2006 IRC Braced Walls (wood)

[bigmig]

I did a search through the arhives at engtips and was amazed at how FEW posted topics I found in regards to braced wall design in the IRC and IBC codes. Am I the only one who has been stumbling through crappy code explanations regarding things like Continous Wood Structural Panel Sheathing (better in the 2006 code) and prescriptive bracing design? My question to all the wood guys:
1) Why is a hold down required on a 2 pack stud (figure R602.10.6.2) for the alternate braced wall panel? This seems like a hinge to me, so how does load even get into the strap?
2) How many of you identify your braced wall panels in your plans, in accordance with IBC 2308.9.3.? Do you use just an abbreviation in the framing plans, like BWP (braced wall panel)or another way of flagging it?

I heard recently that the bracing wall amounts listed in Table R602.10.1 are for seismic only, but have been assigned to both wind and seismic by the code guys. I guess the upcoming codes will have bracing for wind AND bracing for seismic...does anyone else have info on this?

 

[K2skis]

I personally threw all the prescriptive braced wall provisions out the door some time ago. I do engineered wind analysis on all of my structures including single family homes.

Then specify out the shear walls on the plan and am done with it.

Locating a section of wall and specifying it as a "braced wall panel" is all fine and good but how is the force transfered into the wall to begin with and how is transfered back out of the wall? Connections top and bottom need to be specified and at times hold-downs located at the panel ends to avoid rotation.

As far as figure R602.10.6.2 I have to agree with you the strap on the double stud all by itself does not make much sense. This narrow wall bracing was created by APA and was always on both sides of the garage door. Once again easier in my opinion to just engineer the damn thing and forget the prescriptive methods. Also APA has published values for this narrow wall so it can now be used in an engineered design as well.

 

[bigmig]

K2skis,

I am assuming by the popularity of this post that many engineers have come to the same conclusion you have regarding lateral design for residential wood structures, i.e. ignore it, or revert to a designed shearwall or braced frame design.

The problem with designing shearwalls for residential buildings is that this design is complicated, or expensive, depending on how you see it: various elevation profiles, 3 sided box designs, flexible diaphragms, diaphgram deflection, shearwall deflection, wind design loads extrapolated from ASCE 7 and/or IBC...And then after you figure all that out, you now have to spend 2 days detailing it with your drafter. Exactly how many shear planes and overturning paths do I have to trace for EVERY SINGLE shear wall in a 3 story building with holddowns landing on beams etc. Then you have to go into the field and inspect the wall that is built, just to find that the concrete guys left the hold downs out, so now the framers have to drill them in...etc. You get the picture. It is A MESS. There is no way (that I know of) that anyone can convey the lateral construction requirements of a 3 story residential building in "plan" only. Heck, we have 1 entire sheet that is designated for lateral details only.


I would personally rather see an UNDERSTANDABLE code that I can apply a totally legit prescriptive design ("speak into the microphone Mr. Engineer" said the court) and then, as you said, be done with it. Unfortunately, this depends on the ICC codes...

 

[K2skis]

bigmig,
Agreed it is a pain in the butt, but no one said it would be easy! I don't get to crazy with theory here between differential shear wall deflections and rigid vs flexible diaphrams. Clients don't want to pay for all that not that want to pay for much of anything. I have set up spread sheets to help with the calculations and generic details for the various conditions you mentioned. I would say I can knock out wind design for a McMansion (typ fancy house) in about 4 hours. This does not include the countless hours of spread sheet creation, drawing of details, or shear wall theories thought through.

 

[jwilson33]

Interpretation of the prescriptive method is further polluted by the ongoing revision of the supporting documents by APA. I appreciate that they are doing all this research, but Ii have found it difficult to simply follow the Code without deciphering which interpretation or APA test data applies.

I am seeing enforcement of wall bracing slowly improve in the PA and NJ areas. I only get involved when an architect has a question or a builder has a problem. I do see NJ architects identifying wall bracing locations and some do have wall bracing details on their plans. The inspectors are requiring it, but they are not getting too excited about the details.

If you can get involved early in the design phase, I find it easier to recommend moving a window or widen the garage wall a few inches. But then again, engineered plans are not required around here - I just get brought in on specific issues to offer my opinion, rather than to engineer a whole house.

 

[K2skis]

jwilson33,
What part of PA are you from? My office is straight south of York or Harrisburg a couple miles from the MD line. I have done this type of braced wall work over in NJ in the higher wind speed areas. But have since turned down future work due to the arch. that I was working for only wanted me to do the lateral wind design. He designed the conventional cut up roof framing based on what I have no idea but the design was so bad I didn't want to be apart of the project at all. The building officials don't seem to flag anything as far as I know they are putting them up under designed as I type.

 

[jwilson33]

K2skis-

Poconos. Most inspectors here do ignore wall bracing as well as other important foundation items. One inspector, however, knows the wall bracing inside and out and has put me to task in defense of some judgment calls. I really respect that and have learned from it.

Architects around north/central NJ are being checked on their plan submittals for wall bracing. Hillsborough over to Clinton areas. But they aren't too elaborate in their requirements. A good faith effort is usually enough.

I would decline if I had a call from the shore area where they have to use the IBC and do whole-house calcs. I don't know if they are actually enforcing that in the 110mph areas.

 

[DonPhillips]

When I review plans, if there is a seal, then the design professional takes responsibility for the lateral bracing design. For single family not selaed, the drawings must show the prescriptive provisions in the IRC otherwise I issue a correction letter.

The only time I question a design professional's design is when it is way out of whack - like portal openings (garage doors) without a specific design to handle the lateral loads. I told one guy about the APA standard design that he never heard of.

Just this week, I pointed out to an architect that the new 2-2x8 header spanning 8.5' carrying a roof load needed to be confirmed as to the designer's intent, since no species and grade was indicated, SPF#2 does not comply with NDS, and Southern Pine SS is rarely used. If the design does not change, I'll probably call him to talk it over, but at the end of the day, it is his responsibility.



Don Phillips

http://worthingtonengineering.com