ANCHOR BOLT UPLIFT DEVELOPMENT LENTGH IN PIERS 8

[F1955]

I have a base plate on a concrete pier with #8 bars & 1" bolts in the base plate. Does anyone have literature as far as the best way to calculate required development length of the anchor bolts? Would you develop the vertical bars? Would you use the 160 psi bond strength to figure development length? I know some people who develop the vertical bars & end up with anchor bolts 24"-36". Thank you for you comments.

 

[SperlingPE]

See the AISC Design Guide for base plates. There is a discussion in there concerning anchor bolts in tension and anchor bolts in general.

 

[chichuck]

F1955,

ACI 318-02 Appendix D contains the ACI requirements for designing anchor bolt embedments. It reflects current thinking on the subject, and is somewhat different than previous practice.

It is based on shear "pyramid" failure planes, previously capacity was based on a cone. The capacity is dependant upon an embedment of something like a bolt head, nut or sometimes a plate attached to the bottom of the anchor bolt. It is quite different from the development of a bar, and usually will be a good deal shorter embedment than would be calculated for a bar development.

chichuck

 

[UcfSE]

If you are using hooked (L or J) bolts instead of headed bolts, note that the ACI App D puts a limit on the capacity of the hook and the length of the hook. In effect you don't need much embedment before the hook strength controls and that strength itself is pretty limited and usually controls the capacity when I have designed them. I recommend you use headed bolts or have a nut and washer at the end, or something other than a simple hook unless of course the numbers work out for you.

 

[bjb]

overlapping stress cones can be a problem with headed bolts in a column pier. I usually embed rods with double nuts and a small plate washer deep enough so that they are developed by rebar in the pier. You also get ductile behavior that way. If you are developing the anchor bolts with pier rebar, and have significant tension to transfer, it is not uncommon to have large embedment depths.

I have heard of using the 160 psi bond between concrete and an udeformed bar, but the anchor bolts may be oily from being fabricated, which would destroy the bond.

 

[structuresguy]

One thing you need to be careful of is to make sure that the pier bars (not anchor bolts) are developed in the pier above the location of the intersection of the failure cone with the pier bars. If not, you risk simply pulling a chunk of concrete off the top of the pier. This happened in many cases, particularly with pre-engineered metal buildings, here in FLorida during the hurricanes this year. Perfect chunks of concrete were ripped from the top of the pier, and along with them went the buildings.

We detail our pier bars to have 90 deg hooks at the top, and angle the hooks toward the center of the bolt group. Then we make sure to have sufficient length to develop the hook bars. This prevents a chunk of concrete from being ripped off the top of the pier.

 

[bjb]

structuresguy, I use basically the same approach that you do. I make sure that the tension in the anchor bolts is resisted by vertical rebar in the pier, and I do that by embedding the anchor bolts deep enough so that the development length of the rebar is satisfied. I also consider spacing between the AB and the rebar, and cover from the top of the pier to the vert rebar. There is a procedure in one of the AISC design guides for this that I follow.

Here in upstate NY our bottom of footing elevation is 4 feet min. below grade for frost protection, and the wind loads are based on 90 mph, both of which I think greatly contrast with your conditions in Florida. I don't always have to hook the top of the pier vertical steel, but there have also been times that I have when due to thick footings and high loads I needed the hooks.

 

[VTPE]

ACI Appendix D is a comprehensive look into the new thinking of anchor rods. However, it is limited to anchors less than 25" long and less than 2.50" in diameter. I have found that is is satisfactory for most small buildings (under 5 stories). The limiting factor tends to be the interaction between shear and tension. A couple of the sections of Appendix D that deal with shear fail to account for the strength of the ties in resisting shear forces. This tends to cause the piers to become large unless you use "engineering judgement" and account for the steel yourself. In addition, with the rods only being 25" long, it becomes difficult to develop any bar bigger than a #4 bar without hooks. When you hook the bars,you a great deal of congestion in the top of the pier.

BJB - I am also from upstate NY and found some higher wind problems near the great lakes leading to some pretty big anchor rod patterns.

 

[bjb]

vtpe-I used to live in Buffalo, but that was in the days of the "old code". Have you run into wind problems near the Great Lakes because of being in Exposure D? The basic wind speed according to the NYS Building Code is still 90 mph. I like to contrast what we have to do now under the IBC 2000 based building code with the "old building code", where all you had to do was pick a wind pressure out of a table as a function of height above grade. One size fit all.

I currently practice near Albany, and most of the buildings I work on are fairly small. I designed a special moment frame for a fire house with seismic design category "D" that had large anchor bolts and embedments. I also had to use hooked bars at the top of the column piers, but when possible I avoid doing that. I agree with your comment about congestion at the top of the pier from hooks at the top of the pier vert reinf.

 

[epr]

For large anchor bolts in piers, you are generally unable to develop high tension stress using only a shear cone and plain concrete analogy, as the cones usually overlap and also penetrate the sides of the pier.

Consequently, I have always used a method similar to "bjb", where I develop the tension by lapping the anchor bolt with the vertical bars in the pier. It has always seemed logical and practical to me, but I never saw any literature to confirm same. In fact, I have had discussions with a few engineers who do not find believe this method is suitable.

Recently, an article in Structure Magazine, July 2004, entitled "Alternatives to ACI 318-02, Appendix D- 'Anchoring to Concrete'", by Colleen Bush and Ed Schweiter, confirmed my analysis method. It's an article worth reading. It is especially helpful where you have a drilled pier, with a large overturning moment, such as a large, single-pole sign, or large light pole for highway or playing field lighting.