Truss, box beam, or what? Engineers?
Hi All,
I want to pick your collective brains for a challenge I am facing. I am currently going back and forth with the architect and engineer over pair of beams that are required (according to engineer) in a second story remodel.
I’ve enclosed a section view from the plans to help make this clear, but I’ll describe what is going on also. The beams are shown with the x’s at either side.
Homeowner want to redo second floor on a 1930’s era home. Second floor joists are 2×8 and rafters are 2×4. Width of house is approx 25 ft and length of long axis is 32ft. In order to better support rafters and roof load without bearing on floor joists, engineer is proposing two beams (shown in section at left and right of drawing) that will run the full length of the house (32ft) and bear on exterior walls at either end. Span is therefore essentially 32 ft with no post mid span. Because of the distance, engineer is spec’ing either a 5 1/8 x 22″ deep Glulam or a 10″ flange 45# steel beam. Both of these are way outside the practical scope of site conditions because they are too large and ungainly to move into place given the site conditions and locations of neighbors house (they cannot be craned in from side due to wires, trees, neighbors’ houses, etc.) Engineer proposed cutting open roof and “dropping” the beams in from the top. Again, this would push the project well beyond the budget. If we had the money to tear off the roof, we would re-design completely.
I (the contractor, not an engineer) was wondering about the possibility of building a box beam in place. For example, could it be done by using a pair of much smaller glulams for each beam, placing one above the other similar to a giant TGI, but with the web on the outside instead of the inside? Say, for example, you use a 3 1/8 x 7 1/2 x 32 ft Glulam as a top and bottom chord and skin each side with plywood, with 2×4’s at each vertical joint of the plywood between the glulams. I am thinking the glulams top and bottom to give meat for nailing. Materials-wise this would not necessarily be cheaper, but the advantage is that the much smaller glulams could, with difficulty, be manouvered into place (they are much lighter) and the rest could be assembled in place.
OR, alternatively, you truss designers out there, would it be possible to design and build an open web truss in place to span that distance. Either using simpson plates or plywood gussets for the connectors. Again, the advantage being that the constituent parts could be individually manouvered into place and then assembled there and would be much lighter than these massive beams.
Again, I am not an engineer, but I am trying to think of some alternative ideas for solving this problem. Something that would hold up to an engineer’s calculations and be structurally sound. We are trying to avoid having to cut into the roof or into the first floor and basement to add a bunch of posts and beams and footings for intermediate bearing posts.
Am I at all on a feasible path? It seems to me that the engineer involved is not looking too far past the giant beam option. I’m hoping you guys are up for a challenge on this.
Thanks,
Brad.
Replies
I think you are looking at a tough row to hoe. Ask the engineer, but I really doubt if he would stamp any sort of site built beam like you propose. Just the engineering on it alone would be expensive. Isn't like he can look the thing up in a span table, like he can with any other sort of engineered beam. Clear spanning 32' ain't going to be cheap, period.
i think you are thinking exactly what the engineer is thinking. he can't just look it up in a table. but, i was hoping there would be an engineer(s) who do have some experience in designing custom solutions. i would think an extra $1k - $2k spent on an engineer is better spent than an extra $5k - $7k for cranes, poking holes in the house and patching, beams, welders, etc. I'm just bouncing around different ideas here and I appreciate the input from you all here. Keep 'em coming.Thanks,Brad
We have built a couple of beams like what you describe. Both held up fine but we did the engineering ourselves. There was no building code in effect at the time. We couldn't do that now. It is possible to find an engineer who can design this type of box beam but not everyone will. Ask around. It is a good solution.
You could probably do a 3 ply LVL (1 3/4" x 22") and just assemble it in the house. That would cut down on the weight aspect -- less than 200 lbs per ply.
I would ask the engineer about this option.
And yes, a flat girder truss could be made. It's going to be multiple plies so you would have to assemble it the same way you would a triple LVL.
bc... if it's steel... you can bring it in pieces... get a certified welder to make it whole againforget about a 32' box beam... unless it's steel toothe structural engineer can design the beam to be sectional and fabricate in place
if he can't , he/she knows a fabricator who canMike Smith Rhode Island : Design / Build / Repair / Restore
to me ..the whole idea sounds a bit goofy.....and I certainly wouldn't follow the impractical engineer to the kitchen table.........
redesign for a pony wall......or a couple posts........certainly you have bisecting walls beneath the floor to carry the load? somewhere.
that is going to be the back-up plan, to post down to the foundation via walls if necessary. but we wanted to avoid opening up walls on the main floor to avoid having to cut into some nice plaster walls with a cove ceiling. but,if we have to, we have to.
Are you're sections drawn right?
They appear to be a 32' wide house. If this house is 25' x 32', according to your sections, you should have 25' beams. Am I missing something?
Yeah, the section is correct. The house is not a true rectangle. It is 32 ft long, by 25 ft wide at one end and 32 ft wide at the other end because the dining room cantilevers out over the garage on one of the house. But the 32 ft length of the beam is correct. I just posted two of the pages of plans that were the most relevant, I thought.Thanks,b
I have done 1 plywood box beam , site built and as spec'd by an engineer. So it can be done.
However , if you can do a built Up I have a suggestion.
Open up a hole large enough to thread the beams into at the eave line at the front of the house just above floor joist level. Open up the gable end walls at the ends of the beams locations.
Slip the beams in from the front and shove them around by pushing them out one gable end and then back through the other.
I also would try to talk the engineer into lowering them to sit right above the floor and building a stud wall above them.
this was exactly the first proposed solution. this was the problem we were trying to avoid. the neighbors houses and trees are so close that we would not be able to thread a 32 ft beam in from one end. And, if we have to open up the roof from above to drop it in, we increase our costs quite a bit since we were not budgeting for touching roofing and having to re-frame all that. Plus, the idea of manouvering a 1400 lb steel beam around in the attic with the potential that it slips and goes through the ceiling below or breaks plaster did not appeal to me.
A hole can be opened and closed in a roof for about $100 labor. Bob's next test date: 12/10/07
If the whole purpose of the beams is to take the roof load, why not just sister 2x8 rafters from plate to ridge and be done with it?
Solar & Super-Insulated Healthy Homes
How would one deal with the birdsmouth bearing on the new rafter. Matching up to the old one leaves only the depth of the old ones supported on the wall. Not to say I disagree with your idea at all, because it sounds like an ideal solution, just looking at the mechanics of it.
They can't get your Goat if you don't tell them where it is hidden.
How would one deal with the birdsmouth bearing on the new rafter. Matching up to the old one leaves only the depth of the old ones supported on the wall.
From the drawings attached, it appears that the original 2x4 rafters bear on the attic floor (I assume to a rafter plate on top of the ceiling joists).
Widen the plate and bear the 2x8s on that. Cut a plumb cut just short of the ridge (to be able to slide the sisters in place) and then block between plumb cuts of each new rafter pair.
Alternatively, drop additional 2x4 rafters directly underneath existing rafters and glue-nail plywood to both sides of new/old rafters to make 7" box beams. This would have the advantage of keeping most of the rafter bay open for ventilation and insulation.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I'd be looking at it your way on this too. Seems like something is goofy different with this house or with the engineer.
I think he is calcing what looks good on paper with no view of how it can actually be done.But like JJ said, if this has to be a beam for some reason, there is a way to part it and transfer loads somehow.But keeping to the KISS method - the primary problem with the existing house is undersized rafters - so just re-size them and life is good again
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Riversong makes a good point, but your original thoughts of using a box beam or something similar are certainly plausible.
Go to http://www.apawood.org/level_c.cfm?content=pub_ply_libmain and take a look at Plywood Design Specification, Supplements #2 and #5. They cover the design of plywood box beams and all-plywood beams, respectively. You have to register in order to download the .pdf files, but it's free.
Depending upon the snow load in your area, something could be made to work. It seems like your engineer could design the beam to be the full height of the knee wall, which should be pretty stout.
It seems like your engineer could design the beam to be the full height of the knee wall, which should be pretty stout.
And, if a beam is determined to be the way to go (which still baffles me), then at least a box beam could be insulated.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
i'm waiting to hear back from the architect on the engineers reasoning esxactly for needing the beams. I'm still partial to a box beam if only because i wouldn't have to crawl on my belly all the way to the wall top plate to install a new bearing plate and sister on 2x8 rafters. but that is still way better than big beams. Thanks again for you help and input.brad.
There's a bunch of different ways to accomplish this. I would propose to the engineer a site built glulum/truss. Frame a pony wall with 7 layers of 2x8 bottom plates and 7 layers of 2x8 top plates. Plywood both sides. You can glue and nail the top and bottom plates on the deck then toe nail studs between them. Use diagonal members like in a truss between the 24" on center ponywall studs. If you sheathe both sides with 3/4 ply these walls will be much stronger than the 5 1/2x22 glulams. The material list would be about 70 2x8x16 and 16 sheets of plywood.
Just an update here. The engineer came back with a proposal to use 1 3/4 x 11 7/8 PSL's 16 o.c. for the rafters. The rafter span is 17' with ceiling joists at the tails to resist outward thrust. 6 in 12 slope on the roof. My 2006 IRC shows both 2x10's and 2x12's as being kosher at those spans. We are in Seattle in the city. I could imagine there being some provision for heavy, wet snow occasionally, but not that much. We are having another engineer take a look at it next week. I'm curious to see what he says.Brad.
Gotta love those engineers. But then if it fails, it's his butt, and in this litigious society, who can blame him? I wonder how much the fact that you are in a seismic zone affects his designs? Maybe he is thinking about the event of earthquake and snow load occurring simultaneously.