A new approach of steel frame design under dynamic wind load

Introduction Designing a structure like a portal frame of an industrial low-rise building is an iterative optimisation process. In today's design practice, the designing engineer takes from codes or standards the design values for the external loads, for instance the wind load distribution and the appropriate value of the wind velocity pressure. Depending on the climate at the building site, a design value of the snow load has to be taken into account, too. Step 2 is to choose the basic layout of the structure (e.g. hinged or fixed support) and to make a choice on the individual cross-sections. This step leads to a further load contribution which is the dead load of the load bearing structure. An additional dead load contribution has to be introduced from the roof cladding system. The further steps depend on the design procedure chosen by the designing engineer. In the European Union, the Eurocode for steel structures [1] allows three different design approaches, corresponding to three different ultimate limit states. The first ultimate limit state is the exceedence of the yielding stress (elastic-elastic design), the second the exceedence of the plastic carrying capacity of the cross section (elasticplastic design), the third the exceedence of the plastic carrying capacity of the whole structure (plastic-plastic design). As a matter of fact, the economy of the design increases the more the plastic carrying capacity is exploited. If the static elastic or plastic analysis of the actual structure leads to an exceedence of the specified limit state, the cross sections have to be increased and a further analysis has to be performed until that state is reached where the specified limit state is not exceeded under the specified load combinations. In regard to wind loads, this design procedure includes two short-comings. The wind induced loads are fluctuating stochastic loads, which means the amplitudes of the wind loads are rapidly changing in space and time. The load distribution at a specific time may vary considerably from that distribution specified in a code or standard. Then, it is obviously not clear, if the design wind load distribution specified in the code is the decisive distribution leading to the design-decisive non-linear effects. Secondly, the wind load may induce dynamic effects in terms of additional inertial and damping forces. Then, a static analysis of the structural behaviour is inadequate. For both reasons, the actual design procedure seems to be doubtful.