? Deflection Effect on Stresses

[tristan861]

Hello Guys,

I received this steel Canopy (already designed) to design a sub-frame (grey elements in the sketch attached) for the stone cladding at top and bottom of the cantilevers. The consultant provided me with the deflections values for the steel cantilevers at different locations to accommodate for the additional stresses and deflection on my sub-frame! (grey beams)

Honestly I have no clue on how this can be done!. Any ideas would be appreciated.

Thanks a lot !

 

[Lomarandil]

Essentially, in your analysis, you will need to design and detail your sub-elements for the loads imposed (weight of cladding, wind C&C, seismic), plus notional loads (and moments?) that result in the deflections provided by your consultant (essentially deflection compatibility).

Although at first glance, I'd think a pin-pin or pin-slotted joist would allow you to avoid all of that, and simply design for loads imposed. Assuming that the consultant has not assumed any stiffness or bracing from your elements.

----
just call me Lo.

 

[tristan861]

Hello Lomarandil,

Thanks for the reply!. Of course I have done all that except adding the deflections provided by the consultant. I'm struggling at this particular point.
I'm dealing with their cantilevers as existing structure, so how inserting their displacement values on my model can create additional stresses knowing that the beams have slotted holes.

 

[Lomarandil]

Am I understanding correctly that you are designing the grey joists that fill in the panels created by their cantilever frame?

If so, you'll want to consider the relative deflections provided at each end of (each) joist. If the relative deflections are zero (each end of the joist moves the same), then your joist only goes along for the ride.

Then consider how your joist is restrained in each direction that has a net relative deflection. If there are relative deflections, but your joist is not restrained in that direction (e.g. your slotted holes allow for the deflection listed), again, no additional effect on your joist.

It's only when your joist is restrained by connections in a direction that has relative deflection that you will need to adjust your analysis to match. (And one solution may be to change your connections to reduce restraint - maintaining stability of course!)

----
just call me Lo.

 

[tristan861]

Thanks sir

 

[BAretired]

Perhaps I'm not understanding the issue fully, but:

If the grey beams (or joists) are simple spans, deflection of the cantilever beams will have no effect on their stress or relative deflection. In that case, why did the consultant provide deflections?​

If the grey beams are continuous, there will be an effect on stress due to variable support heights. This effect can be calculated. In that case, how could the consultant provide deflections since they are partially dependent on the stiffness of the grey beams (or joists) which the OP is designing.​

BA

 

[tristan861]

They are simple spans

 

[BAretired]

They are simple spans.

In that case, stresses in the grey beams are unaffected. Relative deflection will be unaffected, but maximum total deflection will be the average of end point deflections plus relative deflection.

BA

 

[rb1957]

are you saying the light blue columns and the other elements that touch the grey beams are deflecting ?

won't these (deflections) be enforced displacements on the grey beams ?

do the grey beams connect to the canopy to distribute the canopy load to the supporting structure ?

 

[BAretired]

@rb1957
Yes, yes and yes!

BA

 

[r13]

If the adjacent beams has substantial differential settlement, the erection crew might have difficulty in fitting the connections. Think ahead.

 

[tristan861]

retired13:
The cantilevers are cambered upward, and the grey beams have horizontal slotted connection.

 

[tristan861]

@BAretired: Actually what you said does make sense. The grey beams are simple spans (not continuous), so they are not going to be affected stress wise. maybe I should calculate the total deflection at their mid spans as a service check only. Right?

 

[BAretired]

@BAretired: Actually what you said does make sense. The grey beams are simple spans (not continuous), so they are not going to be affected stress wise. maybe I should calculate the total deflection at their mid spans as a service check only. Right?

You can certainly do that if you wish, but if the joist deflections are deemed satisfactory when the ends are at the same elevation, they should be considered satisfactory in this situation, particularly if the cantilevers are sloped upward.

BA

 

[r13]

I think the consultant provided the deflection for the purpose of detailing only.

 

[KootK]

Just taking a guess at what the issue might be here.

 

[tristan861]

@kootK actually there are simple joists at top ( grey) and at bottom ( purple with short posts downwards) of the cantilevers to support stone cladding at top and aluminum sheets at bottom respectivly.

@retired13 No. The consulultant requested to check the effect of the relative deflection of the cantilevers on the (lets say the Sub system).

 

[r13]

As pointed out by BA, if the sub-framing members are simply supported, there is no effect in the members, force-wise, due to the relative deflection. However, if the relative deflection occurs during different stages, and the deformation cannot accommodate by the connection, then local stress can be high. For out door installation, fatigue maybe is something to look out for.

 

[tristan861]

@retired13 I dont see how the relative deformation of the cantilevers could induce additional stresses on the joists ends? (Correct me if im wrong please), I see it as the joist and the connection will be displaced together along with the cantilever from one side and that displacment would try to rotate the joist along its strong axis, but I have the joists released by using a shear plate with long slotted holes from one side. So, where the strssses would come from in that case?

As for the fatigue, All the given deflection values I recieved are in downward direction (wind in suction is negligible so I guess cyclic loading doesnt exist to develop fatigue).

 

[r13]

If you have bolted connection, the rotation of the member will cause a moment on the connection bolts if tightly fitted. But the proposed slot hole will relief the problem.