Instrumenting Tall Buildings in Los Angeles, CA

Community Seismic Network
California Institute of Technology

As part of the Community Seismic Network (CSN), we are instrumenting selected buildings in the Los Angeles area. We are using low-cost class-C MEMS accelerometers in order to allow each structure to be instumented densely. The main scientific goal of these building arrays is to develop rapid assessment of damage following an earthquake. The belief is that detailed and density measurements are key to doing this.

The MEMS sensors are packaged in a box with a computer and (in some cases) a backup power supply. The computer is an ARM linux computer known as a Sheeva Plug. It interfaces with the sensor (a Phiget 1056), and with the cloud through an ethernet connection. Time is maintained with an ntp connection over the Internet. The data are packetized and sent every 10 minutes, providing continuous readings at every sensor location.

One of the buildings we have instrumented is the Dept of Water and Power (DWP) in downtown Los Angeles. This is a 15-story steel frame building built in 1964. We have placed 30 sensors in this building in communication closets near the north and south ends of each floor. The data are transmitted in near real time to the CSN in the Google Cloud. On the right is a sketch up model of the building developed by Monica Kohler and her students.

Response to the Hollywood Earthquake. Examples of data recorded from a small earthquake (M=3.0) that occurred 2/8/2014 approximately 4 km from the LADWP building. The three columns are the N-S, E-W, and Z components respectively. The top half of each column are the sensors on the north side of the build and the bottom half are on the south side.

The 52-story building is shown on the left and an ETABS model constructed from plans of the building is on the right. Data from the sensors in the builing allows us to calibrate the model. The model will allow us to simulate damage scenarios that can then be compared to the noise level on the sensors to the changes can be detected.

La Habra Earthquake

Sensors on the 1st 12th and 24th floors recorded the 3/29/2014, magnitude 5.1 earthquake near La Habra, CA. which was about 33 km from the building. The recording for NS and EW components are shown in the columns on the right, with floors 1, 12, and 24 bottom to top. Click on the seismograms for larger images.

Synthetic seismograms generated for each floor using the ETABS model with the measured seismogragram on the 1st floor to drive the model at that point.

A comparison of the predicted seismograms (red) and actual measured seismgrams (blue) on the 12th and 24th floors.

Encino Earthquake

The spectra of the horizontal components show the modes of the building. The numbers to the right of each spectrum indicates the floor number of the sensor. The east components show the fundamental mode (~0.2 Hz - 5 sec). Both components show higher modes and their variation with elevation in the building.

Comparing Spectra to Building Models

The first three modes of the building are computed with the ETABS programs and are compared to the spectra from the Encino earthquake given in the previous figure. The frequencies are close but not a perfect match including that some minor adjustments to the model parameters are needed.

Wave Propagation and Spectra in 52-Story Building

The left panel below shows waves derived from ambient noise correlation propagating in a 52-story building.
The right panel shows waves from the 4.1 Castaic earthquake, approximately 70 km from the building.

The plot below shows the spectra for the sensor on each floor suring the Castaic earthquake.
There insufficient energy in the 0.2 Hz range of the source spectra to excite the fundamental mode, but the translational modes up to order 9 are quite clear. .

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