fixed or pin end? 1

[tcaia]

How to determine if fixed end condition. Is steel plate flange bolted to concrete foundation sufficient or is only a welded connection considered fixed.
This subject in general is neglected in engineering texts.

 

[Qshake]

You've got to go back to statics and ask your self what is there that can reaslistically and reliably prevent rotation. In my opinion, a single bolt on either side of the beam at the support is not sufficient means to prevent rotation. In fact, if the support centerline may be assumed along the vertical axis of the bolt, the bolt does not even provide a resistance to rotation.

In other cases it is not intuitive. Take a large beam framed into a concrete pile cap supported by a row of pile. Is this a fixed connection? No, the pile will afford the flexibility necessary to assume the connection pinned. In the same light, a welded connection may also be pinned if a supporting or framing member is sufficiently flexible in one plane or other.

 

[breaks]

I agree with Qshake - you've got to really think these things out.

Most steel column base plate connections (with a concrete foundation) should be modeled as a pinned connection, especially with 2 or 4 anchor bolts. Most concrete columns tied sufficiently into a concrete foundation should be modeled as a fixed connection. Other situations get much more complex and usually are a combination of fixed and pinned tendencies.

I usually use the most conservative assumption when initially designing members that are in the "gray" area. If assuming a fixed connection gives me the largest loads, I use a fixed connection (and the same with a pinned connection). From there, I end up tweaking the model a bit for real-life circumstances. Sometimes this is too conservative, but it's where I start.

 

[tcaia]

Re: Flexible colums-stiff beams
Thanks to Qshake and breaks for resonding to my last ?
i want to use the above condition to simplify design, with inflection point at middle of column. Is there a rule of thumb for how much stiffer the beam should be to assume no rotation at top- twice as stiff, 4 times as stiff etc, or some minimum deflection of beam allowed? I want to fit column in cross walls, so need small column, but have lots of room in floor joist bays for bigger beams. Column will be about 6x3x3/8" tube or 6x14 WF, but how stiff to make the spanning beams?

 

[breaks]

tcaia,

In order to answer your question, we need a little more information. Is there only one column? Are the beams attaching to shear walls? What kind of building is it?

Assuming that a column is fixed at the top is a rarity (in my experience). Unless the column is somehow directly linked to a shearwall or other very stiff member, the column will experience some sort of sidesway associated with the entire structural system.

The beams should be stiff enough if you design them for the loads and deflection criteria (commonly limited to a Total Deflection of Span/240 - check the code you're using). The Beam-Column connections should then be designed depending on whether you envision a moment-frame type of connection or merely pinned connection (see the posts above for more on this topic!).

I hope I touched on your question at least a little bit.

 

[ishvaaag]

The kind of thing you describe, somewhat like a truss solidary to the end columns is consistent more than anything with pins at foundations. Normally there being a sizeable roof, it is likely you will be passing in box action to trusses in the walls, and so your columns, pinned or not, can be thin, since will be axially loaded either from being part of trusses or columns leaning on one main against lateral loads stiffening system.

If you want just frame action, of course you can strive for fixity at bottom, but will be rarely met.

Also I don't see fit guess a moment on just this basis; not that as long you meet equilibrium and the thing is well built you will have problems, but if you want to economize, please analyze the thing reasonably thoroughly. Simplification and economy whilst meeting strength and serviceability are not the most compatible items.