Headed anchor bolts have a forged head at the embedded end which provides pull-out resistance. The head on the end of the anchor bolt is part of the round bar, providing a structurally-sound anchor bolt that is easy to set. Headed anchor bolts are available in square, hex, and heavy hex shapes. Portland Bolt can forge heads on anchor bolts through 2½” diameter in virtually any length. Certain grades of bolts must be heat treated after being headed in order to develop the proper strength per the ASTM specification.
Advantages of forged heads:
- More structurally sound than a threaded rod with a nut.
- More cost-effective than a threaded rod with a welded or peened nut.
- Provides greater pull-out resistance than a 90° bent anchor bolt.
- When dealing with space constraints in a foundation, a bolt head takes up less space than the hook of a bent anchor bolt.
Welded nut anchor rods are made by assembling a nut onto the threads of the embedded end of the rod. The components are then heated and bonded by adding a filler material of molten metal. Without following the proper welding procedures, certain* grades of bolts should not be welded because the round bar material has been heat treated to certain mechanical values. If the material is heated by welding in an uncontrolled environment, the material strength may be compromised.
* ASTM A449, ASTM A193, ASTM A320, ASTM A354, ASTM F1554 Grade 105
First and foremost, the term headed anchor bolt implies a bolt with a forged head, as opposed to a rod with a welded nut.
Disadvantages of welded nuts:
- Most hex nuts are not weldable
- Structural grade nuts gain their strength during the heat treating process, and as a general principle, welding structural nuts should be avoided. Non-structural nuts are not heat treated, although they are often made from steel with high carbon content, the composition of the nut should be carefully considered before welding. For more information, see this FAQ on welding nuts.
- Many high strength anchor bolt grades are not weldable.
- Welding should be avoided, if possible, on any grade of fastener that has been heat treated. High strength fastener specifications which welding should be avoided on include; ASTM A325, A449, A193 Grade B7, A320 Grade L7, A354 Grade BC, A490, and A354 Grade BD. Welding should also be avoided on ASTM F1554 Grade 55 bolts that do not meet supplement 1, for weldability. Fastener specifications where welding is acceptable include ASTM F1554 Grade 36, A307, and F1554 Grade 55 meeting supplement 1. For more information, see this FAQ on welding high strength anchor bolts.
- Rods with welded nuts have less structural integrity.
- Tack welds often break prior to installation, especially when the anchor rods are galvanized. This is due to the non-weldabe chemistry of most standard hex nuts and the fact that the tack weld is being performed on threads. Because of the reduced minor diameter in the theads at the junction of the “pseudo hex head” and the threaded rod, in certain applications, this design may provide less strength than an anchor bolt with a forged head.
- Headed anchor bolts can be more cost effective
- The price of domestically manufactured nuts, especially when they are high strength, large diameter, and/or hot-dip galvanized can be cost-prohibitive, especially when combined with the cost of threading the bottom end of the rod and performing the welding. Often, headed anchor bolts are more cost effective to produce.
Assembled Nut with Damaged Threads
Deforming threads under a nut is a way to provide pull-out resistance on an embedded anchor rod. These parts are made by taking a threaded rod, assembling a nut on the embedded end, and then destroying the threads under the nuts so the nut does not turn off. This function is often referred to as “peening” or “staking” the threads.
Disadvantages of peened nuts:
- Labor intensive
- Not structurally sound because it is virtually impossible to the keep the nut from moving at least a little bit and can sometimes spin over the deformed threads
- Nut is expensive combined with threading the bottom end and performing the labor to peen
90 Degree Bent Anchor Rods
Bent anchor rods are a common configuration for anchor bolts. The embedded end has a short L shaped “leg” which provides pull-out resistance. The anchor is shaped by placing a straight rod in a hydraulic bender that forms the 90 degree angle bend at one end of the bolt.
Disadvantages of bent anchor bolts:
- Can be difficult to set when space is limited
- Can fail by straightening and pulling out of the concrete 1
- May interfere with rebar or may stick out of the concrete footing
- Has a very limited pullout strength compared to headed bolts 1
- Current recommended practice is to use headed bolts for anchorage 1
In our opinion, a headed anchor bolt has several advantages over the alternatives described above and should be considered instead of an assembled nut that is welded or peened.
1 AISC Deisgin Guide 1, 2nd Edition