive bearing against the ground. However, the passive resistance of the surrounding ground can be less than anticipated if the ground is not compacted correctly, and drainage and service trenches alongside the frame can reduce the passive resistance considerably. As an alternative, a bar connected to the column and cast into the floor slab, and wrapped at the end to allow vertical movement, can be relatively cheap. This detail may lead to some local cracking of the floor slab and, where a high specification floor slab is used, the warranty on the slab may be invalidated. The length of the bar should be determined by the ultimate pull out resistance required to resist the horizontal force. A tie across the full width of the frame connected to the column at each side is the most certain way of resisting horizontal forces. It is more expensive in terms of materials and labour and can be damaged by site activities. A full width tie will generally impede the erection of the structure, which will be undertaken from within the footprint of the building. 11.3.4 Base plates and holding down bolts The steelwork contractor will usually be responsible for detailing the base plate and holding down bolts. However, it should be made clear in the contract documentation where the responsibility lies for the design of the foundation details, as special reinforcement spacing or details may be required. Base plates will usually be in grade S235 or S275 steel.