Steel Buildings in Europe

Part 2: Concept Design 2 - 36 4.6.1 Site conditions Increasingly, structu res are constructe d on ‘bro wnfield’ sites, where earlier construction has lef t a p ermanent legacy. In c ity centres, a solution involving fewer, although m ore heavily loaded foundations are often preferred, which leads to longer spans for the super-structure. A confined site can place particular constraints on th e structural scheme, for example the physical size of the elem ents that can be delivered and erected. Access may demand that the s teel is erected directly from a delivery lorry in the road. T his m ay prevent workin g at cer tain tim es in the day m aking the erection programme relatively inflexible. A mobile erection platform provides temporary storage and speeds up the installation process, as shown in Figure 2.2. 4.6.2 Cranes The number of cranes on a project will be dominated by:  The site footprint – whethe r a sensible coverage of the building site can be achieved, including off-loading.  The size of the pro ject – which dictates whether m ore than one cran e is economic. In city centre projects, towe r cranes are often located in a lift shaft or atrium.  Use of additional mobile cranes – multi-storey structures are generally erected using a tower crane, which may be supplemented by mobile cranes for specific heavy lifting operations. As an indication, an erection rate of between 20 and 30 pieces p er day is a reasonable installation rate. W ith aver age weights of the com ponents, this equates to approxim ately 10 to 12 tonnes of steel per day. There is therefore benefit in using fewer, long span b eams. Where possible, prefabrication reduces the number of items to be lifted, and increases erection rates. 4.6.3 Installation of composite floors Composite floors com prise profiled st eel decking, which is lifted onto the steelwork in bundles an d usually man-ha ndled into pos ition. Safety nets are erected im mediately after the s teelwork and before the decking placem ent. Steelwork already erected at upper levels does not prevent decking being lifted and placed, although d ecking is u sually p laced as the steelwork is erected . Completed floors m ay be used as a sa fe working platform for subs equent erection of steelwork, and allow other wo rks to proceed at lower leve ls, as shown in Figure 4.11. For this reason, th e upper floor in any group of floors (usually three floor levels) is often concreted first.