Contractor Forgot Washers 1

[PEStructural]

Does anyone know of a code reference (AISC, UBC, IBC, etc..) that requires flat washers under the nuts of anchor bolts in tension.

The contractor forgot to install washers under the nuts that hold down a column base plate. Due to the forces on the structure, these bolts are in tension in a seismic event.

The bolts are 1 1/2" A36 threaded rod with nuts on both ends (1 end in concrete, 1 end above base plate). The hole in the plate is 1 5/8" diameter. The nuts are standard hex nuts.

As it stands, the nuts completely cover the hole in the base plate. However, I'm concerned with the hole distorting and possibly ripping the nut through the base plate.

The design force on the bolt is 50,000 lbs in a seismic event.

Any thoughts would be appreciated.

Thanks

 

[civilperson]

Why not weld the tightened nuts to the base plate?

 

[dimjim]

Make the contractor fix it.
It depends on what the base plate
material is. How do you know if he
used any in the concrete?

 

[apsix]

I agree, make him fix it, it should be straight forward enough.
The exercise will help concentrate the mind next time he is bolting down baseplates.

 

[GregLocock]

Other option would be to do a test and demonstrate that his construction technique meets your requirements. Onus is on him, of course.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jt12]

Don't know of anyplace where a washer is mandated code wise. The latest steel code says it is likely necessary due to the beyond oversize holes likely in baseplate. If it shows up in your construction documents as mandatory, that is probably enough. With hole only 1/8" bigger than anchor rod, the nut will always be bearing on plate. Going back to steel basics, and analyzing the potential failure mechanisms might let you know if you have a problem. Welding a hardened washer could result in weakening of the nut / potential britle failure.

 

[Dinosaur]

Our DOT Construction Specs require heavy hex nuts and washers. This goes for HS bolts as well as anchor rods. We even have the pay item adjusted to account for the washers and nuts.

I recommend you look at the standard specification language in the contract you use to see if it is in there.

 

[SperlingPE]

Who needs a code reference? Demonstrate that the surface area of the washer (not enough area is in contact with the bolt) is required to resist the tension forces.

 

[dik]

The hole in the base plate is not oversized and the steel is mild. There should be no problem with the anchor rod and nut developing the force.

In most jurisdictions, the EOR is responsible to verify any alteration to the anchorage system.

Dik

 

[EddyC]

I thought that one of the purposes of a washer is to prevent nut/base galling. Such galling would affect nut torquing, possibly resulting in less tension.

 

[dik]

With mild steel, galling shouldn't be an issue... hardened washers are often used for slip critical A325 high strength bolts...

Dik

 

[Lutfi]

Hardened washers are required for A325 and a490 as dik said. They are to be placed under the turning element. They are used so that the hard material turning element does not eat/chew into the softer steel.

I strongly recommend that you get a copy of the “Specification for Structural Joints Using ASTM A325 or A490 Bolts”. It can be found at this AISC link:

http://www.aisc.org/PrinterTemplate.cfm?Template=/ContentManagement/HTMLDisplay.cfm&ContentID=26429

Regards,
Lutfi

 

[UcfSE]

If you don't ask them to fix it, they may well ask why you tried to cost the owner so much money by specifying washers that apparently were not necessary. That can get you in trouble with the owner and loose you some business the next time around as well as give the contractor a way to save face and pass the buck to you for what he screwed up.

Even if it works, make them do something. I think they should have to just put the washers in, as others have said.

 

[samdamon]

PE:

Anchor bolt washers are mentioned briefly in a couple places in AISC Steel Design Guide Series #1 "Column Base Plates", but no specific washer design recommendatons are given beyond using a heavy plate washer 5/16" to 1/2" thick when anchoring with oversize holes. In Table 14-2 of AISC steel Manual 13 th Ed., recommended washer sizes and thicknesses are given for various combinations of anchor rod diameters and baseplate oversize hole diameters.

Dumb question- Is your anchor design tension load of 50k a factored load? Allowable service load in tension for 1 1/2" dia a36 would only be 34k. Regards.

 

[JLNJ]

Are the anchors pretensioned?

 

[dik]

JLNJ... don't know if they are tensioned... likely not with mild steel other than snug tight. If they are tensioned, then they need some space for 'stretch'...

Dik

 

[MikeE55]

Just a note to Civilperson, I don't think you should weld nuts to the plate. AISC's position is that nuts are not necessarily made of weldable material. AISC will not accept nuts to be welded when there is insufficient thread engagement, so I would think this isn't a good fix for this problem either.

 

[boo1]

The function of the washers is:

1. To protect the outer surface of the connected material from damage or galling as the bolt or nut was torqued or turned
2. To assist in maintaining a high clamping force in the bolt assembly
3 To provide surfaces of consistent hardness so that the variation in the torque-tension relationship could be minimized
4. Reduce the contact stress through increased surface area
5. Reduce stress risers form the discontinuity of the hole and bolt
6. To increase the bolt clamping surface area
7. To allow bolt shoulder to clear hole

It was not discussed what were the bolt type, the tightening method and connecting metal:

A325 bolts in general do not require the use of washers when using the turn of tun method. If a calibrated torque wrench is used, a washer should be used under the turned element, the nut or the bolt head. Washers are required under both the head and nut of A490 bolts when they are used to connect material with a yield point of less than 40 ksi. Also when bolts pass through a beam or channel flange that has a sloping interface, a bevel washer is often used to compensate for the lack of parallelism.

 

[PEStructural]

Thanks to all who have commented. In this case, the contractor has hired me to find out if they need the washers. This time, I'm on the other side of the fence.

A few answers:

The anchor bolt is 1 1/2" A36 threaded rod. The plate is also A36. The nuts or not pretensioned. The reason why is because they are anchor bolts for a steel tank which only go into tension in a seismic event.

The anchor load of 50k is the yield strength of the anchor. In steel tank design, the anchorage (base plate, concrete pullout, etc..) is designed for the anchor rod yield strength so the bolt will yield before the anchor attachments fail.

Thanks again for all the valuable input.