A practice-oriented method for estimating elastic floor response spectra

Abstract

A practice-oriented modal superposition method for setting elastic floor acceleration response spectra is proposed in this paper. The approach builds on previous contributions in the literature, making specific recommendations to explicitly consider floor displacement response spectra and accounts for uncertainty in modal characteristics. The method aims to provide reliable predictions which improve on existing code methods but maintain simplicity to enable adoption in practical design. This work is motivated by recent seismic events which have illustrated the significant costs that can be incurred following damage to secondary and nonstructural components within buildings, even where the structural system has performed well. This has prompted increased attention to the seismic performance of nonstructural components with questions being raised about the accuracy of design floor acceleration response spectra used in practice. By comparing floor acceleration response spectra predicted by the proposed method with those recorded from instrumented buildings in New Zealand, it is shown that the proposed approach performs well, particularly if a good estimate of the building’s fundamental period of vibration is available.

Seismic design of acceleration-sensitive non-structural elements in New Zealand: State-of-practice and recommended changes

“…Equation 15 implies that the total floor acceleration depends on the modal properties of the structure and can vary from floor to floor within a building. It also depends on the ground shaking, which can be critical at long periods and at lower levels of a building [36,37]. Figure 3 is an illustration of Equation 15, obtained numerically, which shows that the distribution of peak floor accelerations not only varies across buildings with different dynamic characteristics but also within a single building type [31].…”

“…Figure 4 shows the spectral shape coefficient (component amplification factor in NZS 1170.5), which is given as a simplified floor response spectrum and has a peak value of 2.0 for component vibration periods less than or equal to 0.75s. Multiple studies on instrumented buildings, numerical models and experimental investigations have shown that the amplifications can be well in excess of 2.0 [35][36][37][42][43][44].…”

“…A more rational approach to the estimation of component ac- celeration demands that takes into account the proximity of vibration periods of the supporting structure and the component, and is adjustable for different damping ratios, should be adopted. Recently, Haymes et al [37] proposed an alternative method for estimating component acceleration demands accounting for building and component vibration periods and damping. This procedure recommends that if only the fundamental structural mode is considered, the floor response spectrum should be set equal to the maximum of ground level spectrum and the product of peak floor acceleration and dynamic amplification factor, as given by Equation 16.…”

“…where, S GA = spectral ground acceleration ordinate correspond- The variation of DAF with the period ratio is illustrated in Figure 6, where the period ratios can either be taken from ASCE/SEI 7-16 [26] or the ratios adopted by Haymes et al [37] can be used.…”

“…The maximum dynamic amplification factor can be approximated by the following expression as given in Haymes et al [37]:…”