Can F1554 GR 105 anchor bolts be field welded to the nuts and to A992 base plates as a fix for anchor bolt problems? What precautions should be taken and what electrode do you recommend? Since these anchor bolts are heat treated to achieve their tensile strength, how much loss in strength can be expected from welding?
Scole,
Try the "search" at the top right. Go back about 200 days. I know that the EIC will have to give you a go-ahead before you do anything because of OSHA requirements. I also doubt that you can ever really come up with a "LOSS" formula. Have you looked at any other options, say a coupling nut or a custom shoulder nut?
Anchor rods are used primarily to provide a pre-positioned location upon which to erect the column and to provide stability during erection. They are also used in conjunction with the dead load of the structure to resist uplift forces. Subsequent welding of anchor rods to the base plate will not serve the first two purposes, but can be helpful in providing uplift resistance. Have you talked to, and offered any suggestions to the EOR? The use of a coupler may be precluded if there is insufficient space to thread the anchor bolt and fully engage the coupler. Welding may be precluded if the anchor rod is not weldable. Determine what the carbon equivalence of the anchor rod is prior to welding, because welding the nut to the anchor rod is not always allowed, particularly if high strength heat treated materials are used and the rods are subjected to tensile loads. If welding is not an issue, and because the base plate holes are oversized and the anchor rod is rarely centered in the hole, a heavy plate washer welded to the base plate and to the anchor rod might be an acceptable alternative. The welding of the anchor rod to plate washers involves a fillet weld profile with a weld length that is equal to pi times the rod diameter, which develops relatively little strength. Welding to the threaded portion of a rod is permissible. If larger uplift forces are present, an alternative column base detail should be considered and designed by the EOR.
By scole
Date 12-20-2005 23:57
Thanks gkcwi and swnorris.
I am the EOR. The columns are already erected and are carrying 3 stories of steel. The anchor bolts have to carry 100 kips of seismic uplift each. The nuts are only halfway on because the anchor bolts were allowed to sink into the concrete when the piers were poured. The easiest fix, in my opinion, is to plug weld the 3/4" depression in the top of the nut using E100xx electrodes to fuse the top of the bolt and nut together. There is a 2-1/2" grout space under the base plate with leveling nuts which are holding up the construction. I certainly am not going to let them burn any holes in the base plate now.
The bolts are 1-1/2" diameter ASTM F1554 GR 105 heat-treated and tempered to 1100 deg F. ASTM F1554 says hot bending on heat-tempered bar stock shall not have the temperature come within 100 deg F of the tempering temperature which seems to indicate that the bolts can be heated to about 1000 deg F without affecting strength. Someone sent me a copy of a qualified welding procedure for a butt welded 3" diameter high strength heat-treated rod using a double bevel, a preheat of 500 deg F, an interpass of 450 deg F, and 29 passes.
Do you think that, if we preheated the nut and bolt to 500 deg F, a good quality plug weld could be obtained? What are the temperatures normally achieved during welding?
The chemical analysis test reports for the bolts says C=0.40 percent by weight. Is that good for welding?
The maximum temperature attained during welding is above the melting temperature of the base metal. For your purposes, that will be above 2600F. The lower temperature of the heat affected zone, adjacent to the weld, will be above 1330F.
It is usually considered to be good practice to keep the actual carbon content to 0.3% or less for ease of welding. The carbon equiv. takes into consideration more than carbon alone. There are several carbon equiv. formulas in use, each with their own set of welding recommendations. I like to use the one in the appendix of AWS D1.1-2000 (or later) in Annex XI Table XI-1 for repair work. The upper limit on the Ce is only 0.38, lower than the % of carbon you listed. This formula takes into consideration the diffusable hydrogen introduced by the welding process, material thickness, and the degree of restraint offered by the weld joint detail. I like to use the actual chemistry of the base metal.
You can also refer to "Weldability of Steels" by R. D. Stout. You can probably get a copy off E-Bay or Amazon. The book contains a listing of ASTM and AISI steels and recommended welding practices. You will have to compare the chemistry of your anchor rod to one of the base metals listed in the book.
Keep in mind that the anchor rod and nut materials are not listed in AWS D1.1, so a welding procedure has to be qualified inorder to meet D1.1. That will also provide you, the EOR, with the UTS of the welded assembly.
Best regards
Good points by 803056. I personally don't think that plug welding the partially engaged nut to the anchor rod is an effective means of attachment, especially with the type of seismic uplift you mentioned, but you're the only one who can make that conclusion. If welding is an acceptable means of correction, and it is suitable for these heat treated anchor rods, I would recommend a threaded extension of the same material properties welded to the top of the existing rod. The threaded extension is prepared for welding by beveling the contact end to a chisel point and is subsequently welded using suitable electrode material. The surface of this welded transition is typically non-uniform and will most likely necessitate the use of plate washers of sufficient quantity to allow free rotation of the nut.
By scole
Date 12-27-2005 19:43
Thanks for all the good input, guys.
I checked the carbon equivalency per AWS D1.1 Annex XI using the actual chemistry of the metal. The Composition Parameter Pcm (per Table XI-1) is 0.52 for the bolts and 0.50 for the nuts. The Carbon Equivalent CE (per Figure XI-1) is 0.82 for the bolts and 0.64 for the nuts. These values are so high that they fall outside the charts in Annex XI. Does that mean they are not weldable?
As an added thought. You can expect to develop martensite in the HAZ if the anchor rod is close to the concrete. The concrete will most likely spall if you use a high preheat temperature and it can act as a heat sink that will increase the cooling rate. The cooling rate of the HAZ and weld will determine how much martensite is formed for a given base metal/weld metal chemistry and its suceptibility to hydrogen cracking.
The reason the anchor rods are tempered at the temperatures you listed is to temper, i.e., reduce, the hardness of the martensite. By reducing the hardness, some ductility is recovered.
The question is not how much strength will be lost by welding, but how will the mechanical properties be affected? You can easily gain strength and hardness in the HAZ at the expense of ductility and increased hydrogen induced cracking potential.
You can weld them, but you can expect martensite in the HAZ, and if hydrogen is not controlled, delayed cold cracking.
Ideally, you would perform a post weld heat treatment after welding to allow the defusable hydrogen to evolve and temper the martensite.
Considering the close proximity of the encasing concrete, you might get away with using a hydrogen bakeout. This entails heating the weld and HAZ to above 550 degrees F and holding it for several hours. This allows the diffusable hydrogen to escape without initiating cracks in the martensitic HAZ.
The amount of martensite formed will be reduced by using high preheat and allowing the weldment to slowly cool. The hydrogen bakeout immediately after welding is advisable.
If the temperatures involved for a hydrogen bakeout are prohibitively high (spalling of the concrete), hold the weldment at as high a temperature as practical for several hours and allow to cool slowly.
Again, a procedure qualification will provide you with the knowledge that the proposed WPS is viable or not. It must replicate the field conditions if it is to be meaningful.
Considering the fact that the anchor rod material is tempered at 1100 degrees F, it would support the thought that the mechanical properties of the weldment should be close to those of the unwelded material if matching filler metal is used.
Last, but not least, perform NDT no sooner than two or three days after the welding is completed. Fluorescent magnetic particle testing is quick and will detect very small cracks should there be any. It can take several days for hydrogen cracks to grow to detectable sizes.
Good luck - Al
