Wind loading question

[CEmonkee]

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?

 

[RFreund]

I would say yes, that you would apply wind pressures associated with B and D to the canopy when wind load is 'blowing' on the garage face as shown in your attachment.

Just out of curiosity how are you providing lateral stability for the roof in that direction?

EIT

http://www.HowToEngineer.com

 

[CEmonkee]

Thanks for your reply RFreund, I appreciate it.

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!

 

[RFreund]

If we are looking at wind load perpendicular to the ridge of the porch:

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

http://www.HowToEngineer.com

 

[CEmonkee]

For the lateral loading... can I take out all of the lateral load at the building wall (if the roof diaphragm is detailed properly)? Then for the posts out front only consider them as carrying vertical and uplift loads?

 

[RFreund]

I would say yes, most likely. This situation would fall into the cantilevered diaphragm case. Basically you have a shear and moment at the wall that need to be transferred to the main structure. The moment is then divided by the distance between chords (T=C = M/d) and you have a tension / compression force that should be transferred into the main structure.

EIT

http://www.HowToEngineer.com

 

[AELLC]

I would use 16" square CMU columns with large footings to make the CMU fixed at base in lieu of the wood posts, and detail the overframe roof deck to the existing roof deck to fasten securely, and take part of the shear. I wouldn't use straps to connect to existing roof because those would tend to straighten anyways, negating any tensile capacity.

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.

 

[CEmonkee]

Hi AELLC - thanks for your reply.

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!

 

[AELLC]

Not including the overbuild portion of roof - I come up with shear = 1970 pounds.
Is that close to your calc?

 

[AELLC]

Here is the detail

 

[AELLC]

oops, I showed the GLB as flush - should have been top of GLB = top of wall plate, but no change to basic concept.

 

[AELLC]

Also, the force in the strap is (1970x8.5)/17, not as written on detail sketch. I.E. 985 pounds approx.

 

[CEmonkee]

I'm getting about 2900 lbs total shear (a really rough calc)... quick question about something that I find somewhat confusing - when I'm calculating the loads on the porch, do I use the mean height of the entire house, or the porch roof height? I guess my question is when you are analyzing a house with a large extension like this porch, do you use the entire building to come up with the wind loads, or just look at the porch?

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!

 

[AELLC]

Here is a revised sketch and wind calculation.

BTW, you should use mean roof height of main house to determine pressure zone on patio roof.

 

[AELLC]

Here is what we do in lieu of a "skinny" wood post supporting the patio roof beam. It looks better, supporting all that roof mass.
Pretty much SOP around here on all semi-custom and custom homes.

 

[RFreund]

AELLC -

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

http://www.HowToEngineer.com

 

[AELLC]

RFReund,

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.

 

[CEmonkee]

Hi AELLC -

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?

 

[AELLC]

Just modify one of these 2 dtls to suit...

 

[RFreund]

I see, I thought you were using them in lieu of providing the strapping.

EIT

http://www.HowToEngineer.com