Anchorage to Concrete (ACI 318 Appendix D) - D5.3.5 Bearing strength at head of anchor

[Deener]

My team is designing a group of pre-tensioned anchors. The anchors themselves are basically headed bolts with their threads protruding out through holes in the baseplate above the concrete surface. The nut on these threads is torqued after the concrete cures. To ensure we don't apply too much torque, we use equation D-15 to check the pressure on the concrete bearing against the embedded bolt head. Logically I would think you can take the bearing area and multiply that by the compressive strength of the concrete. Not the case below. The ACI has added a factor of 8. Does anyone know the reasoning behind this? My only guess is that there's empirical evidence to back it up?

Thanks in advance

 

[BridgeSmith]

The concrete directly under the bolt head is confined by the surrounding concrete, so even if it crushes, it has nowhere to go, until enough is crushed to allow the head to move through the crushed concrete. I assume the factor of 8 is based on empirical test results.