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Hoist Backpropping. Overdesign??

Hoist Backpropping. Overdesign??

Hoist Backpropping. Overdesign??

Hi there, I am a graduate engineer so any help on this would be great as the senior engineers at my work are also puzzled with this one!

We are doing a backpropping design for this hoist and coming up with some ridiculous cumulative loads that need to get backpropped for a 3kpa total slab kPa (2LL & 1DL).

There is a 38kN compression load per hoist leg per level for the 4no. mandatory slab levels.
Working with a shoe size of 800mm and overall hoist width of 5m I have taken a (non-conservative) tributary area of 2.25m2 (1.5m X 1.5m).

2.25m2 x 3kPa = 6.75kN usable slab capacity

My resulting calculations per leg are as follows:

level 4 38 - 6.75 = 31.25kN

level 3 31.25+38-6.75 = 62.5kN

level 2 62.5+38-6.75 = 93.75kN

level 1 93.75+38-6.75 = 125kN final load to be backpropped

Now my peers and I have done plenty of backpropping designs before and never come across loads like the above. Is there anything I am doing wrong or missing?

Any help would be much appreciated, thank you in advance hourglass

RE: Hoist Backpropping. Overdesign??

I've never seen loads of that magnitude assumed at ALL floors before. Perhaps there is an opportunity to seek clarification on if the load is applied at all floors simultaneously or if they are trying to convey that it could occur at any given level.

I think you could really justify engaging a larger area of the slab vs. just the nominal footprint of the attaching shoe if you can make and enforce restrictions about usage of the levels adjacent to the lift. Obviously you will need to consider the weight of the materials being staged and loaded through the slab area adjacent to the loading, but if the slab locally can support the concentrated load, then much more of the area can be engaged to support the loading, if other operations that apply loading can be suspended at the time of usage - meaning, you can't use the lift while placing the slab at the forming level above when reshoring engages the area adjacent to the lift.

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