live load - alf living room

[eng003]

I have an existing assisted living facility renovation where flat wood roof trusses designed for 40 psf live load will be use for the floor system for a second story addition. Part of the addition will be bedrooms and other part common areas (living room) although not specifically addressed in my state code and/or asce 7 it seems 40 psf would be appropriate for an ALF bedroom but 100 psf would be needed for a common room any thoughts? Appears I may have to require the existin trusses be replaced just for this reason and/or worse try to modify or augment the floor system for additional strength...any thoughts? Could 40 psf be justified as adequate for this use?

 

[CELinOttawa]

This wouldn't meet the NBCC, but you should be able to supplement the trusses IF they haven't run significant mechanical through them... Anything beyond simple electrics and you're looking for trouble.

I once accomplished a significant upgrade through sheathing existing trusses in plywood to create box beams. You may want to try to think outside the box for this one...

 

[JAE]

I would agree with your 40 psf in rooms and 100 psf in corridors/public rooms

 

[XR250]

@CElinOttawa - How did you take care of the additional tension in the bottom chord splice plates?

 

[CELinOttawa]

Hi Excel Engineering: Added a continuous steel strip to the bottom cord on each face, so in the end this was under the plywood. The same fastening for the plywood to timber added to the placement "tacking" which was done to put the steel in place. It was an adaptive job, but the result was easy to construct. The only tough part was that we required the contractor to have the steel strips under tension when fastened. They fastened one end and then used a pully at the far end to draw the strips towards the opposite end.

 

[msquared48]

I thought the 100 psf floor load applied only in room occupancies of 300 persons or more...

Mike McCann
MMC Engineering

 

[jayrod12]

msquared,

I'd be interested to know where you read that. It would lighten a fair amount of loads.

 

[msquared48]

Can't find it either all the way back to the 1973 UBC. I will keep looking though. I know a big firm I was in in the past did use that though...

Mike McCann
MMC Engineering

 

[AELLC]

Were you thinking of this and had that good old memory gurgle?


TABLE 16-X: Values for Occupancy Importance Factor I
I
Type of Occupancy
SNOW
1. Essential facilities. 1.15
2. Any building where the primary occupancy is
for assembly use for more than 300 persons
(in one room). 1.15
3. Agricultural buildings, production
greenhouses and other miscellaneous
structures. 0.9
4. All others. 1.0
16-04-

 

[msquared48]

I think so. Good catch... Thanks.

Mike McCann
MMC Engineering

 

[hokie66]

I would be concerned not just about the strength, but also the stiffness. Roof trusses wouldn't have the same deflection requirements as floor trusses, unless they were originally intended to be used as floor trusses.

 

[eng003]

Thanks all for the responses! Sounds like general consensus is 100 psf for a common room am I right? I anticpate far less than 300 people as this is a small alf structure but it sounds like the 300 person reference was a red herring??(although msquared48 in my opinion offers some of the best responses on this board). Great thoughts on retrofitting trusses although it sounds a little involved (could you post some details?). I was thinking of leaving existing trusses in place (holding up gyp. board below) and just putting in new trusses between the existing to take the whole floor load. Wastefull and not clever I know but dummy proof...which I may need . Hokie66 good point need to check deflection design criteria if going to try to use existing in bedroom areas, thanks.

 

[AELLC]

I think you could collapse a single truss with a lot less than 300 people if they were standing close enough to each other along the entire span of the truss.

Actually we did a load study years ago and we came up with about 75 psf, but try explaining that to a judge or jury. I would go with the 100 psf.

 

[CELinOttawa]

eng03: Be careful in supplementing the new trusses between old... Like I said above, you need to have very little services to make that option work!

 

[XR250]

@CElinOttowa

"Added a continuous steel strip to the bottom cord on each face, so in the end this was under the plywood. The same fastening for the plywood to timber added to the placement "tacking" which was done to put the steel in place. It was an adaptive job, but the result was easy to construct. The only tough part was that we required the contractor to have the steel strips under tension when fastened. They fastened one end and then used a pully at the far end to draw the strips towards the opposite end."

Nice idea. I'll keep that in mind. I have added an additional flat 2x4 to the bottom chord before. Had a dropped ceiling so the extra depth was not an issue.

 

[CELinOttawa]

Excel: Thanks for the vote of confidence. It was an early job in my career and my mentor remarked that I likely had the idea because I had so little experience... Can't know that somethings "not done" if you've done very little in the past. Just seemed obvious at the time. We did have some hesitation in how to justify it by code, but this turned out to be quite easy under CAN/CSA-O86 (Wood Code).

I really loved practicing in New Zealand, where it is all performance based code with detailed review by the city. Show them what you're doing, show that you did the calcs, and build whatever you want. No mandatory anything, other than loading, just make it safe and practical. Yes, the same flexibility is available here, but it is not really practical. The building permit process practically grinds to a halt if your job isn't 100% typical.