Moment Frame to Slab Connection 1

[rowe]

For a 2 column 1 beam moment frame designed as pinned at the slab-on-grade (w/ monolithic edge beam) foundation, I assume there will be some moment transferred to the slab.

Is there a prescibed method to determine the amount of moment to resist, or just design for the cracking moment?

Thanks for any help.

 

[Lutfi]

I wonder why you ask this question? It sounds like someone is trying to massage some numbers to reduce stress in a column or justify an overstressed column! Just speculating. If you design a bent as pinned, construct it as such. For example, if you provide a base plate with four anchor bolts, you will get some moment resistance. Although the analysis was done on the basis that there is no moment, the presence of the four anchor bolts will have some moment capacity.

You can do 2-D modeling on STAAD and play the fixity of the base and see how much moment you can live with for a given column size and base plate detail.

The pre-engineered metal building designers and suppliers use these types of frames to resist lateral loads in lieu of X-bracing. They normally do not attach the bent/frame to the foundations. Instead, they terminate the bent column few inches above the slab. Thereby, all of the column reactions are transmitted through the rigid frame column base.

I hope this helps

 

[rowe]

Lutfi,

I appreciate your response.

It is a pre-engineered metal bldg. The plans indicate the structural steel columns have base plates welded to the columns and are bolted to the foundation slab (slab designed by others - me) using 4 bolts.

I could analyze the frame, but that would involve considerable time to model the structure (members are not prismatic) and reproduce the wind loads, etc.

I am considering analyzing the base area of the column to determine the moment capacity, and then designing that area of the slab to resist that moment.

Any other suggestions or comments are appreciated. Thanks.

 

[Lutfi]

If it is PEMB, the building designer have to supply you with building plans, anchor bolt layout and frame and end wall column reactions. Look for the reactions and see if they indicate bending moment. If they are fixed, the PEMB designer will supply the base moment. If it is not present, then they are designed as pinned. All you have to design for is for gravity load (downward and the horizontal "kick"). Most of the POEMB I deal with require uplift resistance due to their light weight. Therefore, make sure that you design the footings to resist uplift load. Use no less than 1.5 safety facto against sliding and uplift and sliding.

Hairpins are usually used to resist the horizontal component of the frames.

It has been my experience that they will design the frames as pinned conditions. The building designer will also supply you with the anchor bolt design. All you have to do is design the foundations to resist the reactions. Watch out for the following:

1. The reactions and their combinations
2. X-bracing reactions
3. Bent/moment frame, if used, to resist lateral wind loads
4. Any other conditions that are peculiar to the PEMB building and the site.

Good luck