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?

 

[CEmonkee]

Thanks for the details AELLC, I'm looking at them now...

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.

 

[AELLC]

The EPC connectors are OK but a bit overkill because the beam is sandwiched into the wall stud framing. I don't expect much gravity load on the patio beams so I would design them as 3-1/8" wide GLB supported by dbl 2x6 studs as a post. Normally, we just strap the beam to the studs/post in this case.

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.

 

[CEmonkee]

I see what you mean about the beam being sandwiched into the stud wall... so then for the base of the post, do you specify anything special there, are is it just toe nailed into the sill plate?

 

[AELLC]

At least 2 Simpson H2.5's nailed to the sill plate unless you have enough uplift to require more

 

[CEmonkee]

I'm looking at the uplift now... I'm using ASCE Method 1, and I was going to assume that the porch roof is an overhang in the longitudinal direction. That might be a bit conservative...

 

[CEmonkee]

I'm calculating about 2900 lbs per post. I think the way I'm calculating the uplift is very conservative... I'm considering the porch to be an overhang in the longitudinal direction of the house (along the ridge line of the house). But the ridge of the porch is perpendicular to the ridge of the house, so when the wind is in the longitudinal direction of the house it would be in the porch transverse direction... And with the steepness of the porch roof, the wind pressure would be acting downward on the windward side of the roof. Also the pressures on the leeward side of the roof are not as large as the overhang pressure I was using.

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.

 

[CEmonkee]

Hi AELLC - I looked over your detail for the outside post. That would work, however the client wants to use glulam columns, so I'm looking at using a Simpson PBS66 post base (see attached PDF).

 

[CEmonkee]

Since the Simpson post bases I was going to use out at the front of the porch do not provide lateral capacity, I'm thinking it might be a good idea to add some knee bracing out front... So I would span a beam that runs parallel to the house between the posts, and then put the knee braces between that beam and the front posts. I know that I'm considering the porch roof to be cantilevered and on paper that works, but I think the diaphragm may deflect more than I want before it starts carrying the loads back to the house.

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...

 

[AELLC]

I don't think knee bracing is req'd - the length/width ratio of the patio roof isn't that bad, plus the nailing of the overframed portion to the main roof provides addnl support against drift.

 

[CEmonkee]

Okay... I don't think I mentioned that the porch roof is framed with scissor trusses. As far as I know there was not a plan to sheath the underside of the trusses. Also, I just found out yesterday that the contractor was planning to hang the beams on the outside wall (with hangers), instead of burying them in the wall.

 

[AELLC]

1) Scissors trusses are OK, no bearing on this discussion.

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.

 

[CEmonkee]

Hi AELLC - thanks for your reply.

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!