Screened Porch End Wall Question
Screened Porch End Wall Question
(OP)
Attached is a typical end wall of a screened porch that Arch's send me all the time. Since I do not have a continuous king post, I typically use a 5 1/2" thick solid girt running the width of the porch to brace the bottom of the king post for out-of-plane stability.(typically, these are 18 - 20ft wide.) Most contractors would prefer me to use a 3 1/2" thick member or nothing special at all as it makes their boxing more consistent and is cheaper. I see these built without any consideration to any out-of-plane bracing and have yet to witness an issue in my 25 years of engineering. (Let's not discuss lateral in-plane stability at this point.)
I prefer they use a solid 6x6 king post from the floor to the ridge, but many Arch's do not like the way it looks. Does anyone have any further insight or alternative facts on this matter?
Thanks!
I prefer they use a solid 6x6 king post from the floor to the ridge, but many Arch's do not like the way it looks. Does anyone have any further insight or alternative facts on this matter?
Thanks!






RE: Screened Porch End Wall Question
Note: I'm trying to invent new terminology here that will confuse your Architects.
Mike McCann, PE, SE (WA)
RE: Screened Porch End Wall Question
I have done that in the past. I try to avoid it as I have had issues with it getting built correctly due to the amount of detailing. Might give it another shot.
Thanks!
RE: Screened Porch End Wall Question
I HAVE seen problems with this as built and I can't let it go without something that is capable of spanning horizontally out to out. Usually I want the girt/beam to be a solid piece (glu-lam)
Sometimes I use a steel flitch (placed flat) or plates made of LVL's flat, ripped to 5 1/2" and built up if needed.
Usually I'm not as "strict" on the out of plane deflection calc (especially if it is screen, seems like L/180 is plenty) but I need to have something that I know is adequate for stress.
18-20 ft is far! I seldom have two big triangles of screen above the beam/girt. Usually I can convince all to frame the two side posts full height.
I hate this condition and I know what you mean about having "yet to witness an issue" still, you only need to have an issue once...
RE: Screened Porch End Wall Question
Mike McCann, PE, SE (WA)
RE: Screened Porch End Wall Question
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Screened Porch End Wall Question
RE: Screened Porch End Wall Question
I like the two queen posts full height (no knuckle joint per Msqaured)as well with the girt just acting as a show beam between the two.
I was wondering about that but I cannot wrap my head around if it will perform any useful function. Wouldn't that just put the girt in torsion?
RE: Screened Porch End Wall Question
Yeah, technically it does. Unless you can moment connect the girt to the upper post somehow. Of course, your stability demand from the post always did require torsional resistance from the girt. What can I say? I'm going for gooder here rather than all the way good.
Suppose that you made the gable frame, including the girt and some doubled up rafters, into a simple truss that was able to support the glulam clearspan. Then you would no longer have a stability issue at the bottom of the king post. You'd just have to find homes for your out of plane wind loads.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Screened Porch End Wall Question
If we used AISC as an example, I believe your girt would be classified as a "Nodal" brace as the two brace points are not connected together (like diagonal brace).
Therefore, I believe you would design for brace strength of Pbr = 0.01 Pr and brace stiffness of Bbr = Omega (8*Pr/Lb).
Assuming 2,000# reaction.
Pbr = 0.01 * 2,000 = 20# (this seems easy to meet)
Bbr = 2.0 * (8 * 2 kips/48 in) = 0.67 kip / in. (assuming 48" tall king post)
Assuming DF wood with 4x12 in the flat direction, the strength requirement is relatively easy to meet. Stiffness seems to cap out at about a 14'-0" span for the girt to meet the 0.67 k/in requirement. 5 1/2" wide beam appears to meet requirements for 20'-0" span.
RE: Screened Porch End Wall Question
Yup, thought about that. It just gets a little messy at the posts as they want the "girt" to be at the same elevation as the eave headers -so the rafters would be on top of the bottom chord.
That is about what I have come up with in the past - hence my 5 1/2" width
I might have to go back to msquared's idea and start doing the cantilevered doing rafter thing.
Thanks for your help.
RE: Screened Porch End Wall Question
Actually, I could use some diagonal straps at the heel if the beam was the same width as the dbl. rafters. Just got to sell them on the looks if it ain't cladded.
RE: Screened Porch End Wall Question
Seems like there are (at least) 3 "sources" for a lateral load that could/should reasonably be considered here:
1. A force to provide stability to the girt/beam (due to the vertical load)
2. Out of plane wind load (I thought ASCE-7 says screens pick up 100% of the wind)
3. Something to account for incidental loading on the interior columns (even if it is just a 200# or 50 plf handrail load)
Are we squinting too hard on this one in order to make it look OK?
I've done the end rafters as a truss thing before (to support the ridge). There was a very nice article in Fine Home Building 20 or 25 years ago about that strategy.
I've also uses C5 turned flat to make the horizontal span.
I see this condition often enough and I'm usually "the bad guy" so... very interested in the thoughts of others.
RE: Screened Porch End Wall Question
A second problem i have with something like this: If it is all screen or screen and posts then you have no shearwalls or lateral system, so we are relying on a hip roof to act as a cantilevered diaphragm, i rarely see any substantial chord connection back to the main house or proper shear transfer from the sheathing.
Honestly, i think these do fail when exposed to full design load (not common to see full design wind) and there are multiple redundancies that we cannot model or calculate. spring-hinges, load sharing, wood's large safety factor, duration factors, weak axis bending, shear transfer of screen material (
RE: Screened Porch End Wall Question
Yes, but that is a subject for another thread. I typically just specify lots of nails on the overbuild and hope for the best. I kind of have to stay within the standard of practice in my area or I will be labeled as the "Bad Guy" as Houseboy put it.
RE: Screened Porch End Wall Question
As for the kickers, I got them once for a bigger span and when it was all painted really they weren't too noticeable and they were above window 'king' studs so it kind of blended in.
So it sounds like girts is the primary option or some fancy moment connections with a girt or magic.
RE: Screened Porch End Wall Question
We typically would have 5 1/4 PSL beams that would be wrapped framing into HSS cantilevered columns for lateral. So no matter what you do, you can say it's cheaper than that...
Then again typically the architect would want the porch vaulted and would not want to see the girt at all so we would end up with a kinked steel beam at the gable end to support a steel ridge as a wood ridge would have hung down 1/2" below the rafters or something and this would be "unacceptable".
RE: Screened Porch End Wall Question
(Don't meant hi-jack the thread, mainly wondering how common or open to solid sawn lumber a builder might be for this application.)
RE: Screened Porch End Wall Question
I also rarely use or see Glulam, LVL is the primary. (My hijacking is officially over, sorry XR250)
RE: Screened Porch End Wall Question