Large shear with continuous 2 span - am I analyzing this right?

[jengis]

I have a 29' column primarily supporting lateral load at the upper 25'. 1.5' from the base of the column will have steel members and steel plate framing into 3 sides of the column (with welds all around). I'm analyzing this as a continuous two span (beam) and it's coming up with huge shears at the base connection due to the short 1.5' span. I pinned all three supports.
Am I modeling this correctly? It is possible to look at this as just two simply supported members therefore eliminating the large shears I'm getting? Or because it is a continous member it has to be looked at as a two span continuous member?
Any input would be great.

 

[msquared48]

If the member is continuous, it must be modeled as such to obtain an accurate result. Otherwise, you are just kidding yourself.

To eliminate the extra leverage though, I would consider, if possible, putting a lateral slip connection at the 1.5 foot mark and designing it as a single span member.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Draw a sketch of the column and beams.

BA

 

[Lion06]

I ran into something similar when I was looking at my basement. I wanted to move the posts closer to the foundation walls. When I got one of the posts about 3 ft from one of the walls, the reaction (and shear) at the wall 3' from the post just blew up.

As Mike says, if you can provide a slip connection, do it. You're not modeling anything wrong, it's the geometry. If you can't do that, then consider pinning the column at that location. A typical column splice detail from AISC would qualify as a pin. This will provide axial continuity, but discontinuity for moment which will take care of the shear problem.

 

[rb1957]

i think you're over compliating it ... isn't this (just) a beam on two supports, with a long overhang ?

take moments about the lower support ... Pu*1.5 = P*25 ...
Pl = P-Pu, and (naturally) there's a large moment at teh upper support.

 

[Lion06]

rb-

No, the column has three supports. One at the base, one at 1.5' from the base, and one at the top (29'). At least that's the way I read it.

 

[BAretired]

That is why we need a sketch. Words are easily misinterpreted.

BA

 

[paddingtongreen]

I read it the way SEIT did. It then makes sense that he gets a large force, the two supports at 1.5" act as a fixed end while the other is pinned. It is, in effect, a propped cantilever.

Michael.
Timing has a lot to do with the outcome of a rain dance.