Progressive Collapse - Alternate Path Method in UFC

[pallencn]

Anyone familiar with UFC 4-023-03 (2009), particularly the Alternate Path Method as it applies to Structural Steel framing?

A couple questions:

1. The load combination is listed as (1.2 OR 0.9) * Dead Load. Typically a 0.9 factor is used when you have load acting opposite of dead (wind uplift), but with progressive collapse, its all gravity loads. Still, using a 1.2 factor on such an extreme event seems like too much of a penalty.

2. Do you include the floor diaphragm in the building model? The diaphragm tends to take some in-plane forces while still allowing the frame members to be in flexure. It seems unreasonable to neglect the diaphragm, as the slab will be there in the event of a collapse.

3. Any idea what "Gn" is in Figure 3-13 ?

Thanks in advance,
Pete

 

[ishvaaag]

Just checking the code...

Well, respect 1.2 D they just want to be conservative. These installations are manytimes overseas and the verification of the dead weights are not always stricts. I can say you that in Spain there is a customary unconservative weight evaluation of terrazzo floors, well above the 20% excess (but only for that part of the dead weight).

Respect the second I would include anything that is able to restrain the progressive collapse that is not expressly forbidden by the code. Make a seart for the word "diaphragm" in the pdf, will make you clear that the intent of the check wants the diaphragm action retained if available.

3-2.12.4.1 Loads.
GN = Increased gravity loads for Nonlinear Static Analysis

Use always the find tool in pdfs for these questions, use to help.

 

[pallencn]

I understand what the UFC says. What I'm more interested in is what it DOESN'T say. Anyone have any practical real-world experience with this?

 

[cntw1953]

The attached paper provides some basics, quite interesting.

 

[cntw1953]

Try this one.

 

[cntw1953]

Sorry, don't know why it wouldn't appear here.
You may find it by word search on "Alternate Path Method".
Title of the paper "UFC 4-023-03: Design of Buildings to Resist Progressive Collapse", by K. A. Marchand, D. J. Stevens, B. Growder & T. Campbell. It was published on April 2006, Structure Magazine.

 

[UcfSE]

1. It's not inconceivable to have a floor layout in which the dead load would cause uplift on a column. In that case, more dead load helps resist the downward movement at the failed column, hence the 0.9 factor.

2. The examples in the UFC have diaphragms modeled as rigid.