Anchor to Hollowcore Precast Plank / Panel

[BSVBD]

There are a variety of anchors for hollow core material applications to CMU, brick, plank and panel.

However, depending on the application:
1. The allowable loads are very low to accomplish the necessary load.
2. The tabulated allowable loads are for anchors installed in hollow core concrete with a minimum compressive strength of 7,000 psi. Spancrete specifies its concrete to be 5,000 psi. Therefore, how can i specify an anchor with such a waiver? Can we interpolate the values? And if so, the result would be that much lower.
3. On-site testing is required to determine the actual performance of the anchor. This does not make it practical to specify an anchor in the office to obtain state approval.

Does anyone have a dependable and satisfactory method or anchor manufacturer to recommend?

Among others that i do not recall at this time, I've specified the following:
- Rawl / Powers / Simpson Drop-ins which actually have a "better" allowable tension, but, the main contractor i have been working with complains that the drop-ins tend to spin within the material. That was years ago and i will need to re-investigate that with more than one site foreman. Has anyone else had this complaint? The allowable tensions for drop-ins are better than most anchors for hollow core, BUT, i've stayed away because of ONE complaint. In general, ONE failure is typically enough! Any comments on this?
- Tapcon / concrete / masonry screws which have a low tensile value for hollow core material.
- Hilti screen-tube sleeve anchors with threaded rod and epoxy. Again, the allowables are low.

I DO understand the proprietary reasoning behind the low values. I'm just researching for a better solution and resource than what i currently have.

Any assistance will be appreciated.

Thank you!

 

[Agent666]

The concrete capacity in fastener design is typically proportional to the square root of the concrete strength. i.e. refer ACI appendix D for design equation examples assuming you are in the US. So in your example you would scale down the concrete capacity by ~0.84. Note however that the capacity may not be governed by a concrete failure mechanism.

If you are using a reputable supplier, they will have software that will let you look at specific design scenarios including modifying the concrete strength, I know Simpson & Hilti have programs that do similar things.

The reason the capacity is low, is the embedment is low in these types of anchors, you can't have larger capacities without engaging significantly more concrete through deeper embedment (again refer to the governing equations in ACI appendix D for example).

Drop in anchors are very sensitive to being installed correctly, you need to 'set' them correctly in the drilled hole with the correct tool prior to screwing in the bolt. I was involved in a job years ago where a structure installed to the underside of a hollowcore floor with drop in anchors was being removed/modified and replaced. The contractor found about 50% of the anchors were not installed correctly, some were very loose basically not taking any load. I have not used them since.