wood deck issues 1

[SLTA]

All right folks. I feel like my brain is going to melt out my ears and I have thought myself in circles, so I'm appealing to those who can straighten me out.

I'm working on a rebuild of an existing deck changed to commercial loads. Gutting the deck and starting over but the post locations stay. The owner also wants a post removed, which is creating all sorts of overloading issues. I did a lovely design for a steel beam, connections, everything, and of course they don't like it.

Referring to attached sketch:

PSLs as joists: 12" oc, 24'-6" long. The deflection is almost an inch at design loads. This seems to me it would put some torsion on both the ledger at the house (marked C) and the edge beam (marked B), not to mention the connection of the edge beam to the existing post and edge beam next to it (marked A). Everything is just *barely* squeaking by deflection-wise but also stress-wise.

1. Are there torsion issues connecting the joists to the ledger (C) and the edge beam (B)? If so, how to deal with them?

2. Are there torsion issues connecting the edge beams at the post (C)? If so, how to deal with them?

3. Should I just tell them to do it in steel? All these issues go away then... but of course, then it raises new issues with time involved.

thanks!

 

[MiketheEngineer]

Your decking material with get rid of most of the torsional problems. Still worried?? - Put some rows of "X" bracing between the joists about 8' oc.

1'' in 288 is a bit high if based on live load - I like 360 but overall it may be OK. How much "bounce" can you stand?? What is your "commercial" load and why are you using it on a residential site??

 

[ztengguy]

can you do a top bearing type ledger and beam, that should reduce the torsion issue correct?

 

[SLTA]

It's transferring from residential to commercial. It's 349 out of 360 for defl.

 

[ztengguy]

@Mike, how does the decking relieve any torsion @ the ledger at the house or beam?

 

[CANPRO]

Agree with ztengguy that bearing connection will take out the torsion issues...and easily done on he beam side.

on the ledger side...even if your joists are supported with hangers I would think that the hangers would allow enough rotation to eliminate the torsion in the ledger. But if you're still concerned with torsion in the ledger couldn't you add an extra lag screw (or whatever you're using to fasten ledger to house) in directly beside each joist?

 

[msquared48]

slta:

Depending on how wide the ledger is, and I assume the PSL's are hung with connectors off the ledger, there will be some torsion, the wider the ledger the more torsion on the ledger. This is normally taken care of a few ways:

1. With the lag screws staggered vertically on the ledger to allow the top lag to take the tension;

2. Pedriodic horizontal tension tie straps from the PSL's to the existing floor diaphragm beyond; and

3. Periodic wood columns at the house to help take the vertical load, minimizing the torsion the lags or straps have to take.

If there is an existing rim joist (not blocking) at the house location, you could hang the joists off that instead of using a ledger. In that the rim joist would bear on the wall top plate, there would be no torsion.

Regarding the post connection @"C", usually the joists would bear on top of the beam framing to the column at "C" and there would be no torsional effects to worry about. Are you saying that the PSL's are going to be hung off the side of this beam, the same as the ledger at the house? If so, if the PSL is flush with the bottom of the beam, you could periodically put small steel straps at the bottom of the PSL to the beam. Between the PSL hanger tand the strap, a couple will be created to transfer the torque to the beam. Note though, thaqt you do not have to worry about the column as the beam cannot twost due to the presence of the two connectors. No worries.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

M^2 has it right. I thought you were worried about torsion in the joists. But at the ledgers - good connections properly spaced will either tie into the building or the face of the joists - negating any real problems.

 

[focuseng]

Not meaning to hijack the poster's original 3 questions but is this an exterior deck? Are you using Wolmanized PSL? In my experience the PSL material would not be a good choice for an outdoor environment unless it is 100% waterproofed with a membrane. The creep factor alone would kill your deflections in a wet service environment. And then I have seen too many PSLs looking like pulp after 5 years of in-service on a deck. Some were even wrapped to help trap the moisture. Lumberyards around here won't even sell a wolmanized PSL anymore. Sorry, back to your regular programing.

I agree with M^2 as well. Tiebacks to the floor framing are in order. Consider your lateral loading of the deck and you will need the tie-backs anyway. Look up the deck construction requirements in new IRC and also material published by AWC (DCA 6). I think the Magazine "Deck Builder" just published something on this too.

MAP

 

[MiketheEngineer]

Focus - GOOD POINT

 

[dhengr]

Slta:
I thought we had this problem solved a week or so ago. You can’t go from a residential deck loading to commercial loadings, double the joist span lengths over part of the deck and remove a few columns and beams, without some fairly drastic structural changes. And, you gotta splain this to your client. Rack your brain if you wish, but you still may not be able to do the impossible, and you certainly may not produce a good, serviceable, design. That 24.5' span will work just like a trampoline along side the other 12' span and the bldg. on the right. A static L/349 is still very springy when you are watching the wine in your class slosh around. If that’s a commercial seating area, any foot traffic will drive dinners, or whatever, up the wall. I’ll bet our ‘lovely’ box beam spanned from the corner of the bldg. found. wall, below point ‘A’ on your sketch, to the left, to the column they want to eliminate now. And, that was a sensible structural system and should be reconsidered, and resold. That 24.5' jst. span is just a killer and I’ll bet you’ll have a heck of a time making the jst. connections and the ledger and its connection to the bldg. rim jst. work out.

You could span PT’ed. 2x jsts. in the 10.5' direction in the ‘B’ to ‘C’ area, with a flush header below ‘A,’ and to the bldg. corner, but you will still need a large flush fascia beam on the far left, from the bldg. found. wall up to the existing post position. Maybe just turn the framing and decking direction in that corner of the bldg., about 10.5x12.5. That might look more appropriate. Commercial LL’s are greater, lateral loads and vibrational loads, and hand rail loading are all more problematic, and fraught with potential liability, than are residential loadings, so you shouldn’t skimp. Do look at the AWC (DCA 6) publication that Focuseng mentioned, you can find that on the internet and print it out. It is a good deck builders starting point, but you’ll have to add a little Structural Engineering to this one. You also have to tie it into the bldg. laterally and torsionally, as others have mentioned. The fear is that you get a bunch of drunken dinners all swaying, in unison, to the same music; or that you get a drunken football lineman trying to impress a cheerleader by running into the handrail, head first. He won’t get hurt, but you want the railing to still be there afterwards.

 

[SLTA]

dhengr, I know all this. I have repeatedly explained all this to the client. they have a crapload of money and therefore do not care. the client asked me to look at this other possibility, so I did, and will be billing accordingly.

after hearing all that I've been through trying to make all this work, and learning what it would take to make it work, the owner suggested "well maybe we should just put the post back". um, yeah, good idea.