Progressive Collapse Design

[amaidj]

Hello leds,

I am designing Precast Stairs connected at landing with Bolted Angle, now i need to do Progressive Collapse Design, does it means i need to apply 34kN/m2 pressure as describe in BS8110 : Part 2 :1985 2.6 Robustness?? and then strengthen my connection for the reaction i get from this pressure? and my stairs suppose to be design for this much Pressure?

Best Regards.

 

[mronlinetutor]

this is the way I interpreted it. progressive collapse means there should be an alternate load path should one structural member fails. usually in cast in place concrete an edge beams is required below the horizontal slab at the top of the stair and below the horizontal slab at the bottom of the stair. so u double up the connections. if the bolted connection fails, then the stair will rest on the edge beam thus provide double safety. another flight of stair on other locations should be provide in case this particular stair fails. so u need two stair cases in one location. I am not sure a bolted connection will work.

 

[BAretired]

I don't know anything about BS8110, but 34 kPa (or 710 psf) seems like a ridiculously high pressure to apply to the stair. I think you must be misinterpreting the code.

BA

 

[MotorCity]

Progressive collapse is a global analysis that applies to main vertical and lateral load resisting members (i.e. girders and columns). It is meant to minimize the risk of the overall building collapse, it is not intended to prevent the collapse of small, isolated elements such as stairs.

 

[rapt]

MotorCity,

If it is an emergency escape stair then it is probably required to be checked for robustness.

 

[amaidj]

Rapt that Robustness requirement is this, 'BS 8110 Part-2 1985 it says in section2.6 Robustness, 'Appropriate Design Loads should be choosen having regard to the importance of the key element and the likely consequences of its failure. but in all cases an element and its connection should be capable of withstanding a design ultimate load of 34kN/m2 to which no partial safety factor should be applied from any direction. A horizontal member, or part of a horizontal member that provides lateral support vital to stability of the vertical key element, should also be considered a key element. For the purpose of 2.6.2. the area to which these loads are applied will be the projected area of the memeber'
Now tell me by reading above Robustness requirement from BS what does it means?
how and where to apply this 34 load and to check what?

Regards

 

[hokie66]

As BA said, 34 kPa is a lot, to the point of being ridiculous. Are you sure you are reading the clause correctly?

 

[MotorCity]

I am not familiar w/ the BS code, but if you follow the GSA procedure, stairs would be classified as a secondary element and therefore not included in the progressive collapse analysis. The idea of progressive collapse is to prevent disproportioned collapse of a structure.....not damage, not collapse of a vital staircase, but collapse of the structure. In the event that a building collapses, whether or not the staircases are still functional won't matter.

 

[Ingenuity]

From this paper Link entitled: The UK and European Regulations for Accidental Actions, by Moore, the 34 kPa (or 5 psi) was adopted after the 1968 Ronan Point collapse:

Finally, if is not possible to bridge over the missing member (which may be the case for a small minority of buildings) such a member should be designed as a protected (or key) element capable of sustaining additional loads related to a pressure of 34kN/m2. The value of 34kN/m2 (5lb/in2) was chosen with reference to a rounded estimated failure load of the loadbearing flank wall at Ronan Point. This estimation was based on observational evidence. In practice the 34kN/m2 is used to determine a notional load that is applied sequentially to key elements and is not a specific overpressure that would result from a gas explosion.

I don't have, nor are familiar with, BS standards, but page 4 of the above paper has three methods of ensuring that the building has a minimum level of strength for accidental loadings, including: tying and bridging, NOT just designing for a minimum of 34 kPa.

 

[rapt]

If you read the whole clause, it says this applies to key elements, and sends you back to part 1 clause 2.2.2.2(c) for the definition of Key Element. Which it defines as "the failure of which would cause the collapse of more than a limited portion close to the element in question".

Would the failure of your stair lead to the collapse of the rest of the building? I would assume the answer is NO. So it is not Key Element. So the rule does not apply to it.

If it was a Transfer beam supporting the whole building, you answer would probably be YES. Then you would have to apply it!

 

[Retrograde]

In precast stair design that I have done previously we designed each landing-to-wall connection to also take the dead load of the flight above. That way if one flight failed it would not progressively fail the entire stair case.

 

[amaidj]

@ rapt: i understand but what would you say if the engineer says that incase this staircase is the emergency exit or only exit way then incase of such accident it should sustain the load and the connection should take the load so it remains there and not damaged

 

[hokie66]

amaidj,

That is an entirely different concept. Progressive collapse has been described above by MotorCity and others.

What you are talking about is robustness, but of an isolated flexural element, which if removed would have much less effect on the entire structure than, for instance, a column.

 

[BAretired]

The building which initially made engineers aware of progressive collapse was the Ronan Point apartment collapse in Britain. Since then, there have been other examples such as the World Trade Center and the Murrah building in the USA. The history behind the 34 kPa (5 psi) pressure requirement in the British code is described in the following article:

https://en.wikipedia.org/wiki/Ronan_Point

Structural engineers engaged in the design of buildings should all be aware of the principle involved with the Ronan Point failure.

BA

 

[rapt]

I think I pointed this out in an earlier post above.

An emergency stair would be covered under the Robustness rules which are covered in part 1 clause 3.1.4. How you provide Robustness for a stair by providing alternate load paths I am not sure. It would depend on the layout. Maybe you should read

Practical Guide to Structural Robustness and Disproportionate Collapse in Buildings - Institution of Structural Engineers - 2010

Manual for the Systemic Risk Assessment of High-Risk Structures Against Disproportionate Collapse - Institution of Structural Engineers - 2013

And No, I do not have copies.

 

[Istructeuk]

You need to design disproportionate collapse for a staircase but I don’t think to design as a “Key Element” with 34kPa.

“Key Element” is to be considered in building Class 2B and above in case where the requirements of the tying method or the notional removal method have not been satisfied.

 

[amaidj]

Thanks a million guyz and yes now i have better understanding of the difference in Robustness and Collapse Design, thanks amillion again to you all guys, as i had no clue in the beginning about Progressive Collapse thats why got confused but i my self have read all the details related and with u ppls help i have no better understanding of it. thanks again guys.

Regards