Determination of structural irregularity limits

Abstract

Structures may be irregular due to non-uniform distributions of mass, stiffness, strength or due to their structural form. For regular structures, simple analysis techniques such as the Equivalent Static Method, have been calibrated against advanced analysis methods, such as the Inelastic Dynamic Time-History Analysis. Most worldwide codes allow simple analysis techniques to be used only for structures which satisfy regularity limits. Currently, such limits are based on engineering judgement and lack proper calibration. This paper describes a simple and efficient method for quantifying irregularity limits. The method is illustrated on 3, 5, 9 and 15 storey models of shear-type structures, assumed to be located in Wellington, Christchurch and Auckland. They were designed in accordance with the Equivalent Static Method of NZS 1170.5. Regular structures were defined to have constant mass at every floor level and were either designed to produce constant interstorey drift ratio at all the floors simultaneously or to have a uniform stiffness distribution over their height. Design structural ductility factors of 1, 2, 4 and 6, and target (design) interstorey drift ratios ranging between 0.5% and 3% were used in this study. Inelastic dynamic time-history analysis was carried out by subjecting these structures to a suite of code design level earthquake records. Irregular structures were created with floor masses of magnitude 1.5, 2.5, 3.5 and 5 times the regular floor mass. These increased masses were considered separately at the first floor level, mid-height and at the roof. The irregular structures were designed for the same drifts as the regular structures.
 The effect of increased mass at the top or bottom of the structure tended to increase the median peak drift demands compared to regular structures for the record suite considered. When the increased mass was present at the mid-height, the structures generally tended to produce lesser drift demands than the corresponding regular structures. A simple equation was developed to estimate the increase in interstorey drift due to mass irregularity. This can be used to set irregularity limits.

Quantifying the seismic response of structures with flexible diaphragms

“…For multistorey structures, ISD w_flex increases with decreasing structural height, making the first storey critical. This can be expected for structures having equal storey stiffness at all the storeys [29]. Figure 7 shows the increase in total displacement at midspan because of different diaphragm flexibility ratios.…”

“…Conduct dynamic time-history analyses by subjecting the structure to a suite of earthquake ground motions. In this paper, 20 SAC (SEAOC-ATC-CUREE) LA records with probabilities of exceedance of 10% in 50 years [29] are used for all the dynamic analyses. Figure 3 shows the response spectra for the record suite.…”

“…For multistorey structures, ISD w_flex increases with decreasing structural height, making the first storey critical. This can be expected for structures having equal storey stiffness at all the storeys .…”

“…Here, to obtain the moment of inertia of the rigid diaphragm, I d_rigid , the value of I d_rigid is increased until the fundamental natural period from modal analysis converges to the target value, T rig . Conduct dynamic time‐history analyses by subjecting the structure to a suite of earthquake ground motions. In this paper, 20 SAC (SEAOC‐ATC‐CUREE) LA records with probabilities of exceedance of 10% in 50 years are used for all the dynamic analyses. Figure shows the response spectra for the record suite. Obtain peak values of the following response parameters at every storey because of each earthquake record: In‐plane displacement of the end wall, Total in‐plane displacement at diaphragm midspan, Interstorey drift (from end walls), Interstorey drift at diaphragm midspan, End wall shear force, Diaphragm bending moment at midspan, For rigid diaphragms on end walls and a central wall, the diaphragm forces and displacements are obtained at quarter span ( L d /4).The maximum for each of the above demand parameters from all the storeys are also calculated.Note: Henceforth in this paper, diaphragm midspan refers to diaphragm node at either L d /2 (for single span models) or L d /4 (for equal two‐span models). Structural modelling and analysis of flexible diaphragm structures : For real multistorey structures, the flexibility ratio may vary over the structure height.…”

Seismic behavior of steel buildings with out‐of‐plumb

“…This approach has been used in a number of publications (e.g., Tagawa et al [10], MacRae et al [11], Sadashiva et al [12], Tagawa et al [13], and MacRae [14]). Earlier studies [9][10][11][12][13][14] on structural modeling have shown that the frames modeled as a combination of vertical shear column and a vertical continuous column (flexural column) can represent the behavior of real structures well. The flexural column represents all continuous columns throughout the whole structure.…”