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SPACING REQUIREMENTS: POST-INSTALLED ANCHORS: MECHANICAL VS. ADHESIVE

SPACING REQUIREMENTS: POST-INSTALLED ANCHORS: MECHANICAL VS. ADHESIVE

SPACING REQUIREMENTS: POST-INSTALLED ANCHORS: MECHANICAL VS. ADHESIVE

(OP)
When specifying post-installed anchors, with construction adhesives such as epoxy or acrylic, it seems as though the spacing requirements are, often, in excess of 3x greater than the spacing requirements for mechanical anchors such as the concrete screw anchor.

It seems as though Simpson and Powers have a similar trend. Their adhesive spacing requirements seem unreasonably excessive compared to their Titen and Wedge-Bolt respectively. Hilti, on the other hand, has much smaller spacing requirements, comparable to their KWIK Screw Anchor.

Even the screw anchor in grouted concrete masonry has comparatively closer spacing requirements than the adhesives.

I have often considered that the use of these high strength adhesives would help “adhere” the concrete material together, after going through the vibration of the close-proximity drilling, thus contributing to the greater prospect of a closer spacing.

Over the phone, one of the representatives from one of these companies, agreeing with me, also thought the spacing requirements were unreasonable and had no answer for me as to why. However, I did not speak with any of their engineers regarding this. That may be my next necessary step.

Can anyone please explain, why, the published spacing requirements are so high in comparison to the mechanical anchors? Is this reasonable?

By the way, I employ ASD. ASD vs. SD, regarding the future of post-installed anchors, is another topic which I will start in a separate post.

Thank you all!

RE: SPACING REQUIREMENTS: POST-INSTALLED ANCHORS: MECHANICAL VS. ADHESIVE

The spacing requirements are what they tested. All that does is change your question to, "Why didn't they test closer spacings?"

RE: SPACING REQUIREMENTS: POST-INSTALLED ANCHORS: MECHANICAL VS. ADHESIVE

I'm glad you touched on the topic of ASD vs SD. I myself work in an area that local jurisdictions are still approving an ASD approach, even though it's not up to par.

My only guess for the adhesive anchors is that the manufacturers have tested these anchors at a higher spacing as mentioned above. They most likely did that because they didn't want any interference of the anchors when tested in a cluster. Recall that there will be a cone of influence from the embedment of the anchor. I believe ACI appendix D illustrates this at a 35 or 45 degree angle from the bottom of the embedment. With that in mind. Testing a group of mechanical anchors or a group of extremely strong adhesive/anchors at the same spacing is not going to yield drastically different results if the anchors are spaced too close together.

The adhesive does not adhere the concrete together as much as it adheres the face of the concrete to the surface area of the anchor. With that in mind, using a high strength epoxy to 1 KSI concrete is not going to do you any good.

From my understanding, most values you come across in an ASD approach are derived from testing. It's with the SD that ACI is applied and more variations of spacing, embedment and edge distance are available. Don't quote me on that but it's worth a little research. If you don't want to go through the SD, consider looking at the anchor design programs for each one of these big manufactures.

What I can tell you is that ASD Values are derived from testing a certain configuration of anchors. Then the averages are divided by a factor of safety according to the governing jurisdiction. That's 4.0 for concrete and 5.0 for CMU installations typically. Whatever is tested is what you get, the evaluating engineer may or may not allow interpolation between data points.

I hope this helps.

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