IsoEstimate Software Program

-------Estimating structural responses of isolated structures with large number ground motions

The program aims at estimating structural responses of isolated buildings with simple structural model and large numbers of ground motions. The results can be used to better estimate mean and variation of responses, and better select a more representative 11 ground motions set for detailed structural analyses. A 2DOF simple structure is used to mimic isolated structure. Bilinear hysteretic behavior of the isolation layer is captured. The software program allows users to input AT2 files from a folder, which is an extremely convenient feature, when dealing with large number of ground motions.

The program is developed for CE227 term project under Prof. Moehle’s instruction. Software program and tutorial material is available through Google Drive. The attached project report is a detailed document that provides all information about this software, including the description of algorithm used and comparison with results from Opensees and E-defense real scale testing.

Software Program Download

Software program can be downloaded from the following Google Drive Link. The Project Report file provides very detailed information about the algorithm and set up of the software. The Introduction pdf file is a 10-page ppt slide that helps one to get started with the software in just 10 minutes. 

Highlight of Report

Variation of structural response, a neglected but significant component

       What can we learn from a set of 11 ground motions? Statistic tells us that 11 ground motions are just enough to estimate a mean response of structure within  range with 90% confidence. This small set of ground motions is not enough to tell engineers the true variation of structural responses, which is a neglected but significant component of earthquake engineering. Moreover, the above statement is only valid, when the geotechs or the seismologists give the “correct and representative” set of 11 ground motions. But how can we know whether they are doing their work without errors?

       Watson-Lamprey (2006) demonstrated at the COSMOS annual meeting two set of “reasonably correct” (with correct seismological properties such as magnitude and distance, and with the correct spectrum shape that matches the target spectrum) 7 ground motions can lead to average structural responses offset by a factor of 1.7. This result is not unique. Goulet et al. (2008) compared the difference between using uniform hazard spectrum (UHS) for ground motion selection and using conditional mean spectrum (CMS) for selection. Fig-3 shows their results. The first 10 sets of results (indexes 1 to 11) are responses from UHS ground motion sets while the remaining 5 sets are based on CMS. All 15 sets have the “correct” seismological properties and “reasonable” spectrum shape, but they produce extremely different structural responses. 

       This variation of structural responses is a very important component in earthquake engineering, as large variation forces engineers to consider more redundant structural systems. If the geotechs and seismologists only tell structural engineers that the mean hazard is 0.4g, engineers will scale everything to 0.4g and their design is done. But if the geotechs also tell them that  range is from 0.2g to 0.8g, engineers immediately become worried and will start running analyses with 0.8g to check if their design is redundant enough to avoid catastrophic failure. 

       Up to now, the variation of ground motions and its effects on the responses of building structures are still unaddressed problems, and tools available to engineers are still very limited. This project aims at adding an additional simple tool that engineers can use for approaching this complex issue. 

A design framework with combined high and low fidelity models

       To approach the variability of ground motions and its effect on structural responses, more analyses are needed with larger numbers of ground motions. However, it is impossible for the code writers to force engineers running 100 ground motions with their detailed structural models, because this task is too time consuming. 

       An alternative solution is to set up two structural models: one with high fidelity level and the other with low fidelity level. Engineers can first run hundreds of ground motions with the low fidelity model. Results from these hundreds of analyses will provide engineers an estimation of the average structural responses as well as the variation of responses. Based on the results, engineers can further select a set of more representative 11 ground motions to run the high fidelity model. With a better selected ground motion set, this proposed method will give engineers a more stable and representative estimation of structural responses.

       The IsoEstimate program developed in this project aims at providing engineers a simplified tool to accomplish the low fidelity part of the above mentioned procedure. It should be pointed out that the objective of developing this software program is not to replace the current practice. On the contrary, the proposed procedure and the software program should serve as a supplementary procedure to illustrate the large variability in ground motions and structural responses. Through analyzing the simplified isolated structural model with hundreds of ground motions, engineers can check if the selected 11 ground motions are indeed those “representative” ones that produce an average response. 

       This program can also be used for selecting initial parameters of the isolation system. A great advantage of using a simple structural model in the preliminary design stage is the short analyzing time. Engineers can run tens of these simplified models within an hour to figure out a reasonable initial isolation design parameters for running more complicated and detailed models. 

Pictures of the Program