1950's era double tee precast roof system 1

[johnwow]

I have a client who wanted me to look at a single story building that he's in contract to purchase to see if it's possible to add a second story onto part of it - office use. The original section was built in 1957. I popped up a couple of ceiling tiles and lo and behold there are double tees! Quite a surprise to me at least for a building of that age. The webs are 12" deep (from bottom of flange to bottom of web) and spaced approx. 2'-0". Looking at the history of double tees, these look to be an original design based on the dimensions and the date this was constructed.

Beyond that interesting bit of news, would it be worth my while to research further into figuring the capacity of these for office live load? My plan B to throw at him was to use the existing CMU walls that extend to a 2 to 3' tall parapet can drop open web joists & deck above the concrete if that doesn't break the deal.

 

[JAE]

What is the snow load for roofs at this site? If more than 50 psf, perhaps there's some justification for converting to a 50 psf floor live load without partitions.
But with partitions, and with 80 psf or 100 psf egress corridors, I'd be wary of doing this without fully researching what these members can actually carry.

 

[johnwow]

Just 10 psf snow... it would be partitioned office space I assume, just like the main floor. Yes there would need to be some thorough investigation if there were any way this could be feasible. There are 2 sections - up to a 40'-6" clear span.

 

[JAE]

My guess then would be that these double tees are only designed for a roof load magnitude.
Probably not possible to use floor loading on it.

It always seems that when non-engineers see concrete structures the assumption is that they can carry almost anything....they are hard concrete aren't they?

 

[johnwow]

I would assume so as well.. I just cannot find any data or literature at all for tees of this size from that period to help me back it up.

 

[JAE]

Usually I try the local precasters. From 1950's you probably aren't going to find the actual precaster that produced these.

I have, in the past, run a load test on double tees using the procedure outlined in ACI 318.
Takes a bit of work but can be done. These were bleachers with nothing but storage/dirt underneath so loading these up to 200 psf was easy to do without danger to occupants, etc.

In your case, with a probability that these tees were designed for only self-weight, some nominal applied roof dead load, and 20 psf roof live load (more than 10 psf snow) then I'd worry about a load test actually damaging them.

Otherwise, a testing lab could possibly come in and explore the reinforcing patterns using non-destructive testing. Or a contractor could chip away concrete to reveal tendon size, number. etc.
All a bit difficult.

You didn't say anything about wall capacity, footings, lateral load capacity of the building - those are other items that come into play. Especially lateral wind/seismic with an added floor and roof.

 

[SlideRuleEra]

...up to a 40'-6" clear span.
I just cannot find any data or literature at all for tees of this size from that period to help me back it up.

With a span-to-depth ratio of almost 35:1 (40'6" / 14"), can you make reasonable assumptions to show that the double tees will not meet realistic requirements for live load deflection or floor vibration?

http://www.SlideRuleEra.net

 

[johnwow]

Yes I was just thinking about the ratio to help make an argument here on deflection. I think I'm going to go that route and guide the design to installing a separate floor system.

The existing walls are 8" CMU. Footings would have to be exposed to verify capacity.. that may be another issue later but this roof system is the primary focus at this time. We could install micropiles or helical piers if we have a foundation issue. 115mph wind and seismic B category

 

[r13]

These may help (from PCI 1sr Ed, 1971).

Link
Link

 

[JoelTXCive]

If & when you start making capacity calculations; then keep in mind that you might have 40ksi rebar in the sections.

I had to evaluate a Texas bridge built in 1963; and it had 40ksi steel in it. Several more seasoned engineers have told me that the big switchover to 60ksi was in the early 1960's

 

[johnwow]

Thanks retired13 - I must have missed those when reviewing the '71 PCI. Based on that, ain't no way I'm getting the live load I need.

 

[r13]

Sorry to bring in the bad news, but glad to be helpful.

 

[JAE]

Good links, retired13. You might see if you can pinpoint the date of these documents.

 

[r13]

Here is the link to PCI historical handbooks. Link