3rd Croatian Conference on Earthquake Engineering 3CroCEE

The existing seismic design codes consider bi-directional effects by applying the square root sum of squares (SRSS) or the percentage rule; the latter simplified way of design is commonly employed by accounting for the 100% contribution in the assumed principal direction plus 30% or 40% other direction’s contribution. Although the potential reasons of widespread building damage and collapses in the Feb. 6th, 2023 Kahramanmaras earthquake doublet have been well documented, there are still questions in mind that whether or not bi-directional effects played a significant role in this devastation. To appropriately address this, bi-directional combination coefficients can be calculated using the strong ground motion records of these earthquakes. The Housner’s spectrum intensity values are calculated by using the pseudo-elastic velocity spectrum curves for both directions of the 60 records in locations where destruction was significant. Following the percentage rule, intensities both obtained from the resultant (i.e., bi-directional) and unidirectional velocity spectra are combined, and the corresponding percent values as other direction’s contribution are computed. Note that pseudo-velocity spectrum curves cover a wide period range representing various types of structures and are developed for damping ratios ranging from undamped to significantly damped systems (e.g. systems with energy dissipation devices). Numerical results reveal that 50% of the selected stations from both earthquakes suggest combination coefficients in excess of the 30% rule, indicating that the 30% rule may underestimate internal forces and drifts for half of the stations. This would justify and could be one of the reasons for severe damage and collapses observed in several types of structures. Furthermore, most cases satisfied the 40% rule (this is used especially for bridges), but some exceptional cases led to higher combination coefficients, particularly those for undamped (or slightly damped) and 5% damped structures basically assumed for RC framed buildings in design codes.