Robustness & Progressive Collapse - Key Element Column Design

[Drapes]

When designing key elements for an applied load of 34kN/m2 (such as corner columns on a high-rise) as a way to satisfy robustness and avoid disproportionate collapse, what effective column length do you consider? Is it acceptable to check the column capacity based on a single story height, or do you ignore the lateral restraint offered by the slab? If the slab is ignored, over how many levels do you disregard the restraining effect of the slab?

With reference to the IStructE document - Practical Guide to Structural Robustness & Disproportionate Collapse in Buildings", we have the following extract:

"The general design approach for key elements is to consider uniform pressure acting over their surface (in each orthogonal direction, one direction at a time) plus the surface of any attached items such as cladding. Likewise, any member restraining a key element should be designed for the specified design pressure or the restraining element benefits ignored. The restraining element can be checked separately to the key element however, if both would be affected by the same event, then it is more logical to consider both elements loaded simultaneously. The pressure is applied in conjunction with the accidental load case"

"It would be unduly onerous to apply the accidental design pressure over large areas, e.g. to slabs attached to (and stabilising) long span transfer beams...so applying the pressure to an area limited to 6m × 6m would seem reasonable, although any such relaxation should be considered in light of the specific circumstances."

 

[HTURKAK]

Single storey ht assumption is acceptable provided that, the column restraining elements ( in this case should be beams ,connected to column classified as key element ) ..
Excerp from the same subject reference ; (BS 59508.3 Clause 2.4.5.4 recommends that any other structural component that provides "lateral restraint vital to the stability" of a key element should also be designed as a key element)..

IMO ,The following para. regarding the design of key elements is self explanatory..

 

[Drapes]

Thanks HTURKAK.

The column is actually being restrained by slab only (no perimeter beams), however seems a bit excessive for the slab to be designed for a 34kN/m2 load applied over a 6m x 6m tributary area. Would be struggling to make this work without incurring significant cost on every floor.

Any other suggestions to work around this? If the restraining effect of the slab needs to be disregarded for this exercise, I presume the column would need to be designed with a larger effective length?

Would be keen to understand how others address this sort of situation in practice without being too unreasonable.

 

[HTURKAK]

I assume that , you know the origin of 34kN/m2 .. If the façade and partition walls weaker than slabs, and the structure is monolithic ,still IMO, you may assume single storey ht .

The following picture is aftermath of natural gas explosion.. I found at web.. Notice that, the columns and the main structure still intact..

 

[Drapes]

Yes understand the origin of this load - Ronan Point I believe had a significant role to play.

If the façade and partition walls weaker than slabs, and the structure is monolithic...

Yes, the structure is monolithic and will be adequately tied at the column/slab interface. But how does the facade/partition walls being weaker than the slab have anything to do with justifying single storey height as your column effective length?

 

[HTURKAK]

In this case, the extreme event ( was also the case Ronan bldg) , gas explosion. In your case, if the FRS is a frame system ( rather than load bearing walls, pc load bearing walls etc.. ) the collapse of weaker exterior walls and partition walls will be fuse for the main FRS during gas explosion. IMO, the ductile design and detailing of the CIP slabs will justify single storey height as your column effective length.

The other accidental and extreme loadings which are applicable shall also be studied.

Pls look also

https://en.wikipedia.org/wiki/2018_Magnitogorsk_building_collapse

 

[MIStructE_IRE]

A rarely take this approach (and rarely have to) as it results in OTT member sizes. I assume you can’t satisfy robustness requirements via provision of adequate vert/horiz ties for whatever reason?

The key element design is just one approach.