Beam overhanging one support with brace...modelling supports

[faromic80]

I have a question regarding a simply supported beam overhanging one support. There is a vertical load and load in the plane of the page at the end of the beam. The support nearest the overhang is actually a brace. See the figure I attached. My question is regarding the modelling of the supports for this frame. The brace and beam will be connected to an existing concrete wall with expansion anchors via baseplates. I ran the model using both pin supports and fixed supports. In the case that the supports are pinned, the tension load increases from the fixed case. I'm thinking this is because of the moment basically being converted to an additional tension load on the supports. Another thing I've been debating with myself is how the baseplates will actually behave? I know it may depend on the stiffness relative to loads but in general I think it's safe to design as pinned. What does everyone think?

 

[BAretired]

faromic80,

It is safe, though not strictly accurate to design the frame with supports pinned. In general, adding restraints makes a frame stronger, not weaker.

It is useful to consider the forces separately. For the load P1, it doesn't matter whether the supports are pinned or fixed. To all intents and purposes, P1 is resisted by pure tension in the vertical member.

The maximum bending moment in the column is P2 times the distance to the top of brace whether the supports are hinged or fixed.

If d = distance between supports and h = height of P2, then when the supports are hinged, the vertical component of the brace and column due to P2 is P2*h/d. Fixing the supports will reduce this only slightly unless the distance 'd' is extremely small in which case bending of the frame legs plays a more significant role.



Best regards,

BA

 

[RWF7437]

From the sketch, how would one know which way is up ?
If this is a real structure, not homework, how would the weights of the members be accounted for ?

 

[apsix]

I agree, it's normal to design as pinned connections. You then don't need to worry about the relative stiffness of the concrete wall.

RWF7437
Member is described as a beam, I'd expect the sketch should be rotated 90deg.

 

[RWF7437]

We could assume that the member is correctly described as a "beam" with axial load.
Or, we could, as BARetired did, assume it is a column with bending.
Of we can assume anything.

Let's assume the correct answer is 12.

 

[BAretired]

RWF7437,

You are quite right. I assumed that P1 points up and P2 points right, but the wording of the OP is more consistent with P1 pointing to the right and P2 pointing down. Sorry for the confusion. The conclusion, however is unchanged.

Best regards,

BA

 

[apsix]

RWF7437
It's described as a beam framing into a wall, shown as perpendicular to each other on the sketch.

How much clearer do you want it to be?

 

[RWF7437]

"If this is a real structure, not homework, how would the weights of the members be accounted for ?"

I choose NOT to complete someone else's homework.

 

[kslee1000]

Faromlc80:

The connection wouldn't know the differences if you (the designer/engineer) do not tell it how to behave. You can design any type of connection - fixed, pinned, guided, to fit your needs. For this case, looks like pinned-pinned is most logical and easy to achieve. I encourage you to do both hand cal and computer analysis, just to have a feel on how simple structure can be quite involved with add'l engineer thinkings.

 

[miecz]

Design your connections as though they are fixed. Design your members as though they are pinned.