X-bracing connection bolted vs welded 1

[strucsteel]

I am curious what the general preference is in connecting x-bracing (tension only) members to gusset plates. The general question is bolted vs. welded. I tend to think that bolts are generally easier/faster to install than welds, and therefore cheaper. If bolts are used, then can they be snug tight with standard holes, or would erection tolerances dictate that they have to be oversize holes and pretensioned bolts (would I have to go so far as calling for slip-critical with faying surface preparation)?

 

[mrMikee]

In my opinion a welded x-brace should be less work and therefore cost less. A bolted connection requires detailed layouts of holes in both the angle and gusset plate. Sometimes during the installation stage gusset plates need to be trimmed in order to fit up in with other members that varied from theoretical dimensions. A welded x-brace arrangement is more tolerant of fit up errors. The companies I have worked for would generally use bolted connections for field assemblies or places that needed to be taken apart for access, and welded everything else that was done in the shop.

Regards,
-Mike

 

[LPPE]

We generally use a single bolt for erection purposes and then weld.

 

[dbuzz]

As a rule of thumb, welded for shop connections and bolted for field connections.

 

[Lutfi]

I must agree with dbuzz. Field welding is expensive and I am always concerned for the quality. This is due to the varying and hard to control factors.

Shop weld and field bolt is good advice.




Regards,
Lutfi

 

[strucsteel]

dbuzz and Lutfi - Are we talking about the same thing? I agree that in general shop weld and field bolt is the way to go, but I'm not talking about something like welding double angle shear connections at each end of each beam to columns. I'm only dealing with x-bracing connections. If you are using bolts for x-bracing connections, do you use oversize holes, and do you specify anything more stringent than snug tight? If I were using the brace as a compression brace, I think (not everyone I talked to agrees on this) it would fall under the heading of "built-up members", E4 in the AISC Spec. The last sentence of the third paragraph of E4.2 reads: "The end connection shall be welded or fully tension bolted with clean mill scale or blast-cleaned faying surfaces with Class A coatings." The commentary doesn't say much about it, but I'm assuming that that rule is there to keep one angle from taking the full compression load until it buckles and then overstressing the second angle. I'm not sure that the issue is the same for tension only, since if one angle were overstressed because of an imperfect connection, the overstressed angle would just yield and then the overload would be taken immediately by the other brace; but still, there's the matter that if only one angle is engaged, then the bolts must be acting in single shear instead of double shear, and there may also be issues in the field if the connection is detailed with standard size holes and accumulated imperfections do not allow the brace to match up with the gusset plate as detailed. Any opinions on this?

 

[mrMikee]

Per ASD oversize round holes are for use with slip critical connections. Slotted holes normal to load direction are allowed but in my opinion of little practical use because they don't help the fitup problem. While some reaming etc. is permitted during fitup, bigger holes will require bigger bolts.

-Mike

 

[14159]

I don't think there really is a better way, but just depends on the fabricator/erector preferences.

If you use bolted, keep in mind that braces are often fabricated intentionally short by a small amount. I think 1/16" is common. Somebody pointed out a mistake(?) that I made once by showing oversized holes to help with fitup. The supposed mistake was that the oversized holes negated the intention of the short brace fabrication. Not sure either way, but it's worth mentioning because some of the others typed about oversized and slotted holes.

DBD

 

[AggieYank]

As someone else mentioned, welding the x-bracing works well because when they are off by 1/2" it is no problem at all. We use welded x-bracing and never have problems with it. It is usually an easy weld as well.