Author Topic: Mixing Bracing Methods on a BWL  (Read 7895 times)

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Offline Medeek

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Mixing Bracing Methods on a BWL
« on: February 10, 2013, 10:13:44 AM »
I've since become more of a Design Professional than a DIY so this post may be a bit technical in nature but I thought it might be useful to post here in the chance that someone else has come across the same issues that I am having with the current IRC code.
Recently as I have begun working on the calculations for the amount of BWP (Braced Wall Panel) I need for one of my latest designs I have run into some problems with the current IRC (2012) with regards to mixing of bracing methods.
I recently purchased "A Guide to the 2012 IRC Wood Wall Bracing Provisions" to help supplement and clarify my understanding of the bracing requirements and make sure my plans meet or even exceed the code.  On page 258 (Appendix E) of the Guide it gives a table that clarifies the allowable mixing of these bracing methods.

I am designing this current design for seismic zoning "D".  With the three garage doors shown below I am going with PFH type bracing (classified as intermittent type bracing).  Next to the man door due to the space constraints I will use holddown straps (as shown in the concrete iso view below) so I would classify this as AWB (alternate braced wall) type bracing.  The rest of the wall to the corner of the building is simply WSP (wood structural panel) or perhaps CS-WSP.  However, for good measure I put another holdown at the corner effectively turning this braced panel into a shear wall. 

My problem seems to arise when I look at this table on page 258 of the text referenced above (or the IRC).  According to table one is not allowed to mix intermittent methods along the same BWL (braced wall line) within SDC D0 seismic zones, only mixing intermittent methods BWL to BWL seems to be allowed.  However, one is allowed to mix continous methods along the same BWL.

If I classify everything as CS-PF and CS-WSP then I'm within compliance with mixing of bracing methods along along a single BWL for SDC D0.  But then what about holddowns?  Note, that the CS-PF bracing method does not use or require holddowns.  If I do use holddowns technically my bracing method then becomes type PFH and it would seem to disqualify it since this bracing method is not  a "continous" method and I am now mixing CS-WSP with PFH on the same BWL which is not allowed.

Obviously, looking at my design it is a fairly conservative structural design with more than enough "shear panels" and holdowns to meet and exeed the code.  My problem just seems to be at the classification of the whole thing and how to interpret the code correctly.

Wall Framing below:



Concrete Foundation showing holddowns and anchor bolts:



Details of the front wall framing (PDF) and all dimensions:

http://design.medeek.com/plansets/garages/GARAGE4828-A6D-3/PDF/GARAGE4828-A6D-3_FRAMINGWALLS1.PDF
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

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Re: Mixing Bracing Methods on a BWL
« Reply #1 on: February 10, 2013, 02:31:51 PM »

I'm not answering from any deep level of understanding of this section. If you're understanding it we need to talk  ;D
Does either section of the wall satisfy the requirements for the entire wall?
Would running the header over the service door change anything?

Offline Medeek

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Re: Mixing Bracing Methods on a BWL
« Reply #2 on: February 10, 2013, 10:17:30 PM »
So I ran the numbers for both the wind and the seismic bracing with the following parameters:

100mph basic wind speed
SDC D2
Exp. C
Eave to Ridge Height: 15ft
Wall Height: 10ft
No. of BWL: 2
BWL Spacing: 28ft
Wall Dead Load: 12 psf
Roof/Ceiling Dead Load: 15 psf
Story Height: 10ft
Interior Gypsum Omitted:  N/A for PFH type bracing (but applicable if WSP bracing were used)

With the adjustment factors applied from IRC 602.10.3(2) and 602.10.3(4)  I get:

14.39ft for Wind Bracing
14.40ft for Seismic Bracing

Mostly coincidental that they landed so close, every other BWL that I calculated varied quite signficantly with respect to seismic vs. wind requirements.

What is interesting is that if I disregard the remainder of the wall and only account for the 48" of braced wall provided by each PFH portal frame I end up with 16ft of braced wall provided so luckily I have enough wall bracing with only the portal frames but it does seem almost unfair to completely disregard the additional bracing provided by the WSP bracing (7.4 ft) to the left of the man door.  In my mind it makes more sense that I really have 23.4ft of qualified bracing.

So yes, the portal frames (PFH) satisfies the bracing required.

I've solved my problem but I still have not answered my question about the mixing of bracing methods.

Maybe I'm missing something, here is the link to the code in question:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_6_par039.htm
« Last Edit: February 10, 2013, 10:33:49 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

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Re: Mixing Bracing Methods on a BWL
« Reply #3 on: February 11, 2013, 04:24:43 AM »
I'll review and try to get up to speed, but my question came from one line of thought.

The portal frame is likely more flexible in response to lateral load even when providing adequate resistance than the WSP fully sheathed section which is probably considerably stiffer. You want to combine their strengths but in responding to load the stiffer component, the service door section, will take the entire load until it becomes flexible enough through some level of failure to let the portal bracing take load. Now we have the true level of resistance. If the rigid section cannot take the entire load it is probably plastic or beyond by the time everything "hooks up". Very often when we think of things helping one another in some type of combined action what in fact happens is one element overloads and fails then the next element takes full load. Just a guess from the armchair at the moment.

That was also why I asked if including the service door in the portal framing would affect the results. I'm not up on the details this would require or if it could be done but that would provide more length to the portal portion of the frame. From a common sense point of view you sure look better than the all door garage faces I see passing.

Don't know if you've seen this pdf regarding dropped garage door headers, good stuff. Google "dropped Header Design Guide" for the magazine article and a few more hits.
http://i-joist.org/pdf/WIJMA%20Dropped%20Header%20Design%20Guide%20(11-7-07).pdf

Meanwhile I'm posting to another thread with no thought to bracing, this section of the code has grown from 2 pages to 50 for a reason, this is where the failures are occurring. We need more people talking about these things and pushing our understanding further... I appreciate your posts  :)
« Last Edit: February 11, 2013, 05:21:03 AM by Don_P »

Offline Medeek

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Re: Mixing Bracing Methods on a BWL
« Reply #4 on: February 11, 2013, 10:02:56 AM »
Seems like sometimes you need to read between the lines with the code.  Another interesting thing I just realized is that the requirement to sheath the interior wall with Gypsum does not apply to certain bracing methods (ie. PFH, ABW).  So my assumption is that the penalty you normally take in calculating the wind and seismic factors with the gypsum omitted goes away with this type of bracing.  Updated calculations for all braced walls show below. 



Based on these calculations the only modification required to the plan is to increase the length of the interior braced wall(s) from 48" to 54" in order to get my 9ft of braced wall panel, originally this was 8ft of braced wall panel.  8ft will pass the seismic requirement but not the wind load.  Initially I thought the seismic loads would dominate with this analysis since I was shooting for SDC D2 but after applying the adjustment factors for the wind it became clear that the wind was just as much a problem.  The reasons for this are: 12/12 pitch of this roof (think large sail), tall wall (10ft), high basic wind speed, large open interior, and high exposure all compound together to make the wind a major factor in the bracing requirements.

Braced Wall Plan below:


« Last Edit: February 11, 2013, 09:41:43 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

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Re: Mixing Bracing Methods on a BWL
« Reply #5 on: February 12, 2013, 02:02:44 PM »
As it turns out the garage I have been working on will be built just south of Fort Collins, CO.  I assumed of course that 100mph basic wind speed would cover me but this particular location has some very high winds that dramatically increase as you move toward the Rockies.  The exact address puts it in a 110mph zone where about a mile to the east would have been 100mph wind speeds.  Braced wall line "B" then does not meet the bracing requirements and I've either forced to make the interior wall longer (which cuts up the garage space = homeowner not happy) or grab an 80% reduction in bracing by adding additional holdowns.  Note that this interior wall is basically a shear wall now, with OSB on both sides of the framing and hefty holdowns at the panel end studs.  Any ideas on how to squeeze any more braced wall out of this without making it longer? 

Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

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Re: Mixing Bracing Methods on a BWL
« Reply #6 on: February 13, 2013, 07:49:36 PM »
What about simpson strong walls? After that I think it'd be an engineered moment frame.

Offline Medeek

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Re: Mixing Bracing Methods on a BWL
« Reply #7 on: February 13, 2013, 09:21:13 PM »
Haven't tried that approach yet, but I just recently ordered a stack of literature from them so I'll see what a Simpson Strongwall can do. 

On the matter of literature, pretty much every vendor that I've needed catalogues or information from has sent me free of charge a tonne of material, usually you just need to go on their website somewhere, put in your name and address and request the material.  Weyerhauser (I-Joists, LVL, PSL Beams etc...), Simpson Strongtie, James Hardie etc...  Most of the material is online but its sometimes really nice to just thumb through a hard copy.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer