Study of Damage Provides Foundation for New Building Codes

The Northridge earthquake raised many questions about the safety of our buildings and what city officials plan to do to make new buildings stronger. Senior Structural Engineer Nicolino G. Delli Quadri, chief of the training and emergency management division for the city of Los Angeles, will play a lead role in guiding the development of new earthquake codes for the city. He was interviewed by staff writer Hugo Martin.

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Question: What, if anything, have structural engineers in the city learned from the quake about the adequacy of our building codes?

Answer: The biggest thing is that the high accelerations, both vertical and horizontal, were more than were ever anticipated and so the codes will somehow have to be adjusted to account for this acceleration. We are still out there looking and investigating these damaged sites to find out exactly how the vertical acceleration affected the buildings and how they failed and why they failed. Based on that knowledge, we can change the building codes.

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Q. What is vertical acceleration?

A. Upward motion. In some areas we had up to 1g, the force of gravity, of vertical acceleration. If you can imagine something being picked up and then dropped. That factor was pretty detrimental to buildings. That was not anticipated.

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Q. Do engineers have any idea what can be done to address that in the future?

A. If you are talking about concrete buildings, it means heavier sections, more reinforcing steel through the columns and beam connections with the foundation. We need some design factor that considers vertical uplift more significant than we’ve had in the past.

If you are talking about wood-frame buildings, you are talking about strapping and bolting connections together so that they don’t pull apart. They can flex but they don’t pull apart.

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Q. Once you find a way to address problems like vertical acceleration, how soon before we see it in our building codes?

A. As far as the Los Angeles code, it could be a matter of a year. First of all, these things have to be looked at carefully to find out what really was the cause and what really can be the fix. We don’t want to just rush in there and say, “Ah, yes, this is the problem and this is the solution,” when in reality it may not be.

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Q. Can you explain how it is that in some cases we saw a house collapse and an identical house next door survive with little damage?

A. There are a number of factors. The most obvious one is that the ground motion hitting the buildings were different. That can depend on whether one building is sitting on looser material than another building. The soil properties below the buildings affect the earthquake force immensely.

Other factors could be the structures themselves, even if they are identical buildings. Maybe there were things that were missed in the construction of one building.

Another factor is the orientation of the building. If the motion of the quake is predominantly north-south, for example, your building may be oriented so that it faces in a direction that helps it withstand the force.

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Q. Is there anything that people can look for to point out a building that is likely to withstand a quake?

A. If you are looking at a single-story building, most do really well. If they have been built since the mid-1940s and have a plywood or stucco sheathing that goes down to the foundations on the exterior wall, they do very well. They will have some cracks and things, but you don’t see them collapse.

You will have problems with multistory buildings that are split-level. They tend to have a lot of problems at the split-level area because the two portions moved in two different directions and they tend to have a lot of damage where the floors come together.

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Q. The city created tougher building codes after the 1971 Sylmar quake. Did homes built under those codes do well in this quake?

A. Yes, because that code pointed out the need, especially on wood-framed buildings, to have hold-down anchors, or bolts, at the base of shear walls--the vertical walls at the exterior or interior of the building that keep the buildings from falling over laterally. What will happen is that wall will tend to pick up if there is enough horizontal force and there is not enough weight from above to keep it down.

Also, the codes upgraded the requirements for shear walls in a lot of building to require plywood as a shear-resisting element. Before then buildings were allowed a one-by-six-inch diagonal brace from one corner to the other corner and that was supposed to take the load. But it didn’t. Those things broke.

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Q. Do you expect building owners will complain that the new codes you will propose will increase the cost of building new structures?

A. That happens, but let me give you an example of why it’s necessary. If you look at those industrial tilt-up-wall buildings that collapsed, we had a retrofitting ordinance that was ready to go. The ones that were built before 1976 did not have positive anchorage--metal attachments between the walls and the roofs. There were a number of buildings that were built without it. The code allowed it. Those buildings in the Chatsworth industrial park area, most of them collapsed. The cost of retrofitting that type of building is anywhere from 60 cents to $2 per square foot. The cost that those firms incurred because of the damage was horrendous.

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Q. Can we assume that with each disaster the city goes through, our buildings become stronger with each building code upgrade?

A. You would think so. But that only happens with new buildings. The existing buildings stay the same. Another concern with existing buildings is people altering their buildings without considering what they are doing. For example, they may take out a major lateral-resisting element, maybe an exterior wall, because they want a nice, big, open window. If it’s not properly designed to handle the loads, that could have an impact on the lateral force resistance of that building. That’s why we require building permits for major alterations to buildings, so that those things can be caught before they’re done.

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Q. After we upgrade the codes in response to this quake, can we assume that all new buildings will withstand the big magnitude 8-plus quake that is expected?

A. When you say 8-plus magnitude, it doesn’t mean much because all that means is the amount of energy dissipated by the slip in that fault. The acceleration and displacement are the concern, because the acceleration measures the forces that goes into the building. So you can have an 8-magnitude quake on the San Andreas 50 or 60 miles away and you have a 6.6-magnitude quake right under you and that 6.6 quake right under you is probably going to have a lot more impact.

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Q. Before the 1971 quake we believed our buildings were seismically sound, but we learned that they were not. Now we are again reviewing our codes. Should we expect that we will continue to upgrade our codes after every major disaster?

A. It’s possible. What we can do is look around the world for the worst-case scenario and design our buildings to withstand that disaster. But that is not the most economical way to go. You may be forced to pay twice as much for your dwelling. Can you afford that? Can society afford that? You may not have that type of earthquake for years.

So there is a lot of judgment that comes into play. It’s one of those things that you look at and say, “Well, what is the maximum possible earthquake we can expect based on history and the quakes that have taken place?” The idea of structural engineering is to design the building so it doesn’t collapse, so it doesn’t take lives. You can expect it to suffer damage. In fact, the damage that occurs is really the dissipation of energy. If a building can absorb all that energy and still not collapse, then the building did what it was designed to do.

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Q. You make it sound as if the problem is not how best to make these buildings withstand a quake but how to make them withstand a quake and still be affordable.

A. That’s right. You can design something that is pretty close to earthquake-proof, but it’s going to cost you quite a few bucks. It has to be something that is inexpensive and works.