Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 8 3.2 Imperfections Frame imperfections are addressed in EN 1993-1-1§ 5.3.2. Generally, frame imperfections must be modelled. The frame may be modelled out-of-plumb, or alternatively, a system of equivalent horizontal forces (EHF) may be applied to the frame to allow for imperfections. The use of EHF is recommended as the simpler approach. 3.2.1 Equivalent horizontal forces The use of equivalent horizontal forces (EHF) to allow for the effects of initial sway imperfections is allowed by § 5.3.2(7). The initial imperfections are given by Expression 5.5, where the initial imperfection  (indicated as an inclination from the vertical) is given as:  =  0  h  m where:  0 is the basic value:  0 = 1/200 1,0 3 2 but 2 h h      h h is the height of the structure in metres         m 1 0,5 1 m  m is the number of columns in a row – for a portal the number of columns in a single frame. For single span portal frames, h is the height of the column, and m = 2. It is conservative to set  h =  m = 1,0. EHF may be calculated as  multiplied by the vertical reaction at the base of the column (including crane loads as appropriate). The EHF are applied horizontally, in the same direction, at the top of each column. § 5.3.2(4) states that sway imperfections may be disregarded when H Ed  0,15 V Ed . It is recommended that this relaxation is tested by comparing the net total horizontal reaction at the base with the net total vertical reaction. In many cases, the expression given in 5.3.2(4) will mean that EHF are not required in combinations of actions that include wind actions. However, EHF will need to be included in combinations of only gravity actions. 3.2.2 Elastic analysis Elastic analysis is the most common method of analysis for general structures, but will usually give less economical portal structures than plastic analysis. EN 1993-1-1 allows the plastic cross-sectional resistance to be used with the results of elastic analysis, provided the section class is Class 1 or Class 2. In addition, it allows 15% of moment redistribution as defined in EN 1993-1-1 § 5.4.1.4(B)