Richard Vernon: You’re not fooling anyone, Bender. The next screw that falls out will be you!
Breakfast Club (1985)
Q: How many New Yorkers does it take to screw in a light bulb?
A: None of your damn business!
Baby Get your Head Screwed On, Cat Stevens, 1966
“Well if you’re feeling low, take it slow
Baby get your head screwed on”
Not too long ago, I was helping a contractor select a post installed anchor (PIA) to fasten a seismic wall tie to the bottom of a deck slab. Being the “frugal” person that I am, I recommended the most economical anchor I could find which was a 3/4” wedge anchor. After I put a lot of time into selecting this anchor, the field foreman asked me to change it because he wanted to use a screw anchor instead. I was surprised, so I asked him why he didn’t want to use the lowest cost anchor. He then reminded me that his guys would be installing thousands of these anchors and all of them overhead. To install a 3/4” wedge bolt overhead, with a large hammer, all day long was going to seriously slow down production. It was then that I remembered setting and testing 3/4” drop in anchors overhead myself. Not being built like Arnold Schwarzenegger, after the third or fourth 3/4”drop in anchor I could hardly lift my arm above my head! The foreman told me that the time he would save simply using an impact driver to set the anchor would easily outweigh the additional cost of the screw anchor (which was only minor to begin with). In fact he sold ME on the screw anchor from that moment on. Now, when I perform shear testing on anchor bolts, I always try to use a screw anchor to hold down my reaction frame. Screw anchors are easy to install and can be easily removed when you’re done.
Light duty screw anchors, or Tapcons, were first introduced by ITW/Redhead in 1975 but heavy duty screw anchors have just been available in the US since 2000. The newest heavy duty screw anchor to come to the market is the Hilti Kwik HUS (KH) and Kwik HUS-EZ (KH-EZ). Both anchors meet the requirements of AC 193, Acceptance Criteria for Mechanical Anchors in Concrete Elements and are approved for use with the latest International Building Codes. The main difference between the two anchors is the KH-EZ is approved for “cracked” and “uncracked” concrete where the KH is just approved for “uncracked” concrete. The KH-EZ also comes in a wider range of lengths and diameters than previous screw anchors, the smallest of which is 1/4 inch. So along with being approved for use in “cracked” concrete, this anchor should be a great help to designers who want to give contractors a range of choices for window and miscellaneous steel applications. Hilti has all of the technical information for these anchors on their website.
From my experience, here are the main reasons for using screw anchors:
- Ease of installation. You simply drill the correct diameter hole and then use an impact driver to set it. Done. Even the non-Schwarzenegger types can install them
- They are easy to remove. If you can set your impact driver in reverse, you can remove the anchor
- Close edge distances. Since the screw “taps” the concrete it does not create an expansion force which could blow out the side of a slab.
- You have a wide range of diameters and lengths to choose from. Hilti’s KH-EZ has diameters from 1/4” to 3/4” and lengths from 2-5/8” to 9”
- Works great in CMU! Many designers select expansion anchors in grout filled CMU but what happens when you hit a void in the grout? Your wedge anchor will not work properly. When you use a screw anchor in CMU, the minimum capacity you will obtain is that of the CMU face. Just make sure your screw threads go all the way to the bottom of the head of the screw.
- Works great in tight spaces. If you can drill the hole, you can most likely drive the anchor in. In tight spaces you may not be able to get a wrench in to properly torque an expansion anchor.
- They are approved for “cracked” concrete. No matter what your governing code, you should be able to find a screw anchor alternative!
While I love these anchors and use them frequently myself, there are some things to watch out for when using them. Here are a few things to keep in mind:
- Watch out for old concrete: Screw anchors work great in concrete and CMU that is in good condition but capacities in older structures need to be checked first. Most of the shear testing I do is on anchor bolts in older structures that are being rehabbed. I always try using screw anchors first when installing my reaction frames because they are the easiest to remove. Unfortunately, I’ve had a couple experiences where the condition of the concrete was far too deteriorated to have the screw anchors tap the concrete properly.
- Have a good impact driver! I first tried installing screw anchors with a relatively weak cordless impact driver. I was most likely hitting rebar and my small cordless impact driver just couldn’t handle it. I was very frustrated and ended up having to use expansion anchors in a very small space. Needless to say it took a lot longer to perform the test than I anticipated. I then went out and bought a good impact driver with enough torque to do the job the next time.
- These are interior and temporary exterior anchors only! Most screw anchors typically have an 8 mil zinc coating which is only a temporary exterior protection. Both the Hilti HUS-H anchor and Powers “Wedgebolt, 410 SS” anchor have been tested to ASTM B117 which achieves 0% corrosion of the base material after 240 hours in a saltwater spray environment. But neither is “technically” exterior since they do not have the same corrosion resistance as HDG or higher grade stainless steels. Unfortunately these two anchors have not been approved for use with AC 193 or the International Building Code.
So now you have one more PIA to think about during your anchor selection process. Please don’t roll your eyes though. This is a good thing! They make more than one type of car for a reason. Is a semi-trailer a better option for hauling supplies across country than a compact car? Of course! Having a lot of options in your anchor selection process can help you save a lot of time and money and the Hilti Kwik HUS Heavy Duty Screw anchor is one more way to do it.
Brian Clarke, PE