Steel Buildings in Europe

Part 2: Concept Design 2 - 65 at the sam e tim e. Furtherm ore, the usual requirem ent that m embers and connections rem ain serviceable unde r design loading is ignored when calculating resistance to tying, as ‘s ubstantial perm anent deform ation of members and their co nnections is acceptable’. Guidance on the des ign of connections to resist tying forces is given in Multi-storey steel buildings. Part 5: Joint design [8] . 6.1.4 Tying of precast concrete floor units EN 1991-1-7, §A.5.1 (2) requires that when concrete or other heavy floor units are used (as floors), they should be tie d in the direction of their span. The intention is to prevent floor units or floor slabs s imply falling through the steel frame, if the steelwork is m oved or removed due to som e major trauma. Slabs must be tied to each other over supports , and tied to edge beam s. Tying forces may be determined from §9.10.2 of BS EN 1992-1-1. 6.1.5 Vertical tying EN 1991-1-7, A.6 provides guidance on the ve rtical tying of framed structures. The clause recommends that co lumn splices should be capable of carrying an axial tens ion equal to the larg est design ve rtical permanent and va riable load reaction applied to the c olumn from any one storey. In practice, th is is n ot an onerous obligation, and m ost splices de signed for adequate stiffness and robustness during erection are likely to be sufficient to carry the axial tying force. Guidance on th e design of splices to resist tying forces is given in other publications in this series Multi-storey steel buildings. Part 5: Joint design [8] . 6.1.6 Key elements EN 1991-1-7, A.8 provides guidance on the design of “Key elem ents”. It recommends that a key elem ent should be capable of sustaining an accidental design action of A d applied in h orizontal an d vertical directions (in one direction at a tim e) to the m ember and any attached com ponents. The recommended value of A d for building structures is 34 kN/m 2 . Any other structural component that pr ovides “lateral restraint vi tal to the stability” of a key element should also be designed as a key element. 6.1.7 Risk assessment Buildings which f all in to consequence cl ass 3 have to be assessed using risk assessment techniques. Annex B of EN 1991-1-7 provides information on risk assessment and B.9 provides guidance specific to buildings. 6.2 Floor dynamics Floor response is assessed first by calcul ating the fundamental frequency of the floor, and checking this against a limit. Limits are given in National regulations or technical guidance, and m ay vary between countries. Generally, if the fundamental frequency of th e floor structure is greater than 4 Hz, the floor is usually considered to be satisfactory. The natural frequency of the floor is then at least twice the natu ral frequ ency due to r apid walk ing. W hilst this is generally acceptable for busy workplaces, it is not appropriate for quieter areas of buildings where vibrations are more perceptible.