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:

 

[phamENG]

Yes. That brace is required for stability. It's typically provided by the diaphragm in building construction, but in platforms and other open structures discreet horizontal bracing will do the job nicely.

 

[jayrod12]

with all of the other members pinned, and no diaphragm or pink bracing, theoretically the structure could twist out of square due to a load being applied off centroid. So something is required.

 

[Compositepro]

It seems to me that all four of the top corners are already strongly braced, and adding additional diagonal bracing from one top corner to another does very little. Sure it adds some stiffness under some load conditions, but I think that saying that it is required is going too far.

 

[canwesteng]

No, it's not required. The structure is stable for all load conditions without it. Where it becomes required in buildings is when you have multiple columns in a braced bay, but this is not the case in your sketch. The structure could twist out of square with or without the top bracing - but in either case it will be stable, unless the load causes something to buckle.

 

[phamENG]

Compositepro - I'm afraid your opinion goes against basic structural analysis theory and the requirements of every building code I've ever read. On a small stand I agree that you would see limited movement. But as the scale of the structure increases, the scale of the loads also increase, and the flexibility of the connection styles the OP is describing will show.

If a lateral load is applied at 45 degrees to the framing shown in the first image (parallel to the pink brace), there is no reliable transfer mechanism to keep the top square. The connections are not designed to take the forces imposed on them in that scenario. You end up with lots of thin plates in bending, which isn't going to give you enough. A closer look at the statics of the connection reveals I'm wrong on this piece. Oops.

 

[Compositepro]

Pham, I'm sure you are right. If keeping the top square at all times is important, then so are the diagonal braces.

 

[dauwerda]

I tend to agree with canwesteng on this one. I see no need for a diaphragm in this instance. Lots of industrial structures are designed this way.

 

[phamENG]

Okay...I may have been a little hasty...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. If you load one column orthogonally, that braced fame will deflect. Because we're calling these pins, the opposite side would have a theoretical deflection of zero. That will naturally shorten the distance between the column to which the load is applied and the opposite corner. If you had a brace (even just one) that relative movement would not be possible without buckling the horizontal brace.

 

[phamENG]

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

 

[canwesteng]

If lateral load is applied in line with the pink brace, it is resolved into axial force into the horizontal beams and vertical braces, not bending of the shear tab/clip angles. This is exactly the same load path it would take if the brace were there. The only difference the brace makes is to share this load between two corners of the platform, and thus it will be stiffer. In both cases your connections need to be designed for the axial force that is occurring.

 

[AskTooMuch]

I'm with castwengeng not required in this situation. I would consider having horizontal bracing like that if it's supporting equipment. For the pic shown, looks overkill to me to add hor bracing.

 

[jayrod12]

If we're talking just for stability, I can get behind it being sufficient. However, for real loads I don't feel it's sufficient without a diaphragm or brace. Between connection flexibility, member flexibility and elongation/shortening of members in tension/compression, there would be enough flexibility to allow it to twist out of square. Which I can't really imagine that being acceptable in most applications.

 

[canwesteng]

How does it twist out of square? Not to mention all of those things occur with the braces there as well.

 

[Logan82]

I felt that it should remain stable even without the top brace. However when putting a horizontal force of 15 kN at 45° without the top bracings, the FEM software mentions it is unstable.

It is stable when adding the top bracings.

Details:
Members are:
HSS254x254x13 for the columns and beams.
HSS152x152x13 for the diagonals.
4 m high x 4 m large x 4 m long.

I don't know how it could lose its square shape and form a diamond shape and be unstable without that top brace, because it seems to me like the bracings on the four sides prevent that from happening.

 

[AskTooMuch]

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

 

[BAretired]

If two diagonally opposite corners rest on pinned supports, while the others rest on rollers, the structure is stable without top bracing.

If two adjacent corners rest on pinned supports, while the other two rest on rollers, the structure is unstable. It can be made stable with the addition of one (not two) pink diagonal at the top.



BA

 

[Tomfh]

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.

 

[BAretired]

It’s already stable without the pink members. In fact you could lose half of them and it would still be stable.

Its stability depends on the support type. (see above)

BA

 

[Tomfh]

Its stability depends on the support type. (see above)

OP nominated pins