Wood frame structure

[rittz]

Attached is a sketch showing a cross section through the roof and upper floor of an existing building (50 + years old). It's a tad difficult to analyze but I have a hunch it would be hard pressed to handle the current specified loads (LL snow 25 psf + DL). The client wants to create a living quarters on this level .The bearing partitions possibly would stay as located if possible. The floor structure (2 x16 @12” oc) is clear spanning the retail area below.

I have kicked around ideas about what I will do to reinforce it ... but I would like to hear what others might suggest.

 

[msquared48]

I would thread in new PSL's to either side of the center vertical with new columns placed where possible. Stitch bolt and block the PSL's together as needed. Do not modify the existing trusses in any way, trying to keep the same t/c forces in the trusses by placing the new PSL's low, close to the bottom chord.

This looks like an old turn of the (last) century brick building... For Seattleites, typical Pioneer Square construction.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rittz]

Thanks Mike …. Sorry to bother .. I’m not certain what you meant exactly. It appears you mean to install the psl’s longitudinally through the trusses at mid span. You mentioned columns. The client wants to keep the space below wide open which precludes supporting the psl’s on columns. The cross partitions are of no value to accept much load since they each rest on one or two joists
Regards
Rittz

 

[dhengr]

Rittz:
There sure is a lot unsaid in the sketch you show us. You need to do much more inspecting of existing conditions and details. If the roof is in good shape and its framing does not show any distress, and history shows some near record snow falls on it, I would try to disturb it as little as possible and get it grand fathered into the new use. Otherwise, it is some sort of a bastardized Vierendeel truss, space frame system. Does it really taper from 4' to 1'? How is it attached to the conc. blk. walls, what’s the condition with these connections and what exactly are the conc. blk. walls, 12", grouted, reinf’ed., and their condition? Since the second fl. was an attic in a retail establishment, there was some LL reqr’mt. at the time, and you say 2x16's at 12" o/c. What decking and how is all this attached to the conc. blk. walls. If this fl. is stiff enough, today’s roof loads and 2nd fl. LL are not much on most conc. blk. walls. Is the floor plan shown the 1st or 2nd fl. plan? It appears that some of the bearing walls are not directly under the pony walls in the roof system. What second fl. support can you get from below without disrupting business? Can you gain access to do remedial and stiffening/strengthening work without disrupting business? You have much to do, but it looks doable.

 

[msquared48]

OK then...Looks like you'll have to analyze the existing trusses without the central support(s) and reinforce them as necessary for the new span length. Aspreviously mentioned, the 1 to 4' dimension at the end concerns me too.

May be able to get away with sistering on to the bottom chord and the diagonals. Connection reinforcing could get tricky.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rittz]

Mike
Your right the structure concept is very old …. but just a tad older than me. You are also correct in saying more work needs to be done.

And yes the distance between the upper and lower chord varies from 4 or 5 ft at the front to zero at the back. The structure could hardly be characterized as a truss.
The LL requirement at the time it was built? Did they have such things back then?

The floor plan shown is the upper floor directly under the “trusses” The ceiling joists (some of which are lower chords) rest on the longitudinal partitions.
The roof joists and the ceiling joists are embedded into the exterior concrete blocks.
etc etc
When clients complain when insist on reinforcement (saying it should be ok it’s n standing fine for many years).. I always say … “ no ! That is the problem) ha

I need to get really creative to fix this thing without running the client out of money I think.

Thanks for listening….
J