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OMF in seismic category D

OMF in seismic category D

OMF in seismic category D

(OP)
I have a few questions for a Steel frame canopy which is over equipment, etc. It is less than 35 feet high (14' high). I can use braced frames on the side and back but I need it open on the front. Therefore, I was going to use a OMF with pinned bases along the open side. It will be attached to an existing concrete wall at the base so I want the loads light for the foundation. First, am I reading this correctly and this is allowed in seismic category D. I would use an R=3.25 (braced frame controls) And if it is allowed, can I consider a w beam with a cap plate over a HSS connection as a OMF connection allowed in this situation? It also has high a wind load and a high snow load and it about 10' by 71'. I don't want to use cantilever columns because of attaching to the existing concrete.

RE: OMF in seismic category D

Since you are not getting much action on this, I’ll give it a shot.

Get a copy of the Seismc Design Manual (AISC 341). There are a few provisions for OMF with which you would need to comply. I don’t have mine in front of me, but I remember that there are column/beam connection requirements to design to at a minimum. You don’t need to use a pre-approved system as you would for an SMF.

RE: OMF in seismic category D

I've used OMF in SDC D multiple times. If you have HSS columns and WF beams I would recommend flange plates or diaphragm plates. If you can't spare the extra room for diaphragm plates on the outside, you can cut the column and build a "flange plate sandwich" that essentially gets welded to the top of the HSS column. The bread is the flange plates and the filling is another chunk of column. Depending on the demand, you can either fillet weld or PJP or CJP the column stub and flange plates to the column.

RE: OMF in seismic category D

Quote (Backtowork)

I can use braced frames on the side and back but I need it open on the front. Therefore, I was going to use a OMF with pinned bases along the open side.

I would use OMF at the front and back to avoid unnecessary torsional response.

Quote (Backtowork)

can I consider a w beam with a cap plate over a HSS connection as a OMF connection allowed in this situation?

I think it could work depending on the required strength. However, if you need pinned base plates, I would not use HSS columns. In my opinion, a nominally pinned base condition is difficult to achieve in practice with HSS columns and four anchor rods (minimum required by OSHA and some building codes). Having said that, I would use W columns instead. In addition, a W column allows you to minimize the distance between anchor rods, which is important if the columns are supported by a concrete wall of limited thickness.

In the following link yo will find an interesting steelwise article about the design of a beam over column moment connection for a OMF.

Link

If HSS columns can't be avoided, you can refer to Section 4.2 of AISC DG 24.

Hope this help!

RE: OMF in seismic category D

(OP)
Thank you. I will look at the suggested connections and see if one will work with the details and the loads given. I would rather use a W column however, if I use a W column it may have to be turned so the moment frame uses the weak axis so I was trying to avoid this. I thought I read you couldn't do that in high seismic zones but maybe it was the SMF, I will have to go back and check. Using HSS also simplifies the details for how they would like to do the siding for this.

That's a good point on the anchor rods and the W column. That may end up deciding what I have to go with due to the thickness of the existing wall.

Another question about moment connections. I haven't had much experience with the cost of the moment connections. Is there one that I should try first due to low cost and easier installation for both W beam to W column and W beam to HSS column?

Thanks for your help!

RE: OMF in seismic category D

FYI. (Excerpt from LA Dept of Building) Link

RE: OMF in seismic category D

Have you considered a 3-sided diaphragm with the front bay just being open and gravity columns in that direction? I'm assuming you have some sort of decent metal deck roof, and it might be workable depending on dimensions.

Why do you need to use the weak axis of the W-column for the OMF? Typical condition is to install strong axis in the OMF direction and weak axis in the OCBF direction for obvious reasons.

I've been told that bolted end plate is a standard low cost connection for W-beams to W-columns. This would only apply for attaching to the flange/strong axis of the column.

For HSS Column to W-beam, the steel manual has several options. If you have a continuous beam over the top, the simplest is running the W-beam over the top of the HSS column and using a bolted cap plate. Add stiffeners to the W-beam as required. If your beam terminates at the column, look at a welded tee connection top and bottom (see details in steel manual). Generally avoid diaphragm plates in the HSS where you can, they tend to be expensive.

RE: OMF in seismic category D

(OP)

Thanks for the comments and the link to the table! I'll use the OMF on the back as well.

[quote Structbells][Have you considered a 3-sided diaphragm with the front bay just being open and gravity columns in that direction? I'm assuming you have some sort of decent metal deck roof, and it might be workable depending on dimensions.
]

I did think about a 3-sided diaphragm however with it being seismic D and high wind and high snow I thought it might be better to use the moment frame. I could run the calc's and see what it actually is though.

[quote structbells][I've been told that bolted end plate is a standard low cost connection for W-beams to W-columns. This would only apply for attaching to the flange/strong axis of the column.]

For this particular job I need the either the top of column and beam flush or the beam to run over the column. Is there a way to use the bolted end plate where it is flush at the top? In the steel design guide I only see where is extends above the beam.

[quote structbells][Why do you need to use the weak axis of the W-column for the OMF? Typical condition is to install strong axis in the OMF direction and weak axis in the OCBF direction for obvious reasons.]

It is the weak axis due to some other details but I think I will see if I can come up with another way to detail it so I can turn them. That's why HSS works better for that but they are harder to detail the moment connections.

Any thoughts on this---I have an example where they took the total lateral load and spread it evenly between all columns (cantilever columns with steel roof deck). With a flexible diaphragm wouldn't it be better to distribute the load so the interior columns are designed to take more of the load based on area or is that an acceptable assumption to just divide it by the number of columns it evenly? I think you can't just assume it is rigid and will distribute the load evenly --that would have to be proved first.

RE: OMF in seismic category D

Look at AISC Design Guide 16 for flush bolted end plates, lots of good information in there. It's even right in the title!

I don't recommend using a moment connection to the weak axis of a wide flange column. If at all possible, it's worth turning the column to make your primary structural system work better. The smaller/other details can be coordinated more easily. If you can't turn the column, you could consider a cruciform column as an alternative to HSS. Probably would be too costly/not efficient for your size of structure, but it's another option.

Yes, flexible diaphragms should distribute load by tributary area vs rigid diaphragms by stiffness. It's possible the example you looked at treated the untopped metal deck as a semirigid or even rigid diaphragm. You have to run some checks to determine that, it depends on geometry and deck profile. There's a lot of threads on this forum on the topic if you do a search.

RE: OMF in seismic category D

(OP)
Thanks, I did miss that, I was looking at design guide 4. I will work on turning the columns and thanks for the info. about the deck. I'm sure they assumed it was rigid or semi-rigid but they didn't ever do the check for it so that's what was confusing to me. I didn't think you could just assume that.

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