Progressive Collapse Tie Force Method Restrictions

[Coty]

Section 3 of UFC 4-023-03 design of buildings for progressive collapse implies that the tie force method is not allowed for steel frames buildings with fewer than 4 bays of framing in each direction (i.e. Minimum 5 building grids with columns in each direction). Does anyone have another interpretation? I'm looking at a building about 50 feet wide by 300 feet and would like to use the tie force method, but I only have 2 bays in one direction. That only leaves the alternate path method.

My confusion comes from the design examples where multiple buildings with 3 bays in one direction are designed with the tie force method. This contradicts there 4 bay requirement. Thanks!

 

[Coty]

I thought this would be a failed effort. I saw older posts about federal requirements fall on deaf ears. Disregarding the UFC requirements, does anyone know why a two bay structure would not qualify for the tie-force method?

The only reason I can think is maybe multiple bays are required to develop the ties, but if that's the case you would think it would've been explained.

 

[MrHershey]

We actually asked PDC about that specific example recently. They confirmed the example does not meet the requirements for tie force design and will be looking at revising it for the next issue.

The reasoning we received for why they have the minimum bay requirement basically boils down to tie force method not being applicable when your remaining vertical supports are located either at exterior or penultimate (first interior) locations. With less than four bays in each direction, every column location will be either exterior or first interior.

Idea (I think) is the catenary action from the tie forces will result in potentially large lateral loads on the remaining columns and with less than four bays in one direction your building may not have enough counteracting moment from the dead weight of the remaining building to resist the overturning moment from the catenary action and you'd thus still have progressive collapse issues. That's not to say that it can't work, especially if you've got a robust and well distributed lateral system. You just need to actually look at it rather than relying on the tie force approximation.