Installation of High Strength Bolts, nut above or under plates? 1

[tmgczb]

As the title, I have a question asked by site engineers.

 

[OldDawgNewTricks]

Unless there is some need to do otherwise, I like the nut on the bottom so in case the nut works loose the bolt won't fall out.

 

[dik]

OldDawg... I suspect it's easiest to install in that fashion, too.

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

-Dik

 

[BridgeSmith]

For fully-tensioned HS bolts, we don't worry about the nut coming off, so for bridge girders it's more about clearances - heads go on the outside. That means nut on the bottom of the top flange, and the top of the bottom flange. Webs we don't care. If exposed to weather, I suppose there's fewer surfaces where rust could form and become an issue over time, if the head is on the top.

Conveniently, for twist-off bolts, having the nut and spline on the insides of the flanges makes rattling them easier, too.

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

 

[OldDawgNewTricks]

For fully-tensioned HS bolts, we don't worry about the nut coming off...

Yes, well that's the conventional wisdom anyway. AISC's Engineering FAQ 6.5.1 explicitly says so, "In general, when properly installed, the high-strength boltnut assembly will not loosen... When fully tensioned bolts are required, as for slip-critical connections subjected to vibratory or fatigue loading, the installed tension and the attendant friction on the threads will prevent the nut from loosening."

However, there have been occurrences of pretensioned bolts loosening when subjected to cyclical loadings (crane supports, etc.). The common explanation has been that the bolts must not have been properly pretensioned in the first place. But the U.S. Army Corps experienced loosening problems with bolts in their lock canal miter gates that had been verified to have been pretensioned. They found that most of the published literature was for automotive or aerospace applications and there was a lack of research into the topic for structural applications. So they sponsored the 2016 report ERDC/ITL TR-16-2 "Causes of Pretension Loss in High-Strength Bolts". It is a free download and is an interesting read. I will probably specify some type of "belt and suspenders" to prevent loosening for critical cyclical load situations rather than relying on the pretension alone.

 

[dik]

For bolts supporting mechanical equipment overhead, I always use Loctite Red 263 or peening threads... for that very reason.

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

-Dik

 

[BridgeSmith]

The common explanation has been that the bolts must not have been properly pretensioned in the first place.

That would seem to be a fairly good assumption. Of course, after something fails, nobody is going to admit to not tensioning the bolts. As I said, for bridges the fasteners are almost exclusively twist-offs, so verifying pretensioning is very straightforward.

...the U.S. Army Corps experienced loosening problems with bolts in their lock canal miter gates that had been verified to have been pretensioned.

We deal with some people from the COE, and generally they're pretty sharp, but not all. I don't know the configuration of canal gates, but there may have been some softening or deterioration of material within the grip, or their cyclic loading exceeded the pretension.

Of course, anything's possible, and in a non-redundant and critical connection, I can see including a backup of some kind. Can't use loctite (or anything else) on a twist-off bolt, though.

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