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Exist T joists @ roof

Exist T joists @ roof

Exist T joists @ roof

2 sketches attached.

I am dealing with a new RTU (aprox. 4000#) over an existing concrete T-joist roof. Ts are 20" deep, 30" oc. Each panel is 5' long with 2 Ts thus. The existing drawings show the moments for which the T manufacturer had to design for but I can't figure out two things:

1)    The design moments shown on the existing drawings: I am assuming these to be factored, is that reasonable?
2)    The moment call out does not clearly say whether this moment is for a single T or the whole panel (2 Ts). I am assuming this is for one T.


RE: Exist T joists @ roof

From the drafting style and presentation, this looks suspiciously like something from the 50's or 60's, not later.  If my memory serves me correctly, USD (factored) design was first presented in the '63 ACI code, with WSD (unfactored) being the design method prior to that.  Any date on the drawings?

If there is no date, do a dead load analysis for the tees, roofing and mechanical (look like double tees) and see what the moment is unfactored.  Compare it to what is listed on the drawings.   

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

RE: Exist T joists @ roof

Mike, the drawings are from 1968. I did a dead load check but there are two ways to go about it. Originally I checked a single T and compared it to the moment on the drawing, which I thought was unfactored. Then it occurred to me, what if the moment on the drawings is for the whole section (2 Ts) and is unfactored.

Both numbers work for the existing loads but are about 15-20% apart which makes my RTU calcs tricky.


RE: Exist T joists @ roof

I would think the moments would be for the complete double tee at centre span, and are unfactored.  These are on a design drawing, with the tee design to be done by others.  If the moments were factored in that period of time, the dead and live components would have been combined and the moment given as Mu.

What I can't understand is why the moments in one span are significant, and in the other span are almost nothing or negative.  Am I reading that right?

Interesting configuration of the "lap tees", with the flange joint over a web.  Never seen that.

RE: Exist T joists @ roof

Hokie, one side is 23' while the other is 64' and hence the different moments.

Heck, I'll just check if the new stresses are within the 5% allowance and call it a day.


RE: Exist T joists @ roof

BTW the reason I got caught in two minds is that there is a note on the drawings (kinda shows up at the edge of the attached) saying " 17 spaces @ 5'-0" = 85'-0"


RE: Exist T joists @ roof

The moments as shown indicate that there is continuity at the internal support.  How is that achieved?  Is there topping with negative reinforcement?

RE: Exist T joists @ roof

hokie, this is a very late reply but actually the joists from each side bear on the beam in the middle and the drawings show a 1" filler between the two. so they are not connected and not designed for any negative moment.


RE: Exist T joists @ roof

In that case, unless my old eyes are deceiving me, the short span says it has a Mdl = -2.3 kft.  How can that be?

RE: Exist T joists @ roof

@hokie66 -- based on the short span Mll of 9 k-ft, I think the Mdl is actually 23 k-ft.  It certainly appears that there is a decimal point between the 2 and the 3, but I think it is likely due to the quality of the scan.

Adam Vakiener, P.E.

RE: Exist T joists @ roof

Hokie: I checked the drawings again and the section over the beam clearly shows a 1" gap. I was actually going to post the section here but the boss gave us half a day off so I left work as soon as I could :)

The - in 23 is too high to be a negative, and the decimal point is similarly because of the drawings being in that condition, as AVak pointed out.

But thank you so much for looking so carefully into it, it made me check the drawings again and be very very sure. Thanks again.


RE: Exist T joists @ roof

No problem.  And thanks for clearing that up for me.

RE: Exist T joists @ roof

By the way, most roof decks, even from that day were designed for 30 psf superimposed live load, you could check the general notes to be sure. So, if your 4000# RTU foot footprint area brings the LL  within the ball bark +-5% to produce a 30 psf LL, this would be a useful design check. Be sure to check wind and seismic on the unit where applicable.

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