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Partial Fixity

Partial Fixity

Partial Fixity

I started working on a project a while ago for an underground concrete vault.  The lid of the vault was initially removable so I was considering pin-pin end conditions.  As the project progressed the vault got wider and deeper so the depth of the lid started getting thick.  In an effort to reduce the thickness of the lid it was changed to a cast in place lid and was (conservatively) assumed fully fixed.  After detailing the rebar, it was suggested that I try to model a rotational spring at the corners to help account for the relative stiffness of the members and try to reduce the negative moment.  I placed a unit moment at each corner (one at a time), found the rotation and got my "spring constant."  I then placed a rotational spring at both corners with their relative stiffness.  This ended up reducing the negative moment in the wall by about 1/3 and increased the negative moment in the lid slightly.  Say the negative moment at the corner was 30 ft-kips, the top of the wall went to 20 ft-kips and the end of the lid went to 31 ft-kips.

In all honesty, after several other complications, the whole thing was eventually modeled in SAP2000, but I could never figure out why the RISA-2D approach didn't work.  Has anybody tried this or have any suggestions?

RE: Partial Fixity

Hiw did you model the rotational spring? Did you add a member between the wall and the slab?

RE: Partial Fixity

There is an important distinction between member connections and joint boundary conditions.

A boundary condition is meant to connect the model to the outside world.  This is generally used at the base of a column. Partial fixity at boundary conditions can be easily modeled with rotational springs by just entering an appropriat spring constant.   

Member connections are usually modeled using the two idealized conditions.... Fully fixed (which is capable of transmitting all force from the end of the member into the joint it connects to) or BenPin (which is capable of transmiting shear and axial forces, but which cannot transmit moment into the joint the member connects to).  

There is no automatic way to model a "partial" end release for a member in RISA-2D.  However, there is a way to do it.... Just add in a small link member.  If the link member has the exact same properties (A, I and J) as the main member then the connection is essentially fully fixed.  But, as you reduce the moment of inertia of the link member then the connect moves closer and closer to a BenPin.  

In order to model these partial releases, it is good to have a the elastic moment-rotation relationship that you expect to get out of this connection. That way, it is easy to validate that you have selected the correct moment of inertia.  

RE: Partial Fixity

No, I didn't put another member inbetween the wall and roof slab... sounds like thats the answer.

Sounds like the rotational spring doesn't apply where there are two members that come in together?

Thanks for the tip JoshPlum, I'll try that.

RE: Partial Fixity

That's correct, the rotation spring does not connect between two members.... Rather it is a boundary conditions that provides outside support for the structure at that location.  

The most common use of a rotational spring is when you want to model a foundation support at the base of a column that is somewhere between an idealized fixed boundary condition and an idealized pinned boundary condition.   

RE: Partial Fixity

I'll give that a shot, thanks.

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