Steel SMf vs. OMF

[ampersand]

I am considering purchasing RISA-Connection so that I can begin designing special steel moment frames easily, instead of the ordinary frames I have designed in the past, usually when retrofitting a wood-framed building. Looking at 'dogbone' (RBS) type connections, the following question came to mind:

Special moment frames are supposed to save on material costs, because they use an R value of 8.0 instead of the 3.5 used for ordinary frames, so the loads are roughly halved, and the required moment capacity will be roughly halved. However, since the beam section is reduced, and the reduced section of the beam must still have enough capacity to resist the design loads, then it seems to me that the beam section used will still have to be about the same size as one would use in an ordinary frame. Am I missing something?

 

[JoshPlumSE]

It is a very short section of beam where the section is reduced. The SMF detailing requirements require a decent amount of bracing near these reduced sections. Therefore, I do not believe that you are expected to check the stresses in the reduced sections of the beam for gravity or wind loads.

If I have time, I'll try to go through the code or commentary to see if it explicitly states this anywhere.

 

[ampersand]

Josh,

That is true, that only a small section of the beam is reduced, but the location of the reduction is precisely at the location of highest moment- the end of the beam.

So, I am left thinking that, at the location where we are designing for SMF seismic loads (R=8), we are also considering a +/-30% reduced moment capacity, which forces us to use a much larger beam section, similar to what we might use when designing for OMF seismic loads (R=3.5). If this thinking is correct, I am just confused about how we realize a reduction in material cost.

Thanks!

 

[JoshPlumSE]

I went ahead and read through the AISC seismic design manual and your comments appear to have merit. Example 4.10 says that the Mp of the reduced beam section is normally assumed to control the design of the beam. The example then goes onto say this:

"However in cases where the LTB strength may reduce the flexural strength of the unreduced section below Mp, the assumption will have to be verified."

To me, this is saying that the end capacity of the member is based on the plastic moment of the reduced beam section. But, if the unbraced length of the compression flanges is high enough that LTB controls, then moment capacity would be based on the regular code equations for FTB (without consideration of the reduced beam section).

Something else I found in the AISC 358 commentary on RBS:
Some concerns were raised in the past that the presence of the RBS flange cuts might make the beam more prone to LTB and that supplemental lateral bracing should be provided at the RBS. The issue was subsequentially investigate.... These studies indicated that interstory drift angles of 0.04 radian dcan be adchieved without a supplemental lateral brace at the3 RBS, as long as the the normal lateral bracing requried for beams in SMF systems is provided (per section 9.8 of the AISC Seismic Provisions).

They go onto say that the additional supplemental brace did provide improved performance (in the absence) of a slab. Therefore, the supplemental brace is now required in those cases.

Therefore, the code detailing provisions have alreadfy taken into acount any potential reduction in moment capacity due to LTB of the reduced section. That's why they have so thoroughly investigated the concept of supplemental bracing at or near the RBS.

 

[sandman21]

There is no way a SMF is cheaper in your situation. The inspection, doublers plates, continuity plates are very costly.

 

[JoshPlumSE]

Sandman -

Doubler plates are not normally required. Instead, the column size might be increased slightly to one with a thicker web.

But, my impression is that OMF frames will still often require continuity plates. It's only when you go down to R = 3 that those normally go away. My impression is that the detailing requirements for OMF frames are now much more stringent than they used to be. Though I haven't looked at this section of code in awhile. So, I could be mistaken.

 

[sandman21]

Doubler plates are worth about 80#/ft of column weight, on paper it looks like it is easy to eliminate the plates, but when you start running through the equations and getting the drift to work including the increase for RBS, the column weight increase can be greater than having doublers and having a deeper beam to help the drift. We do a lot of retrofits, new wood buildings that need moment frames, single story commercial buildings, and use OMF. The requirements for OMF make connections on single story wood buildings more cost effective.

 

[JoshPlumSE]

Sandman -

When I read the AISC 341 spec, it seems to me like OMF connections still need to be designed to 1.1*Ry*Fy*Z.

If that's the case, then most of these are still going to require stiffeners, right? I understand that IMF and SMF some more stringent requirements for stiffeners. But, to me that always seemed redundant because the Cpr*Ry*Fy*Z requirement meant that they probably would have ended up with stiffeners anyway.

Am I interpreting the code differently than you? Or, is the 1.1*Ry*Fy*Z not as demanding as I'm making it out to be?

Certainly when you get down into the R = 3 (and you're not requried to use AISC 341) then you will save some on the detailing requirements. But, as long as you're required to use 341, it seems to me that you'd be required to have continuity plate stiffeners even for OMF's.

 

[jdgengineer]

It's been my experience that almost always moment frames are controlled by drift over strength requirements. While I haven't compared directly the higher R value of the special moment frame is negated by the higher Cd value. I doubt you would have a large cost savings with a SMF as the welding and bracing requirements are more stringent with a SMF. For instance I believe a OMF can even have double sided fillet welds in lieu of the complete penetration weld that has the backing bar taken off and covered with a supplementary fillet.

However, if you are in a high seismic area there are severe limitations on OMF. The performance of a SMF should far outperform a OMF (think 1994 Northridge) and therefore it's recommended to use a SMF in high seismic areas.

I've heard in the AISC seminar than when possible its much more economical to eliminate doubler plates and continuity plates if you can do it with a modest increase in column sizes.

In wood buildings SMF buildings are typically very hard to integrate due to the bracing requirements. A wood diaphragm blows up real quick trying to resolve those forces (in my experience at least)