Steel BM #Fail(ure)

Posted in a design education by DS26 on 08.28.2010

In having began my Master in Engineering at IIT (Illinois Institute of Technology), I find myself wondering :

Why are there some crucial things we don’t learn in Architectural Structures ?

Truth is that the recognition of failing conditions in a structural system has got to be at the top of our abilities, especially when visiting an existing building for rehab.

By that I do not mean that all Architects must be experts… after all that is why we sub-contract Structural Engineers. But, it is important that we are aware.

Can you honestly tell me you recognize a condition? every time?

Since I believe the answer to both questions is likely no, for most people… for my first “Pro Structures (for Architects)” post I will show you what the three major failure conditions are in Structural Steel Beams.

1. Lateral Torsional Buckling

Or LTB, this is the most detrimental, and it will likely quickly lead to complete collapse. It is the one that should be prevented at all cost.

With that being the case, the picture below is only a demonstration of the member’s behavior.

Ultimately, the beam would tilt into an angle, typically from the bottom flange.

Here’s a diagrammatic depiction of the condition :

2. Local Flange Buckling

Local Flange Buckling (LFB). It is a concentrated reaction in an area of either the top or bottom flanges, and it results in a type of lift (vertical buckling).

Here is a picture depicting the condition in an existing structure (at the bottom flange) :

3. Local Web Buckling

Local Web Buckling (LWB) is the least likely to occur. Similarly to LFB, it is a concentrated reaction in an area of the web that results in a type of side lift (a type of horizontal bubbling).

Here is a picture that depicts LWB in a structure :

As the picture above demonstrates, this last seems locally severe. In this case, one actually can clearly see both LFB & LWB.

Note that LFB & LWB both occur when the ratio of size between the web and flanges is too large. To clarify, the web is to thin, or the flanges are.


Next time you (Architect) get a call for an ‘Existing Conditions’ walk through in a steel building, prior to starting a job, you are more equipped (with big words) to include the ‘Failing Conditions’ (if they exist) in your Report.

~ Don’t forget a Disclosure Statement that you have not assessed the structure, rather you have observed and do recommend an inspection.   :)


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