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RE: Anchorage to concrete
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- Subject: RE: Anchorage to concrete
- From: "Silva John (sj)" <Silva(--nospam--at)hilti.com>
- Date: Wed, 10 Nov 1999 11:52:34 -0600
Robert, Let me try this one. The idea of requiring an l/d of 8 was to force some measure of ductility, rightly or wrongly, into the anchorage. It was assumed that for many cases, l/d = 8 would ensure steel rupture, and this should be encouraged by punishing shallower embedments. In many cases, (stem wall anchors, for instance), the concrete capacity is severely limited by the geometry of the anchorage, and l/d = 8 cannot ensure steel failure at ultimate. It is important to note, however, that if the ultimate capacity is limited by the concrete, increasing the bolt diameter will not increase the anchorage capacity, although it may increase the initial stiffness. e.g., assuming you are not limited by near edges or adjacent anchors, and looking simply at tension, your 3/4" anchor (let's assume a standard hex A307 bolt) with 6" of embedment would develop a concrete capacity (concrete cone failure mode) in nominal 2000 psi concrete of approximately 26 kips. The tensile strength of the bolt at the threads (assuming an overstrength factor of 1.25) is about 25 kips. The tensile strength of a 7/8" A307 bolt is 33 kips. Increasing the bolt diameter without increasing the embedment doesn't do much for you in this case. With respect to your recommendation for designing embedments in stem walls or narrow foundations, I would agree that the use of equations based on a breakout cone failure for this case is inaccurate (this was the subject of a previous thread, as I recall). Splitting will be the controlling failure mode for the concrete in many cases, and the equations for development length are based (at least in part) on this failure mode. (The original post indicated reducing the embedment of the anchor bolt to 2.75" to avoid an edge distance reduction. I would strongly recommend against this line of reasoning...) Apropos strength design of embedments, I would recommend becoming familiar with the method adopted for the IBC 2000 (the so-called CCD method). It is reasonably comprehensive and provides guidance for a wide variety of loading conditions. Regards, John Silva > ---------- > From: Robert Shaffer[SMTP:rkdn(--nospam--at)cruzio.com] > Reply To: seaint(--nospam--at)seaint.org > Sent: Tuesday, November 09, 1999 9:22 AM > To: seaint(--nospam--at)seaint.org > Subject: Re: Anchorage to concrete > > A good article on anchorage to concrete was published in Bulding Standards > Magazine, July/August 1993. It runs through the whole shear cone area > developement, including what to do with small edge distances. If you are > asking about the large 7/8" diameter L-hook or J-hook anchors that are +/- > 30" > long, I have calculated those based upon the minimum developement length > in > tension for smooth bar reinforcing per chapter 19 UBC, not the section on > embedded bolts. > > But while were on the subject of bolts, I have a question on shallow > c.i.p. > anchors per section 1632.2 which requires the use of Rp=1.5 versus 3.0 if > the > anchor has an embedment depth 8x dia. or less. > > If a 3/4" dia anchor is embedded 6" we satisfy this requirement and can > use > Rp=3.0 and the connection exceeds our required load. However in an > attempt to > be more conservative in our design we use a 7/8" dia anchor with a 6" > embed ( > nothing else changes ) are we now forced to use the Rp=1.5 and the anchor > no > longer meets our required load which has just doubled due to Rp=1.5 . > > > Robert Shaffer, P.E. > Santa Cruz, CA > > > "Randy Hamilton P.E." wrote: > > > Can anyone lead me to a source to prove that a hooked anchor bolt > (j-bolt) > > can develop the loads typically used for hold downs when cast into a > stem > > wall. > > > > Using section 1925.3.2 ('94 UBC) with a phi factor of .85 (assuming the > > j-bolt is hooks around a horizontal bar) and an edge distance of 2.75 > inches > > (assuming an 8 inch stem wall but the bolt is centered in a 5-1/2 inch > 2x6 > > plate), what should the value be for As? > > > > If I use concervatively a truncated cone with a diameter of 5.5 inches > and > > an embedment length of 2.75" so as not to have to reduce for lack of > edge > > distance, I come up with a tension value of 4045# for 2500 psi concrete. > > For service load design, this would need to be reduced by a load factor > of > > 1.3 for wind and 2.0 for no special inspection which results in a value > of > > 1555#. This does not develop the load for even an HD2A bolted to a > double > > stud. > > > > I assume it is acceptable to use more surface area then this > conservative > > assumption but the question is how much and how is it justified? > > > > Thanks in advance for any input. > > > > Randy Hamilton P.E. > > Tucson, AZ > > > > >
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