Tack welding rods

[penpal97]

I'm detailing anchor bolts for column base plates. I've got a threaded rod embeded in the concrete footing. For anchorage of the rod, I do not want to bend or hook the rod. 2 questions for everyone...

1 - I've seen an embeded nut, a nut and washer, 2 nuts sandwiching a plate, and 2 nuts with 2 washers sandwiching a plate. Any opinions?

2 - To keep the nut in place, I've seen tack welding and burring the threads of the rod. Preferences? What rods can and cannot be tack welded? (ASTM A36, A193 B7, A307, A354, A449, A572, A588, A687, F1554)

I guess it would be nice to know what the difference between all the materials is as well (if you have some extra time on your hands).

 

[Lion06]

1.) We show a tack welded nut for our typical detail. You can provide a plate washer if you need a larger bearing area (this will help with pull-out capacity, and to some extent with tension breakout if you need it to). When using a plate, be sure to design the thickness for bending from the bearing pressure of the concrete when the rod goes into tension.

2.) We show it tack welded. I don't see a reason to burr the threads (if it's welded), but it won't hurt anything if they do. F1554 is the standard material for anchor rods. Gr 36 is weldable. Gr 55 is only weldable if you spec the S1 supplement. I don't believe Gr 105 is weldable.

F1554 is the comm

 

[Ron]

Almost any grade of steel is weldable, but some are easier than others. All of the mild steels A36, A307, F1554(36) are weldable without any difficulty. One of the problems with tack welding high strength steels is that you get a significant metallurgy change at the weld interface, so cracks can initiate easier at those points.

Tack welding a nut or nut+plate washer is not a problem

 

[Lion06]

My apologies for the welding comment. I didn't realize you were only asking about the tack welding, I thought you were asking about welding in general (e.g. anchor rod extension).

 

[SteelPE]

I went to a seminar recently (yesterday) where the speaker mentioned this detail. He states that tack welding to prevent the nut from backing off of is acceptable as long as you do it to the underside of the nut. Any changes to the metallurgy will not have an affect on the system because it is on the underside of the nut where no load demand is required.

I believe the speaker mentioned that when you order F1554 36ksi rods you are more likely going to get 55 ksi rods (because the suppliers think giving a higher grade is better). He said, “If you are going to need the rods to be welded then you need to spec F1554 55ksi with the S1 supplement”. Not sure if this is true or not.

Our standard detail is to use double nuts and washers…. But you run the risk of these not being installed (happened to me once before).

 

[penpal97]

Thanks everyone. If you use the double nut, do you still need to tack the bottom nut? I would imagine the clamping force of the nuts would hold them in place. Based on SteelPE's comment, you can not tack the top nut?

 

[SteelPE]

The detail we use, the double nut and washer, does not need a tack weld. When the nuts are put together and tightened they should be impossible to back off.

 

[Ron]

The point of tacking the nut is not to hold it in place during concrete placement, it is to make sure the rod does not turn during tightening of the anchor nuts.

Bring the nut flush with the end surface of the rod. Weld it on the end. You can add or plate washer or not, though the washer will add load capacity as StructuralEIT noted.

 

[sandman21]

Since there is no produce for welding a nut to an anchor bolt we don’t spec. it. Some agencies will not allow it, since most of the time you are welding a high strength nut, there is some concern over its effects. Plus using a double nut is so easy, I would bet that it’s faster as well.

 

[penpal97]

If there is no tack weld, will a double nut (w/ or w/o a plate/washer) prevent the rod from turning while tightening? If so, does it make sense to always use a double nut, and add a plate/washer only if needed?

The double nut would then be the ideal detail. It would eliminate the need for tack welding the rod (which may or may not be OK), and avoid welding onto the nut and washer (which no one likes).

It also sounds like no one is particulary concerned about tacking to the rod (even if it is "weldable" or not), nuts and washers as long as the tack weld is away from the loaded areas (like the bottom of the nut).