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Strut and Tie for Crane Outrigger and Plate

Strut and Tie for Crane Outrigger and Plate

Strut and Tie for Crane Outrigger and Plate

Good morning,
I am doing a check for some pretty heavy crane loads on an existing structure. I am finding that the columns are able to take the additional crane loads, but that the steel beam connections do not have enough capacity. The outriggers are situated close to the columns, but with offsets of around 1 ft from the column centroid.
The contractor has offered to bring in a very thick plate to sit beneath the outrigger pad, and I also have two layers of slab (10" slab and 8" slab sitting directly on top of each other, but not connected by dowels along the interface).
With all of this depth, I am wondering if a strut and tie model would be appropriate? I am not sure that I have my model correct, with the three layers and different materials, but have attached my sketch here. If anyone has done a similar check or has any input / thoughts, I would love to hear from you.
Thank you so much in advance,

RE: Strut and Tie for Crane Outrigger and Plate

I’m not really sure what Strut-n-Tie would do for you, but there are some other issues you better take a look at. Is the 10” slab a permanent part of the structure, along with the steel framing? Is it a solid slab or a hollow core plank? If the latter, you better check the solid ribs/webs for shear and crushing. Is this slab permanently and solidly bearing on the top of the stl. col. so that it really transmits the load to the col., or should any gaps be shimmed and wedged to assure this? Is the 8” slab solid or hollow, and how long? Your sketch suggest a 5’ long (maybe 5’ sq.) stl. pl. 8” thick. At some point what does this size and thk. do for you? You don’t want to try to distribute to much of the outrigger load out onto the stl. bms. You might offset the 8” slab and 8” stl. pl. to the left, so at least their weight would tend to drag the resultant load toward the col. centerline. You might be better off looking at 8x8 wooden cribbing above the 10” slab, since the wooden cribbing crushes to account for slab irregularities. Add a 3’ sq. by 2” or 3” thk. stl. pl. atop the cribbing. You might also look at shoring immediately around the stl. col. in the sketch and under the conc. slab since this would take some of the total load down to the next level and help with any issues of the col./slab bearing and the connections in the sketch.

RE: Strut and Tie for Crane Outrigger and Plate

100kip = ~450kN, and with a 1.0ft offset the moment due to eccentricity is 136kN-m .. That is a very large load, you have to make sure that the column, beam, and beam-column joint can handle it. Also check the layer of the slab for punching and bearing.

There is no strut and tie happening here, there is a strut but I don't see any tie.

The next question is how about uplift? does the reaction from the Outrigger reverse?

RE: Strut and Tie for Crane Outrigger and Plate

Yeah, I'm not loving this either. Might be an awful lot of load to accidentally push through a one level shear connection. Maybe if you could weld seat brackets in under the beams and stiffeners above or something. What's the nature of that upper slab? Topping or structural?

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