Concrete Wedge vs. Threaded Expansion Anchors

[VirgilP]

Members,
I wanted to see if anyone knows of a substantive difference with regards to long term “relaxation” of the two common mechanical concrete anchors on the market. Mechanical concrete expansion anchors can of course employ either a mechanically expanded wedge at the tip or a large thread to produce the compression force along the anchor shaft. As long as an ICBO report authorizes the use of either anchor to resist wind and seismic loads I would think that they are both suitable for long-term exposure to occasional cyclic loads. Does anyone have any recollection of a substantially more failures for either type when either anchor was installed correctly and subjected to no more than the published allowable loads as best could be ascertained from a proper direct and prying tension/shear analysis?

Virgil Payne

 

[flamby]

I am with you. They both are same analytically. However do not ignore the creep in concrete, which is a major cause of long term relaxation. Provide additional tension to compensate the creep.

 

[Sciscoe]

Virgil, don't have empirical data on tension and shear analysis, you can find some of that data on the Powers Fasteners web site. We use an 8 inch wedge bolt for one of our applications. We choose the wedge versus the expansion bolt system for three reasons. 1. If the expansion bolt installer is not careful when inserting the expansion bolt, it is possible for the wedge sleve to be slipped up the bolt and never actually engage properly. This conditions can be difficult to detect until investigation after a failure. With the single piece wedge bolt, when the bolt is fully seated it is fully installed. 2. The expansion bolt functions by exerting a continuous load pressure between the expansion sleve and the hole. This continuous pressure is focused where the sleve and expansion wedge occur on the expansion bolt, rather than the entire length of the bolt, which is the case of the wedge bolt. 3. When the wedge bolt is installed, there are no continuous pressures on the hole. Pressure would only occur when subjected to tension, shear or moment forces. Over time, the absense of continuous hole pressure would, in theory, leave a stronger hole. In applications near the edge of a concrete slab (3 inches from edge or 3 inches from a saw joint) the continuous pressure could lead to weakened concrete hole or early concrete slab failures.

Check out the bolt manufacturers web sites for specific specs. Good luck.

 

[VirgilP]

Thanks for the reply all. I typically specify Powers Wedge Bolts but occasionally I will run into a consultant review comment that takes issue with them. I think this may be due to the fact that some of the smaller sizes, say 3/16”, of the previous versions of threaded concrete anchors might have been over-torqued on occasion, rendering them useless in tension. However, it seems to me that the allowable torques for the Powers threaded fasteners are in line with the setting torques of traditional expansion bolts when larger diameters are used. To be safe I think I am going to check a similar nominal size adhesive anchor as a fallback to a stripped out threaded anchor, as the hole required for the adhesive anchor is generally larger than the one created after a concrete base material failure. If I include an adhesive anchor in the calculation package up front I can see a good field guy falling back to the adhesive option for the occasional event of a questionable thread set.