Right now I can't help thinking about all of those tall glass and steel buildings in downtown Chicago.
Hi KathyV,
Disclaimer: I am not an engineer and may not be using accepted nomenclature or describing the events to a scholars level of understanding.
I would not worry very much about those buildings. They are built to a higher standard. It the residential homes, small buildings and highway overpasses that will fail in the Midwest. Skyscrapers are also designed to withstand high wind events. That design criteria can help them survive an earthquake. Steel is wonderful for its ability to twist, stretch, bend, and not fail--if properly connected and sized.
Out west, I watched overpasses being re-built after being felled by earthquakes. They were designed to withstand an earthquake of magnitude 8 (as I recall) on the Richter scale. Many of those same overpasses were dropped by the Northridge event. The motion was different in Northridge, and its acceleration factor was greater.
The midwestern overpasses and bridges will fall like dominos if a 6, 7 or 8 Richter scale earthquake hits this area. They will suffer long-column buckling like in the El Centro event. Or, their abutments at either end will move in the opposite direction from the bridge itself like in Sylmar, Northridge and Loma Prieta.
Modern western bridges are not attached at their abutments. They are in slip joints. After Northridge, these bridges showed signs of large scale movement at their abutments. The blind eye could gauge the slippage from 50 feet away. Most of those bridges survived. Their columns did not suffer long-column buckling (search "Cypress Structure collapse in Oakland California, Loma Prieta event).
My house is 5 years old and built by a major regional builder. It is not attached to its foundation at all. I don't mean minimally attached, I mean at all. After Homestead and Northridge, our standards out west were stepped-up again. Lateral and uplift loading hold-down features are built-in in modern western homes. Not here.
But, but, Homestead was a hurricane event...you DUMMY!
Yeah, it was. Out west, our staple sheathing nailers were banned. We had to use "hurricane" ties to connect our roof structures to shear walls and ultimately, foundations. Homestead Florida changed the UBC and the California Building Code. Seismic events and wind events can apply similar lateral loads to a structure. Roof and floor diaphragms are critical to a structures survival.
The Japanese know how to build. We learned from Kobe and other Japanese events.
I was a western project manager and builder.
Hold downs, PA's, drag trusses, and enhanced shear walls. Every engineered feature is connected to concrete, top to bottom, in western homes/buildings. Not my house in Ohio. Western homes are designed to withstand uplift and lateral motions at the same time. Not my home.
Out west, we did not build homes to survive earthquake events unscathed. We built homes to remain standing long enough for the occupants to evacuate the building. Building officials may very well condemn the homes I built after a major event. I will feel great if the occupants walked away from them, minimally injured.
cheers,
m
