OMF connection design 1

[etiennePr]

We have a huge debate at our design office in regards with a clause in AiSC 342-05. Clause 11.2a states that an Ordinary Moment Frame FR moment connection should be designed to the lessor of 1.1*Ry*Mp or the maximum moment that the system can develop. We have 30m (90ft) single span industrial, one storey building. The building requires a Seismic Design Category D adherence. The dominant loading (according to ASCE 7-05) is not seismic, but wind and live loads. Questions:
1. Can the FR moment connection (bolted) be designed to the maximum loads, as determined by structural analyses, using ASCE 7 loads? (using "the maximum moment that the system can develop" clause)
2. or must the moment connection be designed to 1.1*Ry*Mp, irrespective of the ASCE 7 loads?
3. If there is beam splices (using bolts) in the rafter (beam) of the OMF, to what forces/moments shall the splices be designed to?

 

[JAE]

The maximum moment that the system can develop would be the moment that occurs in your connection when ANOTHER element within the structure fails but which is code compliant.

In other words, it is NOT based upon ASCE 7 load combinations necessarily. Rather, you look to find the weakest link in your chain (your structural system), based on your final design of those links. All the links meet code and may actually exceed code limits.

You are then required to design the moment connection for the loading combination that creates the failure of the code-compliant weakest link. That loading combination may be a bit higher than the ASCE 7 loads simply due to detailing requirements, fastener spacing, etc.

Many times the weakest link might be overturning of the structure, diaphragm shear failure, etc.

An example might be that the floor diaphragm that drags in the shear to your moment frame can resist a lateral force of 30 kips. The ASCE 7 load combination may actually require only 26 kips but you designed the diaphragm to the higher load (30k) simply due to the next most uniform fastener spacing. Thus, the 30 kips causes a moment in your moment frame and this is the maximum moment that the system can develop.

Bottom line - the moment frame connection cannot be the weakest link.

 

[wannabeSE]

The AISC 341-05 commentary for OMF also mentions that the limiting force can be determined by calculating the earthquake force with an R of 1.

 

[JoshPlumSE]

WannabeEIT has a good point, but doesn't really elaborate much.

When you are designing that moment connection for seismic, you are using REDUCED seismic loads. Therefore, you cannot directly compare your ASCE load combinations for wind and seismic to determine which one controls for the design of the connection.

In order for your reduced seismic forces to be valid, the connection needs to be stronger so that your overall system can experience the ductility which allowed you to reduce your design forces.

Does that make sense? Even though the design wind shears are higher than the seismic shears, the connection design must still be done per the seismic requirements.