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Re: PS CONCRETE: Torsion in Double-Tees

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            Bill,

                        I intentionally did not discuss if the very thin open section of a double tee could carry much torsion. And no, it would be a very rare double tee stem that had a closed stirrup. Precasters are always trying to reduce the amount of shear steel provided from what ACI Code provisions require. Historically, many have provided 5’ in the ends of stems, even if the _minimum_ required shear steel extended to 20’ or 25’ from the end of a 60’ double tee. Most of the time, the concrete will carry the shear, but ACI requires steel when the shear becomes half the concrete shear capacity. And, wonder of wonders, under load tests with a little more than ultimate loads, tees without minimum shear steel fail in shear. So they pass the load test, but ACI is correct on the failure mode afterward. And there are lots of warehouse roofs built before the minimum steel requirement that are doing just fine. I know because we get the occasional call from an owner asking if he can add load to his roof. But we do not get calls after blizzards saying those old roofs collapsed. And the Code snow load provisions have increased in the same period.

                        Back to torsion: IIRC, the PCI Journal had an article about three years ago saying that a 60’ long tee could tolerate a 1” warp in the bearings. So if three of the tee’s stems were supported by coplanar points, and the fourth was 1” lower, the tee would set on those four points without cracking. If there was about a 1½” warp, some tees would crack and most tees would crack with 2” warp. Since most tees are twice as wide as their stem spacing, the total warp on the deck is twice the warp of the bearings. The authors had both experience and finite difference analysis to support this conclusion. A twist of 1”/60’ is not much. Not anything like as much as 25% of the load on one stem is shared by the other and maybe the adjacent tees, too.

                        To answer your direct question, model the torsion if you like. If you do, you will be doing more than most precast engineers do most of the time. Some do that kind of thing for lawyers or if they are writing PCI Journal articles, but not for everyday work. Most heavy rooftop equipment sits on curbs and I generally assume they help spread the load and do not go further. I hope this does not sound too cavalier. You can call me or email me directly if you like (540-665-3204).

                        “Don't have my copy of AASHTO here in "Clinton Country."” I thought you were in Arkansas, not New York.

            Jim Getaz

            Winchester, Virginia