HSS Brace to HSS Column

[IngDod]

Greetings,

I have a question regarding connecting an hss brace to an hss column, virtually all connections I have seen in books, AISC, online.. etc... connect the hss brace to the hss column by means of a gusset plate, I understand the way this connection works.. however I wonder why the connection cannot be made by directly welding the brace to the column as if it was a truss.. and using the K chapter specs of the AISC. Is there a particular reason why this is not commonly done?... I ask because using a gusset plate on a hss wall comes with its own problems as the connection strength will always be limited by bending of the column wall... similar when using X-braces there is always some complicated plate detail when joining the braces at the X.. is not easier to just have one continuous brace and the other welded as a truss cross connection?

Thanks.

 

[jayrod12]

The problem with doing it the way you are suggesting is site conditions never work exactly as well as we plan on the drawings. The gusset plate connections allow for some leeway in construction and still act as intended. i.e. if the contractor were to get the bracing all cut and fabbed in the shop and then got on site and the angles don't line up correctly then they would have to site cut etc. gusset plates allow for the hss to be at approximately the intended angle but allowing for construction tolerances.

 

[SteelPE]

The only reason why I think you don't see braces directly welded to columns like you propose is because of erection requirements. Usually an erector would erect the steel, get the building plumb and then make the final connections to the LFRS.

 

[jayrod12]

At least that's how it was described to me when I asked the same question. Sometimes for the internal connection we will have one brace be continuous with gussets pre-welded to each side. so each X is made up of 3 hss members, 2 short and one long.

 

[paddingtongreen]

Field welding? after field fit-up?
Welding on the rounded edge?
Overhand welds? In tight spaces.

Think it through, remember it needs to be constructable and fast and lowest net cost. Time is money.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[IngDod]

Thanks for all your replies. I can see now why this would be a problem and why the gusset plate is preferred. I was thinking design wise only and thinking that perhaps there was a structural reason for this rather than construction. In my area it is common to see connections like this where the brace is directly welded to the columns.. here field welding is the norm rather than the exception so this could be the reason.

 

[TXStructural]

Gusset plates are not only practical for construction (very important) they are necessary for structural function (also really important.)

Constructibility is the number one thing engineers overlook in design. It should be the one of the first considerations in the design process, not an after thought.

The gusset provides the flexibility required to reduce the risk of putting unwanted moments into members. Rigidly welding a brace to a column creates a fixed-end brace and any bending will put unplanned moment into column.

 

[IngDod]

Thanks, I have been reading more and more about this connections and my final conclusion is that they are indeed the only way to go. Specially if there is any kind of seismic force involved as the gussets play a big role in making sure that the connection is much stronger than the brace and that the brace buckles or yield in the intended fashion. Using directly welded the size of the brace would be dictated by the connection rather than by the loads.. and the AISC requires the connection to resist RyFyA of the brace... which leads to some very ridiculous situations if you have a brace that was up-sized twice or thrice its load required size to meet connection requirement.