Braced and moment frame together, what's the clarification then?? 3

[NewbieInSE]

Dear Engineers,
Hope you are all fine.

We have a steel shed.

The moment frames are along x direction. These are actually tapered column and tapered rafters. Column-rafter joint is rigid.

Along Y direction, the rafters are tied using tie beams, pinned I beams. Columns are also tied just below the eve level. At roof there are roof bracings, spanning from tie beam to tie beam.

But, along Y direction, we used X bracings to control deflection. For wind, deflection limit comes like H/500.

The steel columns are pinned at pedestal top. Pedestal base are also pinned. We used grade beams just 1 ft.below the plinth level.

I think, this is a very common, typical shed structure.

My question is, since we used moment frames along X direction, and concentric braces along y direction, therefore how are we going to clarify this? Because based on whether moment frame or Concentrically braced frame, we need to find seismic parameters.

Is their any combined framing comprising both? I'm not aware if there is any in the code.

 

[HTURKAK]

Your case is the use of different seismic force–resisting systems in different (orthogonal) directions of a building.Refer to 12.2.2
Below copy and paste of the same..

(12.2.2 Combinations of Framing Systems in Different Directions. Different seismic force-resisting systems are
permitted to be used to resist seismic forces along each of the two orthogonal axes of the structure. Where different systems are
used, the respective R, Cd, and Ω0 coefficients shall apply to each system, including the structural system limitations contained in
Table 12.2-1 )..

But , make sure that there will not exist interaction between these systems . The system limitations in Table 12.2-1 should apply independently to the two orthogonal directions.

 

[human909]

Codes vary by region so pick and choose if this applies to you. If you seek to obtain benefit from the ductility from a moment frame then there can be more rigorous requirements on the connections that a typical shed portal frame doesn't meet. So it might worth considering that. When I've designed seismic for structures that are moment in one direction and portal in the other I take the benefits of the moment frame being more flexible and having a longer period. But I don't take advantage of the ductility of moment frames as in my local code the additional requirements there are too onerous to implement in many cases.

That said I'd normally expect most of this discussion to be fairly academic because for a shed surely wind would dominate even in high seismic zones. (Except I suppose for extremely low, broad sheds)

 

[NewbieInSE]

HTURKAK, what did you mean by "make sure that there will not exist interaction between these systems". Did you mean I shouldn't use both braces and rigid frames along single direction?

 

[phamENG]

Correct. If you have braced frames and moment frames in the same direction, the determination of loads can get a little more complex if you're in a moderate to high seismic area. But if you have moment frames in one direction and braced frames in the other, you just have to run through your load analysis twice to get the seismic force(s) in each direction.

 

[NewbieInSE]

When I've designed seismic for structures that are moment in one direction and portal in the other I take the benefits of the moment frame being more flexible and having a longer period. But I don't take advantage of the ductility of moment frames as in my local code the additional requirements there are too onerous to implement in many cases.

So you mean that, you don't use response reduction factor which accounts for ductility, to design your moment frames. Right? In that case, doesn't your connections automatically become large and require lots of bolts etc.

And if you allow for high period, then how do you limit your deflections?

 

[NewbieInSE]

Correct. If you have braced frames and moment frames in the same direction, the determination of loads can get a little more complex if you're in a moderate to high seismic area. But if you have moment frames in one direction and braced frames in the other, you just have to run through your load analysis twice to get the seismic force(s) in each direction.

What type of complexity may arise? I think, response reduction factor, overstrength factor , deflection amplification factor may become unavailable in the Code.

Any other problem are you talking about?