Adding dead load to existing floor – struc. engineer question
I have been a member and reader of breaktime for maybe 4 years but rarely post. Always love reading though… I know there are a few engineers here so this question is aimed towards them. Obviously any information will not be taken and used I’m just seeking advice. I have a acquaintance / client who just bought a building next door to my own house. Its a commercial space that was formerly a deli. The one story building (built in the 90’s) sits on a foundation with a 4′ crawl space. Approx. 25’x25′ main area with a 10’x7′ bathroom bump out to the back. We’re talking approx. 700 sq. ft. The floor joists are 2×10 fir spaced 16″ on center with 3/4 tongue and groove subfloor. In The middle of the span is triple 2×10 beam with about 4 16d nails every 16″. There are 2 T’d 2×4 posts spaced evenly under the beam sitting on 2’x2′ concrete footings. The client wants to put a polished concrete floor on the floor system. I do a lot of custom concrete work and so he asked me about it but I was obviously very sceptical due to many reasons such as weight issues, building movement & creep, cracking concrete, stress on building etc etc… The floors we have done are on slabs. He said he had a concrete contractor coming to look at it so I asked him to report back to me with what they said. Said contractor quoted covering the floor with vapor barrier, wire mesh and pouring 1.5″ of polymer concrete then polishing and sealing. There was no mention or discussion of stress on the building. I stopped by to take a closer look and the gc he has working didn’t seem to give a s**t about structural integrity. He jumped and down on the floor and said “look it will be fine” Ughhhh. How many times have we seen that!? I refused to do the job but the guy is an acquaintance I see every day. He shares an office in the same building as my office. The building he bought is right next door to my house. I cannot sit and watch this if its going to be a problem. I done some quick calculations and the 1.5″ of poured concrete will be weigh in at at LEAST 15,000 lbs So this is adding 15,000 lbs of deal load to the existing floor system. The building is going to be rented out as a commercial space again, probably restaurant / deli and this area will have tables, chairs, people, counter space etc etc. The kitchen equipment would not be here. There is a separate area for this on a slab in the back. What are we looking at here for numbers and risks. I think it’s nuts but I don’t know the numbers. I am a builder, not an engineer! Thanks for taking time to read this!
Replies
I'm not an engineer either,
I'm not an engineer either, but that 15,000 pounds over 700 sq. ft. is 21+ pounds per foot loading.
Normal residential live load calculations are done using 40 psf and 10 psf dead load plus additional live loading of beams and walls for ground snow load specific to your area.
For residential loading the thin layer of concrete would reduce the allowable live load by more than half the original designed loads.
Not good for residential occupancy and maybe catastropic for a commercial operation. Most commercial designs are calculated using 50 to 100+ psf live loading depending on end use.
I can't imagin any ahj approving such a plan without an engineers stamp.
"Obviously any information will not be taken and used I'm just seeking advice.
I have a acquaintance / client"
New site sliding into quicksand real fast, is the 'client' also a firend of your BIL
1. stupid to put concrete over wood imo
2. eng? yep, I R 1
3. No 1 coast DF or SYP per your dim will hold 117.03423765 psf to the last 9 insignificant figures. Floor will be deflected enough by the concrete that any bounce will be damped by the concrete mass. #2 hemlock holds only 78.xxx psf, now you in deep doo doo.
4. No engineer needed, jsut gave ya the answer, tell your BIL the contractor is fos, not to be trusted.
5. Unless contractor has also proposed adding 2 more sets of them ther more triple 2x10 beam setups at open span joist points.
1. stupid to put concrete over wood imo
I've seen it done to good advantage in new structures. Apartment I had in NJ had concrete on the floors -- about an inch, IIRC. Real quiet and no bounce. Brand new apartments (I was the first occupant of this one) so it was no retrofit and presumably was engineered with the concrete in the plan. Had to be a lot cheaper than precast or site-cast, with many of the same advantages.
I've built apartments like
I've built apartments like that, but it wasn't a true concrete mix that was used. I can't recall the product mane, but it is a light weight mix that is pumped in and almost self levels. IIRC it is about 1/3 the weight of standard concrete mix.
Like I said, I am not taking on the job. I am just trying to give the guys good advice. I am not the type that can sit back and watch stuff possible nightmares happen. As soon as I met the GC I was completely unimpressed and my "steer clear" lights went on (e.g. the old jump on the floor "oh this will hold good" trick, and his comment on "oh and the sub floor plywood is pressure treated. Pressure treated is a hardwood so its much much stronger."!!!!
Anyways, thank you for the responses. I passed on some of the number thrown around from you guys to the owner and this is what he heard back from the concrete company....
"Here is some info that might help. The cementitous topping we use will weigh 20 lbs/ square foot at 2" thickness. There is an alternative product that we can use that can be placed at 1" thickness. The weight of that is 8.3 lb.s per square foot. Obviously it is your decision either way but I am confident in the process and specific site. Typical jobs that call for lightweight concrete over radiant heat at 1.5" thick in normal residential applications are usually 20/lb.s per square foot. These require no additional structural enhancements."