Pinned and Fixed Supports 1

[imsengr]

My colleague likes to have all pinned supports for his structures, but I prefer a mixture of pinned and fixed supports. If I put in all pinned supports, then my structure becomes unstable. I am working on a baghouse support that is about 30 feet high. It has got a few bays of about 20 feet wide, with all the floor, wind and seismic loads, and a generous amount of bracing.

Q1. Will this structure work if all the supports are pinned? Or do I need a mixture of pinned and fixed supports? Can I design all the supports as fixed, or will this be overkill?

Q2. In real-life construction, is there a huge difference between constructing pinned support and constructing fixed supports?

Q3. My colleague says that there will be no moments at the supports, as we have provide copious amount of bracing in the frame? Is this correct? I say that there will be moments generated in the supports no matter how much bracing we provide and we have to design for these.

Q4. Modeling our structures in software, when all the supports are pinned, we are getting a lot of instability errors, but when I fixed some of the supports, the model runs perfectly. Is this vindication that fixed supports are required?

Thanks all.

 

[chichuck]

GAEngr05,

most of this sounds like pretty basic stuff.

Q1. It will work if there are some lateral braces in each direction. The bracing needs to go down to the ground level, not just knee braces at corners. Then with all pinned connections, including at the column bases, it should be stable.

Q2. In real life there is a large cost difference: pinned connections are simple, clip angle type connections (inexpensive). Fixed supports require something like end plates on beams, full welded flanges or similar details (expensive)

Q3. If the structure is fully braaced, you will have very small moments at joints. This happens because the braces keep the lateral deflections down to very small levels. Try modelling a fully braced truss in your software, and use no end releases on the members, you should see very small moments in all the truss members, small enough that you should be able to ignore them in design. This is why classic analysis of pinned trusses works as you may have been taught in school.

Q4. That is what I would expect if you have inadquate bracing in your model- the software is finding instabilities in it. You can fix it by adding more bracing in the correct locations or by fixing some member ends against rotation. Either technique will eliminate those instabilites and make the model run.

All of that said, you still have to make the decision to use braces or moment connections to make your structure stable. I generally favor bracing myself, but sometimes there are job requirements that make this impossible. (access requirements, equipment or piping interference, etc.)

You have to make that call.


regarsds,


chichuck

 

[JAE]

chichuck is giving you good answers. I would further emphasize that fixed supports are not typically done in most buildings. When you fix the base of a column, you then have significant moments to transfer into the foundation system....then, you have to have a foundation system that can properly transfer that moment to the soil below.

 

[minorchord2000]

We are designing a rather large steel framed structure with all pinned connections, relying a cross bracing in the walls in both directions. The story drift is rather small when the structure is braced from the foundation up to the intersection of the roof girders and beams. Keep in mind that if you design the cross bracing to take compressive forces, the l/r ratio is limited per the AISC Code.

If you attempt to fix the beam-column joint at the top, the drift might get excessive in the direction of the rigid frame. Most pre-engineered metal bldgs use moment frames in the transverse direction but brace the walls in the long direction. All of their foundations are pinned. If the moment connection is not sufficiently rigid, you will get increased thrust (or shear in the transverse direction) to resolve at the foundation.

 

[archeng59]

I try to avoid fixed supports whenever possible to avoid increased foundation costs due to transferring the moments into footings or piers.

 

[ironmon]

Dear GAEngr,
I would avoid the fixed bases, especially when you are able to use vertical bracing (king of vertical stability!)

I do have a question regarding you computer model.
How are you distibuting the loads to the vertical bracing... horizontaly (ie, horzontal bracing, diaphrams, like decking or concrete slab).
The lack of a horizontal system could be causing your instability issues.

 

[VTPE]

One other thing to consider in the model. Recognize the difference between member end fixity and boundary conditions. If you model the bottom of column as pinned and then attach it to a pinned boundary condition, there is an inherent instability. A pin-pin connection has no rotational or torsional stability and will rotate endlessly. Some computer programs recognize this and lock the joints without telling you. The better way to model the base of the column is have the column end fixity with no releases and frame it to a pinned boundary condition. This may stop your instability problem.

 

[UcfSE]

Are you using a 2D model or a 3D model? There's more to think about than a simple pin when you are makinga 3D computer model, meaning fixing all of your translational DOF will not necessarily provide a stable structure even though the reality is you are modeling the same thing as you would with a 2D pinned support. Remember to check rotational DOF as well, something you may not always think about when modelling a structure.

 

[JStephen]

The model should reflect the actual construction that is proposed. If the actual construction will use support points that cannot resist moments, then the model should not be fixed at those points. It really sounds like you are having problems with the model, and not with the structure. If you model a typical structure correctly, you may find structural inadequacies, but an instability error implies something else is wrong.

 

[patswfc]

As mentioned alreadly, if there are no clashes with windows, doors, services, etc., then I would always go for a frame with pinned connections and bracing (usually cross bracing) to provide stability.
It will be easier to analyse and design, fabrication will be much cheaper and foundations will be cheaper.
Also the frame will be much stiffer than a moment frame which will reduce p-delta effects.

One point to note, depending on the design code you use, is beam end reactions on columns in the computer model. In the UK were BS5950 is the code for design of steelwork, the code states that for simple construction (pinned connections), beam end reactions should be taken as acting 100mm from the face of the connection to introduce nominal moments in the columns. The majority of frame analysis packages simply transfer the load for the beam as a pure axial load on the column without an eccentricty.