Top bracings 5

[Logan82]

Hi,

Do all 5 sides (4 sides and top) of a structure need to be braced if all connections are pinned and the supports at the base are pinned connections?

Here is an example to illustrate my question:
Would you say that the bracings colored in pink are necessary if all the connections are pinned? There are vertical loads (Y) and also wind loads in two directions (X and Z).

Without top bracings:

With top bracings:

 

[canwesteng]

That could be the source of the instability though, error in support input in the model.

 

[dik]

If you can imagine a horizontal load on opposite top corners... it will want to do a parallelogram. Cross-bracing can be replaced with a single diagonal brace in tension or compression. Not certain, but I think all four corners have to be pinned to prevent a 'parallelogram' shape (pinned opposite corners would likely prevent that).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

OP nominated pins

So he did. I missed that. In that case, I agree that the structure is stable without a top brace.

That could be the source of the instability though, error in support input in the model.

Yes, that is a possibility.


BA

 

[KootK]

Sometimes it's just model error where you need to allow support to carry torsion eventhough torsion is minimal.

That would be my first guess. Perhaps the horizontal braces inadvertently added torsional restraint to members originally lacking it. Torsional restraint is responsible for more than its fair share of modelling instability issues.

 

[Tomfh]

It’s model error for sure. The structure is stable.

 

[BridgeSmith]

I agree with Tomfh. As a theoretical model, not only are the braces in pink unnecessary, but only one diagonal brace on each side is necessary for stability.

As a real structure, with connections and members that have limited stiffness and strength, the necessity or advantage of adding additional members to resist the applied loads depends on the connection and member stiffnesses and strengths.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Logan82]

Hi!
I just got my hands on the model again. You were right that the bracings are not necessary. The problem was that the column torsion at the supports was free. I fixed the torsion at the supports and now it works:

Thank you for your help!

 

[human909]

It is funny how some simple questions that seem like first year engineering trigger such a debate.

It’s already stable without the pink members. In fact you could lose half of them and it would still be stable.
The pink members provide additional stability and help keep it square.

Agreed.

it is theoretically stable for the reasons canwesteng stated. But I stand by my statement that typical shear connections (OP stated they're pinned) and brace gussets would have trouble coping with the resulting forces for eccentric loading. I also disagree that the braces wouldn't keep it from twisting out of square.

It isn't just theoretically stable. IT IS STABLE. Furthermore the braces to keep it from twisting out of square as the bottom is effectively fully braced. Of course adding bracing at the top would significantly reduce the twist from an eccentric load but it wouldn't remove the twist.

As far as your comments on the shear connections and brace gussets I don't where your concern is. The shear connections and brace gussets are seeing negligible difference unless your structure is starting to twist into a spiral.

For what it's worth, all of the free standing industrial platforms I've designed included the brace.

For what it is also worth I have never included this brace in the industrial platforms I've had built. And they generally don't have anything that could function as this brace.

These structures are typically squarish with a height 5-6 times the width. I have a design that I am finishing up one now. I HAVE included this brace because due to significantly eccentric loads. Before the foundation had 24% difference in loads, after including the brace it had 7% difference at ultimate wind loads. I included it mainly to reduce localised uplift on the foundation. As far as my steel structure goes the difference in strength and deflection is negligible.
(Of course neither of are wrong here, it comes down to what design criteria you are working to.)