I’m continuing with the front porch design that I have posted several questions about already and have another. I plan on a wide RO at the entrance to the porch in the 18′ to 20′ range with ganged posts at the outside similar to the bungalow that was in the past issue of FHB houses. I’ve just been to the Trus Joist site where they have a sizing table for their various engineered lumber products. I need to size the roof carrrying header beam that will be on the outboard side of the porch.
According to the tables for a dead load of 15 and live load of 25 I can go with several combinations of LSL, LVL, or PSL that range from 3.5″x14″ to 5.25″x11..25″ for a RO of 18′ 3″.
I have a few questions….
What are the relative merits of the three beam types that they offer LSL(Timberstrand), LVL(Microlam) and PSL (Parallam). Is one better for an exterior application such as this in which it will be wrapped? What are the economies of each?
In addition the sizing tables had you put in the width of the house( perpendicular to the direction of the header in question) in determining where you would go to size such a header. The smallest width was 24′. I’m not sure why they would use this for a header that is at one outside wall. At any rate I assume that since my application is a shed roof attached to the existing structure that the 24′ width category would be plenty conservative in sizing my header beam.
Sorry for being so wordy. I look forward to your responses.
Replies
I'd say the best one to use is the one that's locally available. Around here LVLs are all that virtually everyone carries. Everything else is special order.
The company that sells you the beam should be the one to size it. They'll know the local codes and loading requirements.
Using a 14" deep beam to span 18' or more is pushing it, IMHO.
Thanks for the reply Boss. One thing I don't have a clear understanding of is beam depth or height relative to width. For example if these LVLs are 1.75" thick how much more stiffness do you get out of say a 4 ply built up beam as opposed to a 3 Ply for the same depth, or conversely how much more depth do you need for a 3 ply assembly to be equivalent to a 4 ply etc. ?
The rule of thumb that's thrown around for beam depth relative to span is one inch of depth for every foot of span. So in your case, an 18' span beam would need to be roughly 18" deep. But that's just a starting point. You have to consider how much wieght the beam is carrying and how much deflection is allowable. There's no easy comparison for how many plies will work at 3 ply with a deeper beam versus a shallower 4 ply beam. But in general, depth is better. A deeper beam will deflect less in general. A deeper beam with less plies will almost always be cheaper than a wider, shallower beam. But sometimes there isn't enough height or a deep beam. Then you might have to look at adding plies and/or going to steel.
A man is but the product of his thoughts. What he thinks, he becomes. [Gandhi]
Check. Thanks.
OK.....I just talked to someone at Trus Joist who took my numbers of dead load 15, live load 25, shed roof rafter span of 10' @ 3/12 pitch w/ rafter overhangs of 24". He came up with a 3.5" x 11.875" wolmanized parallam beam which would have 1/2" of deflection w/ the full design load.
I can't get a feel for what that amount of deflection really means. Is that a reasonable amount for a porch shed roof? Your comments please.
Offhand, 1/2" of deflection isn't much. Particularly for a roof system. I still don't like a beam that shallow spanning that far, though. But I tend to be realy conservative. You could always bump the depth up to 14" just to be on the safe side if you were concerned about it.
A career is born in public, talent in privacy. [Marilyn Monroe]
I was thinking just that if I can fit that extra height into the plan. As always thanks fo your help.
I'm a little bit biased toward Laminated beams in an application like yours, over the other options (LVL's, Paralams, etc.) although I've used all of them at one time or another.
If you do have a concern about deflection, you can have a lam beam made with camber....the supplier/mfr. should be able to do the calcs based on your span and loads. The cost difference should be minimal....some plants lay up beams with a standard camber and there is no upcharge.
Just be sure, if you go that route, that you pay attention to the arrows stamped on the beam and set it right side up. :-)
Thanks for your response. By camber do you mean essentially having a crown built into the beam? When I spoke to the Trus Joist people they first recommended their microlam beam but were concerned about using it in an exterior application even though it would be under roof and wrapped. Their parallam beam can be had in a wolmanized version but that does derate its stuctural qualities slightly.
In a previous post responders didn't seem to be too concerned about an untreated beam being used in this case but the Trus Joist guy thought that even ambient moisture could be a problem. This didn't make much sense to me as I thought that one of the hallmarks of these engineered products was their dimensional stability with changing atmospheric humidity. Obviously it would need to be protected from direct water contact. Your thoughts?
"I thought that one of the hallmarks of these engineered products was their dimensional stability with changing atmospheric humidity. "
Not at all. They shrink and swell like crazy with changes in moisture content. Just like conventional lumber, if not worse.
I've never heard of any problems with LVL beams that were kept under roof and protected from rain. Guess you could paint the thing if you had a mind to...
In Palm Springs, they think homelessness is caused by bad divorce lawyers. [G. B. Trudeau]
I had some paralams delivered for a job a couple three years ago that had, apparently, been stored at the bottom of a unit near the ground. While the storage area was covered, the beams had picked up ground moisture and had swelled IIRC nearly 3/4" over dimension.
Weyerhaeuser's engineering people acknowledged that the beams would never shrink back and were structurally compromised.
Weyerhaeuser made good on them, but I've avoided paralams since.
Other engineered beams, like LVL's and lambeams will swell when wet, but the glue is tried and true and they will return to dimension when dry.
Camber is an arch put into the lam beam when it's layed up. I've built some heavy docks and piers with PT lam beams that were cambered for anticipated live loads (heavy cranes and forklifts). I'm not an engineer, but just a slight camber adds incredible strength.
The stock cambered beams I've used in residential construction had about 1/2" camber in 20' IIRC.
I wouldn't hesitate to use a lam beam in the application you've described. You can order a "header grade" beam, which is ugly, but made to dimension and less expensive, or a "finish grade" beam which can be sanded, stained, painted, etc. The header grade is fine if you're going to wrap it with some other finish material, or if it's going to be buried in a wall, ceiling, etc.
Thanks Boss and Nman. I feel good about proceeding with my design now that this hurdle has been cleared with your opinions and information. Like I said in a previous post I usually do my own work but in this case I'm going to have a framing crew I know do the rough frame so I want to make sure I have all my i's dotted and t's crossed.
I recently went through the same kinds of decisions regarding a beam in my basement. While I ultimately ended deciding on ripping the floor and joists out and starting over with trusses, I first went all the way through the deliberations of varying beam products.
Had we not decided on the tear out, our next choice was a 3-ply LVL with 3/8 steel flitch plates. Given the loads calculated by our engineer, the need to limit the depth of the beam, and local availability of products, this combo priced out the best.
My local steel fabricator was going to take the LVL supplied by me, drill the LVL and steel and bolt everything together, delivered to my site.If I remember correctly, we got the load and deflection properties of 16" deep LVL in one 11 1/4" by adding the steel flitch plates.
My info is posted as an HO and not a qualified builder or engineer, so take this for what its worth.