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RE: Base Plate anchors after the fact

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Joseph,
 
Sorry for the confusion.
 
Anchors are typically tested in shear with a flat steel plate separated from the concrete only by a thin Teflon sheet (see for example ASTM E488). Therefore, the reported allowable and ultimate values for 'shear' are based on minimum eccentricity of shear load (typically 1/2 of the anchor diameter). If your connection mimics this condition, then no adjustment for bending is necessary. Many connections involve some standoff distance which increases the bending moment in the anchor and allows for greater (unrestrained) concrete crushing at the bearing side of the anchor. This further increases the bending moment. Bending stresses are of course critical for the strength of threaded rod. Baseplate grout may or may not serve to reduce this effect. Our own research indicates that thicker grout pads are effective in resisting shear only if the concrete under the baseplate is roughened prior to placing the grout (not standard practice, as far as I am aware). Reinforcing the grout pad greatly increases its effectiveness (also not std practice). If the loading is seismic, we would advise caution in considering the effectiveness of the grout. Of course, many elevated and double-nutted baseplate connections are not provided with baseplate grout (typical for freeway light standards in many areas). In these cases there is no question that bending should be considered.
Regardless of whether you are using ASD or LRFD, a bending check for standoff connections is necessary as it reduces your effective global safety factor.
 
Note also that mechanical post-installed anchors that are used for standoff connections should be provided with a bearing washer and nut at the concrete surface to carry compression loads.
 
Regards,  John Silva, SE
Hilti, Inc.
-----Original Message-----
From: Joseph R. Grill, PE [mailto:jbotch(--nospam--at)micron.net]
Sent: Monday, April 02, 2001 05:03 PM
To: SEAINT
Subject: Base Plate anchors after the fact

There has been some similar threads in the past.  "Hilti" has a paragraph showing reduction calculations in shear capacity of their anchors, they say to be used, due to bending moment in the anchors.  Their diagram looks similar to a situation with a steel base plate.  The also say something to the effect that their reduction should be used if anchors are subject to ultimate shears causing crushing of the concrete base.  Does anyone understand this?  If the anchors are designed to service load levels are the anchors subject to "ultimate shears" in their words?  Would strengthening an in place base plate require their reduction if the base plate is grouted?

 

Thanks,

Joseph R. Grill, PE