ACI 318-99 Beam tie requirements

[SperlingPE]

I have 20 3/4" deep concrete beams. I am in an IBC 2000 jurisdiction. Seismic Use Group III, site class C, design category A (low seismic region). What are the requirements for the concrete beam ties? Can "open top" stirrups be used? If closed ties are required, can they be 2 piece ties? What are the requirements for the two piece ties? Any immediate help is appreciated.

 

[JedClampett]

You can use "open top" stirrups as long as the top of the stirrup has sufficient anchorage (hook, development length, etc.). Make sure you have satisfied the minimum shear reinforcement requirements (11.5.5) and spacing (11.5.4). The first stirrup is usually placed 2" from the support face.
For more information on stirrup detailing and design, see the CRSI "Reinforcing Bar Detailing." It gives some excellent examples of beam details.

 

[JAE]

In addition:
ACI Chapter 7 includes provisions for Structural Integrity where beams with open stirrups require different (more) longitudinal reinforcing to provide continuity in case of a failure of a specific beam.

 

[SperlingPE]

There are stress reversals at or near columns, so I thought closed ties were required. Spacing requirements and steel requirements will or have been satisfied. The question about ties or stirrups is being brought up by the iron workers. Two piece closed stirrups would help speed up placing the rebar. So if closed stirrups are required, is there a way to detail two piece closed stirrups for this shallow beam?

 

[JAE]

Some years ago there was some thought that the closed end of a "U" shaped stirrup had to be on the tension side of the beam - but I don't believe that this is borne out in research and certainly isn't in the code.

Per JedClampett above - check out the CRSI book. Also, ACI has, or had, a good detailing book as well - check their website

http://www.aci-int.org

 

[SperlingPE]

see ACI 318-99 section 7.11 (all sub sections). I believe I have found the answer to my question (after closer review) and following on to section 12.13. Thanks for all replies.

 

[BertNeily]

Since ACI 318-99, Section 7.11 has been mentioned.... I'm involved in an office controversy regarding the applicablility of Figure R7.10.5 to flexural members addressed by Section 7.11. Section 7.11 calls for compression reinforcement to "... be enclosed by ties or sturrups satisfying the size and spacing limitations in 7.10.5 ..." Some staff members read this to mean that beam compression bars must gain lateral support within the limitatins shown in Fig. R7.10.5 - in other words, tied like columns, as explicitly required for columns in Section 7.10.5.3. However, the rest of the world doesn't seem to hold this view. I've reviewed multiple texts & the CRSI Manual and cannot find a specific BEAM or GIRDER design that explicitly applies the Section 7.10.5.3 requirements. Does anyone have any thoughts on this?

 

[JAE]

If you are counting on compression bars in your beam capacity, then yes, you would have to tie those bars per 7.10.5.

In most cases, however, engineers do not use compression bars as part of their capacity. There are, perhaps, bars that are in compression in a beam, but the engineer has chosen to ignore them for capacity calculations. These bars, in my opinion, do not require the 7.10.5 ties.

If you have compression bars in your beam (where you are counting on them for strength) then you usually have a condition where your beam is undersized and heavily reinforced, both of which lead to servicability problems as well as higher cost.