Wind loading question
Wind loading question
(OP)
I have a question about applying the wind loads on a house. Please see the attached elevation schematic. The client wants to extend the back porch and support it with glulam posts (the posts will be located where the red line is shown). The porch roof pitch is 8:12.
My question is when applying the wind loads in the longitudinal direction of the house (along the main ridge), what should I do for the porch roof? It's what I would consider to be a transverse direction because the wind load would be perpendicular to the ridge of the porch roof. In Figure 6-2 of ASCE 7-05, I think I would use the pressures associated with zones B and D, does that sound correct?
My question is when applying the wind loads in the longitudinal direction of the house (along the main ridge), what should I do for the porch roof? It's what I would consider to be a transverse direction because the wind load would be perpendicular to the ridge of the porch roof. In Figure 6-2 of ASCE 7-05, I think I would use the pressures associated with zones B and D, does that sound correct?






RE: Wind loading question
Just out of curiosity how are you providing lateral stability for the roof in that direction?
EIT
www.HowToEngineer.com
RE: Wind loading question
Attached is the roof plan of the porch area with an isometric. The porch will be framed with trusses and will be about as long as it is wide. At this point I am just coming up with the loading and have not considered how I am going to brace all of this. If you have any advice / suggestions, I would appreciate it.
Thanks!
RE: Wind loading question
I think the easy (and probably typical) solution is to provide Kickers. But it seems that most do not want to see kickers.
So, you could analyze the roof diaphragm as a cantilevered diaphragm, similar to a balcony/deck (or a shearwall for that matter). If you go this route you need to have a load path at each 'chord' of the diaphragm (the glulams in your case) and provide some sort of lateral 'hold-down' back into the main structure/diaphragm.
Last you could try a wood moment frame or some type of modified wood moment frame. But this is almost always a 'no-no' and it is not warranted here. I probably should not even mention it in this case.
EIT
www.HowToEngineer.com
RE: Wind loading question
RE: Wind loading question
EIT
www.HowToEngineer.com
RE: Wind loading question
If you supply plan dimensions of the porch and the roof slope, and your design wind speed and exposure category, I can see if the wind shear I compute is similar to what you compute.
For new construction, it would be easy to tie this to roof trusses lined up to the
porch eaves. Another technique used frequently is to make the columns 16" square wood-framed columns with two Simpson STHD14 hold down straps each column.
RE: Wind loading question
The porch is roughly 17' wide by 16' deep, the roof pitch is 8:12. Wind speed is 105 mph, exposure C.
This is new construction... the client is just changing the porch part of the plans. Could you elaborate a bit more on your statement about it being "easy to tie this to roof trusses lined up to the
porch eaves"?
Thanks for all of your help!
RE: Wind loading question
Is that close to your calc?
RE: Wind loading question
RE: Wind loading question
RE: Wind loading question
RE: Wind loading question
I looked at the PDF and I'm not sure if I understand your detail... the porch trusses rest on the glulam beams, and the glulam beams frame into a bearing wall on the back of the house (green line in attached .jpg file).
Thanks for your help - I really appreciate it!
RE: Wind loading question
BTW, you should use mean roof height of main house to determine pressure zone on patio roof.
RE: Wind loading question
Pretty much SOP around here on all semi-custom and custom homes.
RE: Wind loading question
I've been interested in the wood column you propose as I have seen it before but was curios about the following:
It is basically a wood structural panel shear wall with holddowns. However it does not meet the provisions of slenderness ratio for a shearwall. I've been interested in research regarding what the failure modes are for wood shear walls - buckling, etc. Then trying to justify how to design to prevent these failure modes. However with the column you could design it as a built up section and space the fasteners based on shear flow and reason that the extra stud members prevent buckling of the plywood.
Any insight to the design/justification of these built up wood columns?
Thanks!
EIT
www.HowToEngineer.com
RE: Wind loading question
The framed columns are not required strictly for shear, they are just extra insurance against drift of the patio roof.
The strapping of the patio beams to the roof trusses is providing 90% (so to speak) of the resisting structure.
We use them a lot where there is a long porte-cochere at custom houses. The shear panel diaphragm ratio may be exceeded, but it is a closed, box structure altogether so I am not worried about the "rocket science" of it, and not capable of analyzing this all on a "PhD" level. Simpson exceeds the ratio with their wood Strong-walls, but they do load-test them.
Sorry for not being technically correct, but it is the result of doing houses for so many years, it is the pragmatic nature of me to get problems solved efficiently. It is pour nature to want to justify everything thru rigorous calculation, but IMO we need to think outside the box and at least give something some capacity instead of just a wood post.
RE: Wind loading question
Thanks for your replies and sketches. It looks like I am close to your calc for the total shear...
Regarding the detail showing how the GL beams will frame into the wall - I'm attaching a .jpg that shows how the trusses are framed around the porch. Both GLB's are offset from the full length trusses. GLB#1 is aligned with a bearing wall. What would you suggest for this situation?
RE: Wind loading question
RE: Wind loading question
EIT
www.HowToEngineer.com
RE: Wind loading question
I'm also attaching a PDF with some Simpson connectors that I had been considering using - would something like this work as well? The Glulam beams will be supported on 6x6 posts in the exterior wall.
RE: Wind loading question
However if you still use the EPC and 6 x 6 post that is fine, just specify plywood shim because a GLB is a bit narrower than the corresponding solid sawn lumber.
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RE: Wind loading question
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I guess there are different ways to look at it, but my feeling is that it's better to error on the side of being conservative.
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RE: Wind loading question
A sketch is attached - the location of the beam and posts where the knee braces would be installed are the light blue color. Any thoughts would be appreciated...
RE: Wind loading question
RE: Wind loading question
RE: Wind loading question
2) You can't leave the trusses exposed to weather, even though there's a roof overhead. Moisture and sag-resistant gypsum ceiling board is required.
3) Insist on burying the beam ends in the wall. Hangering them off the wall is nonsense. I have no confidence in this contractor, don't let his ignorance go on.
RE: Wind loading question
I agree on the hangers - not a good idea. I was looking a little further into the possibility of taking the chord forces out in the building (so I wouldn't have to embed the outside posts). On one side there is a bearing wall that also serves as a shear wall, so I could strap to that. However on the other side there is a truss that runs the width of the building, but no shearwall or foundation underneath it that I could get the load down to.
Thanks again for all of your comments, etc. You have been very helpful!