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Introduction to the RC building example. Modeling and analysis of the design example.
P. Fajfar and M. Kreslin
Figure 2.2.3 Modelling the peripheral walls
2.3 Structural regularity
Regularity of the structure (in elevation and in plan) influences the required structural model (planar or
spatial), the required method of analysis and the value of the behaviour factor q (EN 1998-1/4.2.3.1).
As shown in this section, the test structure can be categorized as being regular in elevation and in
plan. A lot of work has to be done to check the criteria for regularity in plan (see section 2.3.1) and, in
practice, a designer may wish to avoid this work by assuming that the structure is irregular in plan.
(Ir)regularity in plan may influence the magnitude of the seismic action (via the overstrength factor
D u /D 1 ). In the case of the investigated building the overstrength factor does no apply and there is no
difference between seismic actions for a plan-regular and plan-irregular building. The test structure is
regular also in elevation, if we do not consider the irregularity due to basement. For a structure regular
in plan and in elevation, the most simple approach can be applied, i.e.a planar model can be used
and a lateral force method can be performed. Moreover, the reference value of the basic behaviour
factor q 0 can be used (see EN 1998-1/Table 4.1). Nevertheless, in this report, the standard (i.e.
spatial) model and the standard (i.e. modal response spectrum) analysis will be used.
2.3.1 CRITERIA FOR REGULARITY IN PLAN
In general, the regularity in plan can be checked when the structural model is defined. The criteria for
regularity in plan are described in EN 1998-1 (4.2.3.2)
o the slenderness of the building shall be not higher than 4 (O = L max /L min ),
o the structural eccentricity shall be smaller than 30% of the torsional radius (e 0X d 0.30r X , e 0Y d
0.30r Y ) and
o the torsional radius shall be larger than the radius of the gyration of the floor mass in plan
(r Xtl s , r Ytl s ).
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