Boomerang Beam

[Aldar]

Hello,
I have a problem with designing precast concrete beam of boomerang shape. If you are not familiar with the shape, i attach the picture of it. The problem is finding moment diagram. Solving by hand, i get max moment at the mid section as for normal beam. However, i tried to model the beam in Strand 7 and SAP2000 and moments diagram are completely different. There was negative moment at the mid section. Does it mean i modelled it wrong o i am solving by hand in a wrong way. If somebody designed this kind of beam before, could you give advices o any examples. Thanks!

 

[MiketheEngineer]

How and where is it supported?? Not enough information.

You might want to re-post this in Strucutral Engineering

 

[patswfc]

I assume your drawing shows an elevation of the beam. If so your
problem is likely due to the supports that you have applied in the analysis models. If you have pinned both ends this will give a negative moment at midspan. If you use pinned at one end and roller on the other this should give results similar to your hand calcs.
You need to know your actual support conditions as these will be somewhere between the two cases of pinned-pinned and pinned-roller
to assess the forces in your cranked beam, or simply design the beam for both cases.

 

[csd72]

Yes it is definately the supports.

Most hand formulii assume that one end is a roller support but most analysis packages default to all supports being restrained.

In the first case yo have what is basically an odd shaped simply supported beam, in the later case you have an odd shaped arch.

 

[Aldar]

Yes, it was the support definition. I tried the pinned-roller support and it gave results like from hand calculations.
Another questions. The beam will be supported on RC column( there's a neoprene plate between them). So, how do i model this kind of support? I get it should be something between pinned-pinned and pinned-roller cases, like patswfc said. Is pinned-roller case is reasonable assumption? thanks

 

[csd72]

Pinned roller, unless you are going to design the columns to resist the horizontal thrust.