Combining Local and Global Effects For Member Utilization 1

[StrucPEng]

Hi All,

I have a bit of a conceptual question. What is the thought on combining local utilization of a member with global utilization of a member.

As an example, say we have an HSS tube that is in bending and is utilized to 55% of its capacity. And attached to that HSS we have a connection that is causing local bending in the wall with a utilization of 80%. Is there a need to consider the interactions of these too utilizations or are they separate failure modes? A few other considerations:

- If the connection that is locally 80% is causing the moment that utilizes the global member to 55%, is that different than if the connection is a shear connection and the bending is primarily from an external load?

- Is this ultimately a local stress issue? Combine the global stress with local stress?

- this does not seem to be considered much in the parts of the country where connections are delegated out to a separate engineer

I am only using an HSS as an example and I know there are some well defined equations that account for global bending with local wall effects but I am thinking more general for all types of shapes.

Thanks in advance for the input!

 

[MotorCity]

Your code should tell you how to consider combined forces. Not all limit states are lumped into a single utilization factor. Obviously you select the highest utilization factor. For example, AISC considers bending + axial as a limit state. Generally speaking, global limit states (axial, bending, compression, tension, shear) are not combined with local limit states (flange local buckling, web local buckling, etc). If you are using software, take a look at what it is telling you the controlling combination or limit state is.

 

[cliff234]

Chapter K in AISC 360-16 deals with this. The demand-to-capacity ratio, U, is considered when designing the HSS connections. If U= 1.0, then there will be no reserve capacity to deal with the secondary stresses at the connections. See also pages 13-17 and 13-18 (in the 15th edition AISC Manual) for additional discussion. There is a recommendation to keep U well below 1.0 when designing the members to avoid overstresses (due to secondary stresses) at the connections.