IBC 2000 code question 1

[mjohan]

I am looking for other peoples interpretation of section 2305.1.5 of the IBC 2000 regarding use of plywood shearwalls... then compare that section to 2305.3.1.

Section 2305.1.5 rather clearly states that "Wood shear walls,... shall not be used to resist horizontal seismic forces contributed by masonry or concrete construction in structures over one story in height. (some exceptions)

Section 2305.3.1 states "Wood shear walls are permitted to resist horizontal forces in vertical distributing or resisting elements, provided the deflection..."

I would like to give a scenerio and apply these two sections. A four story multi-family building over a parking garage. Bad soil conditions results in a larger seismic force in a very low seismic area. The construction is composite steel joist with a 3" concrete deck (all floors). Walls are light gauge steel, sheathed with plywood on all exterior walls. X-braces "tension straps only" will be utilized, however, in only one direction of the building. The other direction has many openings and a perforated shear walls yielded better results.

Is this a code violation?
Also, any one see problems mixing the two lateral resistive systems?

Thanks

 

[DVM]

Check section 2211, light framed cold formed walls. There aren't the height limitations as for wood framed walls and I have seen, locally, up to 6 stories metal deck and concrete topping supported on this type of wall structure.

I don't care for tension only straps because they cannot be pre-tensioned. Pre-tensioning is required to limit deflections as the wall must deflect to engage the strap initially.

 

[mjohan]

Your right, that section states wood structural panel sheathing may be used. In addition, reference 2305.3.1 and plywood shearwalls seem like a sure thing. The only snag is section 2305.1.5 that states the building height is limited to one story. I have heard rumors that an amendment will be made to this section to agree with what section 2305.3.1 states.

 

[changyichen]

Now some codes has extent the use of wood shear walls to two story building.

 

[ERV]

Your current scenario does not appear to meet code. Not only code requirements, if you did a seismic analysis on the structure, I think you will find that your LFRS will be inadequate. What seismic zone? What about wind?

 

[mjohan]

ERV,

I am not sure why it would be inadequate because seismic is governing the design. So the straps and shearwalls are designed for seismic forces. The building is located in a low seismic design category, so plywood shearwalls are permitted to be used.

As far as an overall stiffness of a building, it is my opinion that plywood fastened to the exterior walls is a much sturdier structure than just straps and dense glass. I do recognize that larger seismic forces have not tested as well, compared to straps (mainly screws shaking out). The only reason seismic is governing is because the IBC 2000 has bumped up seismic criteria. Even on a hurricane shoreline, seismic still governs.

I have not received any amendment, stated in my earlier post, which will allow plywood shearwalls to be used on a four story structure governed by seismic forces.

The reason I initially started this thread was because I was wondering if other engineers have run into this problem with the current IBC code. This building would meet code if designed under BOCA. The IBC is obviously a conglomerate of several codes, and I believe that limiting the plywood shearwalls to one (possibly two) stories is a mistake and will be changed in the future... even if seismic is the governing force. IMO

I appreciate your responses.

 

[rday]

mjohan

I don't think you find an amendment to allow wood shear walls to support seismic lateral loads due to masonry/concrete in structures over one story in height (beyond existing exceptions). Nor do I think that provision will change as it reflects the language in the UBC 97 section 2315.2. I think the language in IBC section 2305.3.1
does more to limit the use of gypboard in high siesmic areas than permit the use of plywood shearwalls in your case.

I believe using wood shear walls in the case you outlined would be a code violation.

I my area we were/are under the UBC and the provisions are similar, so I'm afraid I haven't encountered your problem.

Wish I had an easy answer for you.

Rik

 

[ERV]

IBC 2305.1.5 Wood members resisting horizontal seismic forces contributed by masonry and concrete.
"Wood shear walls, diaphragms, horizontal trusses and other members shall not be used to resist horizontal seismic forces contributed by masonry or concrte construction in structures over one story in height." (IBC 2000)

I think maybe you have interpreted this to mean concrete floors, when in fact this pertains to concrete or masonry wall construction. It is against code to restrain concrete or masonry walls with wood shear panels over one story in height (two story if certain conditions are met).

I don't think that seismic will always govern "even on a hurricane shore line" as you say. Wind can be an awsome force (so can earthquakes).

Check your one-way tension straps (may not be to code).

 

[mjohan]

Thanks Erv,

Can you elaborate as to where in the code this pertains to wall construction? Is this an assumption because masonry is typically used for walls, not floors? I guess curiosity leads me to my next question... Why would you resist lateral forces with plywood shearwalls if you had concrete or masonry walls?

My earlier posts are referring to one particular building located on a bad geotechnical site which has made seismic the governing force. FYI

 

[ERV]

mjohan,

If we carefully look at 2305.1.5 under exceptions, particularly 2.3, it reads "Combined deflections of diaphragms and shear walls shall not permit story drift of supported masonry or concrete "WALLS" to exceed the limit of Table 1617.3".

I have seen instances where attempts have been made to control out of plane lateral forces with wood shear walls (buckling usually occurs under seismic load).

 

[AlohaBob]

It seems to me we couldn't use plywood shearwalls to resist seismic forces from concrete or masonry at all in the past. I think it was for non-ductile response in seismic reversals. Failure modes too lethal in insufficient lead time to collapse. The IBC seems to make exception for structures up to two stories. They provide the formula to calculate deflections.

Nonetheless....
4 Stories > 2 Stories. N.G.

You can use different lateral systems in two directions. The force calcs, base shear and story shears should differ by the R factor, etc. The dynamic analysis should show interesting harmonic effects different than a simplified analysis.

The architects gave us a flavor for suggesting the appropriate seismic system for their buildings. Try to stick with the type of materials in use. If possible, use masonry to resist loads from masonry, steel for steel. In plywood shearwall solutions, light frame construction should be the most common generator of this solution.

I would think that for 4 stories, you are beyond the practical capacities for structural design in wood anyhow. The code would then be helpful to justify or lead you to an easier more efficient design.

 

[cap4000]

Due to a significant difference between the "In-Plane Stiffness" between the wood and concrete/masonry wall combined systems,(NOT THE FLOOR) IBC limits it to a 2 story max. As such,"Torsion" I believe is also a major concern along with the critical "Drift" criteria as stated above.