HI there, we need to know which kind of bolts are proper to fasten an existing stair ramp to wall/beams? please note that the stair is in a subway station and heavily loaded in terms of vibration and the bolts have to be resistant against fire as well. Is there any test results for functionality of mechanical bolts during fire? please note that stairs are critical elements for pedestrian to escape during fire. Thanks,
Hi Davood,
Currently, there are no test standards for dynamic loading of post-installed anchors in North America. However, please reference the “2010 Hilti European Dynamic Design Guide,” which can be used as a reference during the design aspect of the project. Please click here to access the guide from the Hilti UK website. Specifically, please review the summary on Page 14, Section 2.4.6.
The design premise, for the HIT-RE 500 V3 and HIT-HY 200 adhesive anchors with threaded rod, is that in a high-cycle fatigue application with 2,000,000 cycles is that you would perform an ACI 318-14 Ch. 17 static anchor design, and the strength of the carbon steel rod in steel failure is reduced to 25% of the static capacity, and the concrete/bond strength is reduced to 55% of the capacity (for cored holes for HIT-RE 500 V3 this would be reduced further to 40% - see page 9 under Fatigue Behaviour of Adhesive Anchors). In PROFIS Engineering, the design would need to be such that the static utilization does not exceed 25% for the carbon steel rod and 55% for concrete or bond failure modes (40% for cored holes).
Along with the design of the anchorage, there some other items which can help reduce the fatigue forces into the anchor and help to distribute those forces.
1) Making sure the anchor is installed with the published installation torque will increase the pre-stress force and help distribute some of the fatigue forces into the fixture and concrete. Section 2.4.2 and 2.4.3 show how the forces in the anchor are increased when the pre-stress force is low. Installing the anchor with the correct Tinst, and then periodically inspecting the nut and washer throughout the service life of the anchorage, can help reduce the potential for overstressing the bolt from fatigue stress.
Additionally, proper Tinst and pre-stress forces help with group anchorage. In a group, if some anchors are “tighter” than the others the fatigue forces will be distributed unequally and reducing the steel capacity of the “looser” anchors.
2) Similar to item 2, after the provided nut/washer are installed with the correct Tinst, providing a lock nut on top will reduce the potential for the nut/washer coming loose.
3) Installing the anchor as close to perpendicular with the surface of the steel fixture and concrete are important. Installing at angle increases the bending forces in the bolt which will reduce the steel capacity over time, even less than 25% of the static capacity. Additionally, installing at an angle will not permit the proper clamping of the nut/washer meaning even if the nut has the proper Tinst applied the pre-stress force is not distributed correctly, reducing the steel capacity.
4) For anchor groups, it was discussed that the proper Tinst is needed for load distribution between anchors. For adhesive anchors, in shear, it is also advised that the adhesive is injected only after the steel fixture has been placed on the concrete surface. That way, when the threaded rod is installed, the adhesive will fill up the annular gap between the rod and concrete (for bonding) and that it should also fill up the annular gap between the rod and the steel fixture. That way, in shear, you will have equal load distribution between all the anchors in shear since all bolts are bearing at the same time no matter which direction in shear the forces are applied.
Generally speaking, steel will lose strength at high temperatures so it would be expected that the steel strength of the anchor would suffer at these elevated temperatures. Additionally, Hilti coats our anchors to help elevate their performance which may be affected by these elevated temperatures as well. Furthermore, concrete strength would suffer as well which of course would impact the general capacity of the anchor.
Regards,
Ashley