anchor bolt bottom plate welding

[senthil83]

we are using anchors bolts with bottom nut and washer. refer attachment.

in order reduce the anchor rod bottom side threading and nut.

we propose to use only bottom plate washer welded to anchor rod without nut.

IS it OK in terms of pullout strength requirement?

regards,

senthil

 

[Ron]

There might be a few obstacles but it can be done...

Consider the following:
1. The plate will need to be thicker than with the nut
2. The rod has to be of weldable steel
3. You will need enough weld area to develop the rod
4. The welds should be inspected....you only have one opportunity to get it right
5. Develop a welding procedure specification in accordance with code requirements to document your process
6. The welder should be certified to the procedure specification (if in US, AWS D1.1 would apply)

 

[senthil83]

thank you ron

 

[hokie66]

Ron is correct about the welding, but most of us would stick with the nut and washer. Threading rods is a standard fabrication practice, so why the reluctance to do it that way?

 

[racookpe1978]

Threaded rods allows that final 1/4 turn of the lower nut to get the vertical member actually vertical. Otherwise, you're "trusting" the welder - working underneath the "washer" ?? to get all of the anchor plate alignment washers exactly even and parallel with each other. No welder is that good.

If welded, you really, really do not want the vertical rods to be threaded at the point of the weld, but you have to have threads where the upper nuts are going to be tightened down. Welding into threads is extremely poor practice = no cleanliness, can't prep, can't get good fusion down in the "points" of the threads, but the upper part of the thread melts away quickly.

Cleanliness of the irregular melting surface (er, threads) is nearly impossible: if you protect the upper part of the threads from corrosion so you can get nuts on later, you can't get the coating off the lower (welded) part of the threads.

 

[hokie66]

racook,
You seem to be confusing the anchorage (nut and washer) with a levelling nut. The anchorage is already embedded in the concrete when the column is plumbed.

The OP's rod is threaded at the top. For some reason, he wants to avoid threading the bottom end.

 

[racookpe1978]

racook,
You seem to be confusing the anchorage (nut and washer) with a levelling nut. The anchorage is already embedded in the concrete when the column is plumbed.

The OP's rod is threaded at the top. For some reason, he wants to avoid threading the bottom end.

True, I did that.

But my point still stands.

If the lower plate (the washer) is to be within the concrete, then all of the restrain "value" of the vertical rod is going to be carried through that single weld around the rod at the bottom.

Consider: Without the nut on the bottom, the length of the outside of the weld circumference fillet is going to more than 40% LESS length that current configuration. At MOST with the new weld, it will be pi * OD of the rod. Now, it is at MINIMUM 6x length of flat of the current retention nut below the current washer. If the bottom is still threaded, then maximum weld length = pi x minor diameter of the threads of the vertical rod.

At MOST, the leg length of the new weld can be = thickness of washer. Current configuration: Maximum weld leg length = height of nut.

So, the proposed change has less weld length, smaller weld leg height, and needs the weld to be all the way around a smaller length with a much greater change of blowing through the thin washer, or very poor penetration into the base metal of the rod as the weld attempts an all-around weld of two very different thicknesses of metal. The washer will heat past red-hot very quickly, the rod almost none. This means weld quality will suffer.

Now, there is an absolute backup: The nut WILL serve as a retention device quite well if any bad weld occurs, corrosion occurs, or any small crack occurs. With no nut, there is nothing to hold that entire rod in except the skin friction of rod to concrete when a weld crack occurs in the likely-lower-quality but more expensive weld.

Failure mode of the washer metal itself is threatened as well. Now, the all-around fillet weld is (approximately) a hexagon centered at mid-point of the area of the washer, criss-crossing the washer area as the hex nut outside changes. It is farily robust with a large weld area (larger fillet leg height on the washer.) That means the washer loaded in tension has a very small bending moment that must be resisted through the washer thickness. Most of the thrust as the rod is pushed down or pulled up each load cycle of every wind gust is resisted by the combined thickness of the washer and nut area very close-in to the OD of the rod.

In the new configuration, ALL of the resisting force of the washer is transmitted across a much longer radius of the bending moment, and that bending moment is located a much larger distance from the new (smaller) fillet weld. Fatigue loads of the (proposed one-sided fillet weld on the bottom of the thin washer) of must be considered in the design of the new fillet weld and washer thickness.

Ron's design concerns are all valid. This single weld needs to be very carefully designed and NDE'd on every weld. There is no backup if the vertical load is to be resisted. Worse, the loss of any single weld on the washer only increases the load on the other three remaining rods and their welds on that post since pivoting movement of the column increases when one washer fails. There is no redundancy in the baseplate once any weld fails.

 

[TLHS]

If anything, this is backwards. Just use the nut and omit the plate.

The nut will develop very significant loads on it's own when you check the various failure mechanisms.

As noted above, any welds in this situation are normally intended to stop rotation rather than being structural elements.

If you really don't have length for the thread below, get a headed bolt. It's significantly more conventional.

http://www.portlandbolt.com/products/bolts/headed-anchor/