More rigid horizontal seismic restraint in walls 2

[rlflower]

I would like to initate a discussion on seismic restraint in conventional wall framing.

I am intriged by the problem presented by the conventional practice to construct a relatively rigid wall finish (stucco, plaster, drywall, etc.) over a more fexible seismic restraint system (conventional plywood shear wall panels, for example). The obvious result is extensive cosmetic damage to the wall finish in response to a major seismic event.

Agreed, this is not an issue to be addressed by the building code. The primary concern of the building code, understandably, is life safety. What i am addressing here is not a life safety issue; however, it does have considerable importance when one considers the massive extent of cosmetic damage that has occured after the 1994 earthquake in Los Angeles and how it directly affected the local economy for a number of years following.

Cosmetic repair became a major cost concern - an unnecessary cost to be absorbed by building owners. If the engineering community wiould focus upon viable means to construct seismic restraint systems that were more rigid than the wall finishes, then this problem would have a solution.

What type of seismic restraint system would you propose? Or, perhaps this would require a new invention?

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[cvg]

perhaps architects and builders should use more flexible finishes? not sure this problem could be solved without retrofitting all of southern california at an enormous cost. it is part of the charm of living in SoCal

 

[msquared48]

Or, allow these rigid finishes to float over the structural sheathing via some special prescriptive mechanism that probably would be more exensive to install than just replacing the sheetrock.

Mike McCann
MMC Engineering

 

[rlflower]

To cvq:

Thanks for your input.

We should keep in mind the wall finishes most predominately in use: stucco for exterior finishes, and drywall or plaster for interior finishes. Also, drawing from my experience, most construction projects involve existing construction.

There just isn't enough incentive for wide-spread use of new "flexible wall finishes." Rigid wall finishes have been used extensively in the past and will likely continue to be used in the future.

If we accept this as a fact, then we are faced with how to work with it. A practical appoach to innovation should always have an eye on conventional construction practice. Let's not "re-invent the wheel", but rather, let's see how we can improve the conventional practice.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[rlflower]

To clarify:

I would think that innovation is required to create a more rigid lateral seismic restraint within the wall rather than attempting to make the wall finishes stronger or less rigid (which is what I meant by not "re-inventing the wheel"). We need to think outside the box of conventional thinking - that of plywood shear wall restraint systems, etc.

The real downfall, BTW, with plywood shear walls is the lateral drift afforded by the elongation of the nails attaching the shear panel to the wall framing. Plywood shear walls performing at their peak code-prescribed design capacity allow for lateral drift to a degree that is beyond what these rigid wall finishes can tolerate.

So, if we toss out the idea of employing a plywood shear wall restraint system, what other options do we have that would provide a more rigid response?

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[cvg]

structural steel or concrete is more rigid. both are generally only suitable for industrial or commercial use. both require skilled trades, and can't generally be handled by your smaller contractors. concrete block is good but needs extensive reinforcement to handle seismic loading. I can't think of another cheap, lightweight, strong, stiff and easy to work with material that could be used. Or better screws to replace the nails.

 

[rlflower]

To cvq:

Structural steel seems to be the more ideal material to work with. Light gage steel construction, in fact, is cost efficient with only a slightly higher cost than with conventional wood construction.

Light gage steel construction has been common in the industrial, commercial and agricultural markets for many years. However, light gage steel construction has not been introduced to the residential market at a large scale. This is unfortunate, for it has great potential in resolving a host of problems related to wood construction.

Light gage steel construction commonly employs an "X" bracing system for lateral restraint within the steel stud walls. These "X" braces are relatively rigid in comparison to conventional wood framed shear wall panels.

I am interested in other engineers' experience with the specification and design of "X" bracing in all-steel or hybrid construction.

The "smaller contractors" tend to stick with what they know well. Some are very familiar with concrete and prefer to work with it. Others are eager to work with light gage steel, while many just want to continue building with wood.

As far as substituting wood screws for nails in wood shear panels, BE CAREFUL! The properties of the common wood screw tend to be weak in shear capacity. Specify ICC ES approved screws with sufficient capacity and follow up with periodic inspections.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[hawkaz]

The assertion that light gage cross bracing is more rigid than plywood shear walls is only valid for short wall segments, such as perimeter walls of multi-family housing. When you get longer walls, the drift from a plywood shearwall would drop significantly, but the cross braced wall will not see the same decrease. In fact- as the bracing gets longer, the bracing will elongate more.

Personally, I think that more thought put into blocking and strapping around openings- and better QA/QC in the field to make sure everything is installed properly would fix most of the issues noted above.

 

[cvg]

I think it would require a change in mentality for the architects and builders as well as local ordinances requiring better quality construction in high risk areas.

Builders and developers are not held accountable for damages caused by quakes. Homeowners are told that the structure is designed to withstand a quake. But few have the experience to know that only means the structure won't collapse. They don't realize that a)they will be required to pay for major repairs and b)insurance won't cover it. If they knew this they could make an educated decision to either move to a lower risk area or to demand better quality construction.

 

[rlflower]

To Hawkaz:

I've had extensive experience in structural design of custom residential construction during the last housing boom in the Los Angeles area. My experience says that we don't get longer walls; rather, we get shorter - more slender - walls.

However, I see your point. For "X" bracing, the longer wall might see a series of "X" braces rather than one extended set of braces. I would think "X" braces set at an angle less than 45 degrees from the horizontal become much less effective in achieving their purpose.

To cvq:

Quality control! Yes, it is not very manageable. But you've hit the nail on the head as far as the typical building owner's expectations. but, I think the major repairs you mentioned can be largely avoided if a little more careful attention is paid to the details - one of which would be a careful selection of a lateral load resisting system.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.