R Value of a Moment Frame with a Fixed Base 1

[WiSEiwish]

If I have a steel moment frame, does the base fixity have an affect on the R value used for seismic design? The R value for a cantilever column system is 1.5, and I wasn't sure if fixing the base of the moment frame would somehow create a combined system of moment frame and cantilever column requiring the use of the lowest R value.

 

[DaveAtkins]

R will be at least 3 for a moment frame. You may be able to use a larger R, depending on how you detail the moment frame.

DaveAtkins

 

[jdgengineer]

Fixing the base of the moment frame will not effect the R value. As Dave Atkins mentioned the type of moment frame that is chosen Ordinary, Intermediate, or Special is all that effects the R value. However, keep in mind that depending on the system you choose you'll have to meet the detailing requirements in AISC 341 including the requirements of Column Bases shown in D2.6 (assuming you are using AISC 341-10).

The validity of a "fixed-base" is also somewhat debated. It will be pretty hard to get something to behave truly as "fixed". So make sure you pay particular attention to not just strength but also stiffness of the base connection and supporting foundation.

 

[WiSEiwish]

Thanks for your responses.

I was a bit unsure, I guess, of what a cantilever column system consisted of. I was thinking that it may be any column with a fixed base, but I now am understanding it as columns with fixed bases that are not part of moment frames. Would this be a correct understanding, or too simplistic?

 

[TehMightyEngineer]

I ran into something like this on an unusually shaped, low rise building in a low seismic area (skewed loading dock addition). The dock had three lines of moment frames and a rigid diaphragm. Resistance out of plane to one of the frames and to help with the torsion caused by this setup was provided by three fixed base columns. These columns were part of a moment frame and provided both in-plane fixity and out of plane fixity for the frame. We had much debate in the office wither this was a dual system with R = 1.5 or a moment frame system with fixed columns of R = 3. In the end the PE and I concluded that the definition of a cantilevered column system in terms of R factor is the determining factor.

See ASCE 7 section 11.2 for the definition of cantilevered column system. It states that such a system is defined as a system where "lateral forces are resisted entirely by columns acting as cantilevers from the base."

Maine EIT, Civil/Structural.

 

[TLHS]

TME, the cantilever post requirements are intended to apply to things like some sorts of canopy structures where you've got the cantilever action of posts working on their own without bracing or frame action. If the posts form part of a moment frame in the direction of seismic resistance you're examining, then that is not cantilever construction.

 

[SAIL3]

I would think that a fixed base would retard or diminish the typical energy dissipation found in a moment frame with pinned base.However, as was pointed out, it is very difficult to obtain a truly fixed base. I don't think I would go as far as considering the R value of a cantilever system...maybe something inbetween due to the inherent residual flexibility of most so-called fied bases.

 

[TLHS]

Or are you saying that there were frames in one direction and no frames in the other direction?

 

[JAE]

I agree with that - a moment frame with a fixed base is not a cantilevered column system. Think of the failure modes of that system - if the fixed base were too flexible - or failed - you'd have a "better" more energy-absorbent system. The definition posted above uses the phrase "resisted entirely by columns..."



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[msquared48]

A moment frame with a fixed base has double curvature in the columns.

A cantilevered column system has single curvature.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

TLHS, the problem I had that we solved with the fixed base was that wind and seismic effects had a large difference between the center of rigidity and the center of rotation. Thus, I had a large torsional effect which caused some weird out-of-plane effects in my frames due to the non-90 degree corners I had. I resisted a portion of these out-of-plane loads using the fixed bases of the moment frames (with the moment frames and rigid diaphragm resisting the rest). The reason I couldn't do it all with the moment frames was deflection and serviceability of the loading dock addition. Also, because the building was seismic design category B I got out of all the torsional irregularity requirements.

Maine EIT, Civil/Structural.