COLUMN END CONDITIONS 2

[lmcan]

Can anybody clarify my followinG doubts about column end conditions:

1. A steel column with base plate and 2 or 4 anchors and 300 below finish floor is considered as "fixed" support or "pinned" support in analysis.

2. In cantilever beam construction, the column end with a W beam resting on top of column, is considere "pinned" or "fixed".

Thanks

 

[JAE]

1. You can assume either way - and then design the connection accordingly.

If you assume it is pinned, then the design of the anchor bolts will not be considered as resisting the moments at the base and the base plate will not be subjected to an actual moment (in your design). In reality, the bolts and plate do take some stress, but the plate is usually designed to allow bending across its thickness and the moment is minimal and the structure behaves as you assumed it.

If you assume fixity, then the baseplate and anchor bolts will take the moment into the footing and the bolts, plate, connection to the column, footing, etc. all have to be designed with the resulting moment coming through.

2. Similar to 1. in that its is how you model it, and then how you design it in response to the forces that result.

This is really a pair of questions that show the classic design of anything structural. The assumptions affect the results, and the result affect the final design.

In these two cases (column base and a cantilever beam over a column) we usually choose to pin as moment connections are more costly.

Again - in reality, any connection takes some moment - but usually the details can/should correspond to your assumptions in your modeling.