A look at the new earthquake guidelines

A look at the new earthquake guidelines

By JOHN HOOPER
SWMB

A just-adopted version of the Uniform Building Code (UBC) will have a significant impact on the design and construction of buildings in the Seattle area and throughout the West Coast.

The 1997 UBC contains many significant revisions that will affect building owners, developers, architects, and engineers.

Both the state of Washington and the city of Seattle have recently adopted the code, with most jurisdictions to follow. A new "International Building Code" (IBC), slated to replace the UBC, is currently under development and will become the primary governing code of the U.S. by the year 2000.

What are the major impacts of the 1997 UBC on earthquake design, and how do they impact building layout and related issues?

All of the code changes in the area of earthquake design originate in study and analysis of actual poor building performance in recent earthquakes, such as the 1994 Northridge and 1995 Kobe events.

The changes made in the 1997 code will have no cost impacts if building design is done correctly up front. A well-thought-out building will certainly require a team approach of architect, engineers and others working together to conceptually lay out the building so that earthquake conditions are minimized.

But under the new code, if a building is not correctly laid out up front the cost of compliance will be high. It will be almost impossible to make a "bad" building "good" - given the issues that have been revised.

Changes

There are four major areas of change relative to seismic design.

The first is in the definition of "soil parameters." The type of soil underneath a building has a direct effect on how the building will perform during an earthquake. For example, buildings built on rock are affected much less than, say, buildings built on soft soils. For each soil type, there is a code-defined numeric factor that is applied to determine earthquake-resisting design levels for a building, to ensure that it will perform appropriately during an event.

In previous codes, there were as few as three different soil categories, with a correspondingly limited range of code-defined factors. Under the new code, there are six different soil categories, and the range of code-defined factors has also increased significantly. This can result in an earthquake force increase of up to 75 percent.

The soil categories were changed to reflect new scientific evidence regarding the response of soil mediums during major earthquakes, as originally identified during the 1989 Loma Prieta earthquake and further validated in the Northridge earthquake.

In the past, it has not been crucial to determine exact soil information because the range of design factors was small, and had a smaller effect on the building's earthquake-resisting system and cost. But now there is the potential to greatly increase design levels with proper soil information. If this information is not obtained up front, it translates into a correspondingly large increase in the cost of design. It is important that designers become much more systematic in ascertaining soil type.

The second major change is the introduction of a new redundancy/reliability factor.

The more earthquake-resisting elements that occur in a building, the more capable the building is of having one element fail while others are still available to resist earthquake demand, thus avoiding a progressive collapse situation. The intent of this code change was to encourage the use of systems with more redundancy and get us away from those that perform poorly in earthquakes, such as buildings with only one braced bay on each side.

We know from reconnaissance of damage done in the Northridge and Kobe earthquakes that buildings lacking redundancy do not perform well. Actually, the new factor encourages a level of redundancy in our buildings that we've always assumed was there. But we were wrong.

Longer shear walls in a shear wall building, more braces in a braced-frame, or more bays in a moment-frame all are methods of increasing redundancy. It is important to note that this redundancy factor must be incorporated early in design. If it isn't, there can be a code-required increase in the earthquake design level of the building of up to 50 percent. This requirement affects not only the length of shear walls, etc., but also foundation design for the earthquake-resisting system.

The third change is in the detailing aspects of various systems, or how a building is "tied together" piece by piece to become one composite structure resisting earthquake forces. Some examples of detailing include how a floor is connected to a braced frame or how a tilt-up concrete wall is attached to a plywood roof.

The 1997 code has changed the way a building is tied together, and design-force levels have increased (based, again, on building performance in the Northridge earthquake). Depending on the systems used and the level of redundancy, the code changes on detailing may raise the cost of the building.

The final major change relating to earthquake design concerns the code definition of building "drift," which has been modified. "Drift" is how far a building moves from side to side during an earthquake, and this affects how close to a property line a building can be built (in other words, setbacks are required for earthquake design purposes and not just zoning requirements).

The code addresses earthquake drift to guard against the possible pounding of a building against an adjacent structure and to ensure that elements such as cladding and window systems withstand earthquake deformation in a major event.

The 1997 UBC has modified the building drift parameters so that maximum earthquake drift is considered. This increases the amount of a setback relative to past codes, sometimes upwards of 25 percent or more. It should be noted that this percentage isn't across the board - it depends on building height, the structural system selected, the redundancy of the system, etc. But it is an important item that is sometimes overlooked - at great expense.

The new order

The 1997 code will be the last UBC to be published by the International Conference of Building Officials.

This requires some explanation. Briefly, there are three model building code organizations in the United States, charged with the responsibility of developing the codes that govern building design. These three organizations - the International Conference of Building Officials, the Southern Building Code Congress International, and the Building Official Code Administrators - have decided to jointly develop the new international building code. The IBC currently exists in draft form and will be formally published in the year 2000.

Here are some questions and answers about the consolidation.

Why was a decision made to eliminate the UBC and consolidate the codes?

It was too difficult for owners, architects and designers to be going from code to code, depending on the location of their project. The three model codes had quite a few differences.

A decision was made to jointly create a new document that would be valid throughout the U.S. The new IBC will include all aspects of the existing codes, including fire, life safety, occupancies, exiting, etc.

How is the IBC being developed?

The new IBC document is being developed based on the material included in the three model codes. Writers are picking and choosing portions from each. In the draft completed last year, modifications were made in a formal code review process. Any discrepancies are being resolved through committee action.

Why is the 1997 UBC important now, if we're changing to the IBC in the year 2000?

Because there is likelihood that the 2000 IBC will not be ready for acceptance by the local jurisdictions - various city, county, or state governments - by the year 2000. Because of that, the 1997 UBC may be around longer than its normal three-year life. It may be upwards of six years before the IBC is ready. It's a longer-term code cycle than we're used to dealing with.

Is there a chance that we can make modifications to the 1997 UBC?

No, not through the normal process. But the state is always capable of making changes that affect issues important to us, through the Washington State Building Code Council. There is some opportunity for change that way.

Will the 2000 IBC be adopted throughout the world?

There is the potential that this IBC may extend beyond the boundaries of the U.S., but there are currently no firm plans for that development.

Has the 1997 UBC been adopted in all jurisdictions?

By the end of 1998, nearly all jurisdictions in the state of Washington will have adopted the 1997 UBC. You should contact the local building official in the jurisdiction where your project is located to confirm code-adoption status.

Again, this article has highlighted the major code changes relative to earthquake design. There are many revisions throughout the code in all sections. Code users will need to review the sections in detail to determine specific changes made to areas affecting their work.

John D. Hooper is director of earthquake engineering at Skilling Ward Magnusson Barkshire Inc. As a member of the committees instrumental in the modifications to the 1997 UBC, as well as the development of the new IBC, he has given many presentations on the implications of code changes.

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