Just one bolt? 4

[theonlynamenottaken]

OSHA/AISC requires one bolt in solid webbed bracing members for erection purposes, but can the final connection only utilize one bolt? I don't like the idea of any connection relying on one bolt, but this is not my design... just a review. I've searched this site, the web and the AISC manual and cannot find anything stating that more than one bolt is required for bracing connections. (FYI - the bracing is concentric tension only X-bracing using single angles)

 

[UcfSE]

I'm not aware of a requirement specifically requiring more than one fastener but it is always good practice where possible. If it were me and I could only have one bolt, I would put a good helping of extra safety factor on it.

 

[bjb]

I have never seen any finished connection with fewer than 2 bolts. I always use at least 2 even if I only require 1 bolt for some redundancy. The connections are critical, and I think that most structural failures are related to problems with connections. A bracing connection with only 1 bolt is very questionable in my opinion.

If I was reviewing someone else's design and saw only one bolt in the connection you describe I would definitley question it, and offer my preofessional opinion that the connection should have multiple bolts.

 

[Taro]

Per OSHA 29 CFR 1926 Part R Section 1926.756(a)(1):

During the final placing of solid web structural members, the load shall not be released from the hoisting line until the members are secured with at least two bolts per connection, of the same size and strength as shown in the erection drawings, drawn up wrench-tight or the equivalent as specified by the project structural engineer of record, except as specified in paragraph (b) of this section.

 

[Taro]

Sorry, just reread the original post and noticed you said BRACING members.

 

[DaveAtkins]

I was taught in college always to use 2 bolts, in case one of the bolts is defective.

DaveAtkins

 

[Bagman2524]

I agree with the 2 bolt minimum crowd. Most catastrophic failures are caused by connection failures.

 

[dougantholz]

I believe you could easily say the intent of section 1 of ASCE 7 - which is a part of all building codes and these provisions are likely in the code somewhere - that the buiding should be sufficiently redundant to prevent collapse. I don't believe a one bolt connection is the least bit redundant.

 

[hokie66]

The number of fasteners in a connection should be designed based on the load, capacity of the fasteners used, and required redundancy. Consider pin connections of tension members which are connected at their ends by in some case quite large pins. Is there redundancy? The connection may be very conservatively designed, but in a strict sense, there is no redundancy. I agree with provision of redundancy, but small bracing connections can be quite adequate with one bolt, just as large ones can be fine with one large pin. Just don't get hung up on too many unbreakable rules. Think each one through.

 

[nutte]

Section D5 of the AISC specification covers pin-connected members. Make sure you satisfy these requirements as well.

 

[bjb]

Given that most catastrophic failures are related to connection problems, is it worth any perceived economy to only use 1 bolt?

 

[mcutchi]

Bolts are cheap. Lawyers are not. Tell the guy / girl to use two bolts.

 

[theonlynamenottaken]

The contractors in my area are notorious for doing whatever they want regardless of what is specified... especially on small projects like this. If A325 bolts are specified then chances are that hardware store Grade 5 rejects will be installed (we have no contractor licensure or unions here). This in itself demands redundancy and overdesign in most cases. As I said, had it been my design I would have specified two bolts. I'll do my best to convince the specifying engineer to use two bolts (with the support of the redundancy statement in ASCE 7), or at the very least perform a field visit to check the connection after erection if the connection is specified with one bolt.

Thank you all for your responses.

 

[gtaw]

As an ex-ironworker, I always say two bolts are always better than one, if for no other reason than to make it easier to make the first bolt. The typical sequence is to spud the first open hole with the tapered end of the "spud" wrench. The ironworker uses the spud wrench to pry the second bolt hole into alignment so the first bolt can be placed in the open hole and tightened.

If only one bolt is used, the ironworker has to spud the hole, use a "c" clamp to clamp the iron tight, remove the wrench and then hope the clamp holds the alignment until the bolt is place. It is false economy to use one bolt. The extra bolt hole saves time and money.

Best regards - Al

Best regards - Al

 

[PowerRanger]

I designed a portal frame supermarket two years before when I just graduated, at that time, I found one bolt in shear is strong enough to take the bracing force (wall and roof bracing), and I showed only one bolt in my plans, later, an architect review my plans and told me that 2 bolts is more practical in construction, because normally we used the bracing member in tension, so the section not very thick (or I should say it will very easy to sag under self weight). If only use one bolt in connection, the site worker will be very difficult to straight the member to fix into position, but if you use two bolt, it will be more easy to put a rod into first hole, and hold the member straight). Or if you really want to use one bolt, put a turnbuckle in the member then.
After that, I used 2 bolts in my all other design.

 

[VTPE]

Section D3 of the AISC manual discusses requirements for pin-connected members. With only (1) bolt in a tension only connection, I believe this section applies. In addition according to seciton J7, a minimum factored load of 10 kips must be met for the conneciton. If an 3/4" A307 bolt is used, it will not meet the criteria.

 

[nutte]

The 10-kip minimum load requirement (6 kips for ASD) does not apply anymore (in the latest AISC specification, at least). The commentary goes on to mention that this load requirement is unnecessarily conservative for lightly-loaded members (girts, sag rods, lacing, etc).

Depending on what code is applicable to your project, what you have may or may not be okay. Letter of the law versus spirit of the law.

 

[seattlemike]

Back in the day, I was told of the 10 kip (6 kip) minimum that thus required two bolts. Maybe that's the load of the crane falling over something.

Personally, I think 1 bolt is fine for light loads: I mean it is supposed to hold thousands of pounds. But more importantly, a single bolt is predictably a pin... no moment development to force unforeseen loads into the bolt. With two bolts, and a 3" eccentricity, you get like an effective capacity of like 1.1 bolts or something. That seems pretty darned inefficient.

So, if you're concerned about redundancy, and lawyers, and things of that nature, sure, throw a 1.5 FS on it, but at least with one bolt, you know you just have a pin.

And I think you can probably find a long history of single pin connections with clevises and whatnot. Fact is, the brand new seismic system for the 2006 IBC, Buckling-Restrained Braces, those use single-pinned connections (as manufactured by Star Seismic). Although... those are like 5" diameter pins in some cases.

Cheers,
Mike

 

[connect2]

Like gtaw's point the most. Once the science, lawyers, and codes are satisfied it makes alot of sense.

 

[WMO]

My interpretation of this one bolt requirement for bracing members is for bracing that normally would be an all field welded connection thus requiring no bolts. For safety concerns a pin bolt is required so that the brace can be installed before field welding and the hoist cable removed. I never considered it a matter of one bolt being sufficient for the loads.

Regards,
Mark