Extraordinary Event Load Combination 1

[CANeng11]

Has anyone here worked with the load combination for extraordinary events? From ASCE 7 - 2.5.2.1 Capacity - For checking the capacity of a structure or structural element to withstand the effect of an extraordinary event, the following gravity load combination shall be considered0.9 or 1.2)D + Ak + 0.5L + 0.2S (2.5-1)in which Ak = the load or load effect resulting from extraordinary event A.

I'm trying to figure out how the value of Ak is determined.

 

[r13]

Just guessing. For a house facing incoming roadway, maybe we would like to evaluate the impact of a car crashing into it.

 

[CANeng11]

Ya I would think it would be along those lines. I'm thinking in terms of a fire inside a structure. I can't really think of an additional loads created by a fire. Maybe potentially debris falling?

 

[phamENG]

If you're looking for a design that considers fire, take a look at AISC Appendix 4. (It's on page 586278 of the linked PDF.)

It provides requirements for load combination (essentially the same as ASCE's Extraordinary Event LC), as well as thermal properties of steel at elevated temperatures to determine thermal loads and information on the changes that occur in mechanical properties as steel heats up.

 

[r13]

Yeah, that makes good sense. Consider the catastrophic results of several cases of club fire.... Also, collapse of porches, emergency stairways due to over crowd gathering, celebrations.

 

[MegaStructures]

I've used it for blast loads and sudden removal of building columns. I believe the development of Ak is completely up to you, ASCE is just saying consider whatever extraordinary load you have and to the best of my reasoning they aren't putting an extra uncertainty factor on it because the extraordinary loads could vary in nature so widely.

 

[r13]

As the trend goes, ASCE will soon focus on, and bring in, a few special events for designers to consider.