Equipment Frame Analysis

[LSPSCAT]

I have an equipment frame that is approximately a 12' x 10' bay. Historically it was always analyzed as a simply supported beam with two columns. The load is applied at the center span of the bay. Historically the columns were braced to prevent any lateral movement and movement out of plane. The connections are square bolt groups.

Due to seismic considerations there are now lateral forces in the plane of the frame and out of plane that are introduced into the system. If no bracing is provided to a secondary structure would it be appropriate to analyze this as a rigid frame and size the connections to develop the moments. The first analysis would be based on the actual live load on the center span beam. Secondly, we would check lateral loads applied to this system.

Welding is not in the picture because pieces of the equipment are cast iron. Out of plane conservatively I have considered the mass of the equipment at the story height and the resulting out of plane bending moment in the system.

 

[sdz]

Can you post diagrams?

 

[LSPSCAT]

See attached diagrams. Historically (unwritten/documented company policy??) it seems like the first load case analysis was always carried out based on simple framing. Beam simply supported and a basic P/A column check. Typically some form of bracing was supplied to tie the frame back to a secondary structure. I am looking to get away from that and mainly just looking to consistently analyze the frame a certain way. Two load cases are considered separate for design. A vertical load applied directly center span. Second load case is applied as shown, conservatively I would probably apply it at the top beam height. I am leaning to a rigid frame assumption with fixed supports for the lateral load case. Should that same assumption hold true for the first load case?

 

[kslee1000]

Also try to pin point your concerns/questions would help to gather meaningful responses.

 

[LSPSCAT]

Does the rigid frame analysis with fixed supports seem to make sense for the initial design? This would generate moments in the columns and connections to design around. My initial iterations seem like this usually governs the design. I just need to make it stiff enough to limit lateral and out of plane deflections. Bracing can be fabricated to stiffent the frame in the lateral direction. I am looking for some type of general reasoning that the simple framing assumption is not applicable.

If bracing to a secondary structure is provided at the top of the frame, simple frame analysis makes sense and lateral forces are resisted by the bracing.

If bracing is not provided, a rigid frame analysis is performed....If the frame alone is not sufficient, cross bracing will be provided. Just trying to think through it...

 

[kslee1000]

The rigid frame is more logically sound for load case 2. How and why the lateral load is applied at the location shown?

 

[zcp]

Whichever design is most economical (while still carrying the loads) for your situation / components / fabrication techniques would be the way to go. Either the connection to the ground, the upper frame connection, the whole structure (bracing), or some combination must take the loads.

Two routes you can take....1) be consistent throughout or 2) worst case for each member. The first sometimes makes the calcs more detailed for some of the components. The second is often overly conservative.

Hope that helps.

ZCP

http://www.phoenix-engineer.com