Reinforcing Requirements at Top of Pedistals?

In CA it's standard practice..and compliance is met most of the time. We use the CBC which is an ammended version of the UBC.

The UBC provision for added ties(to engage ABs attaching WF bms or cols) at top of RC frame columns in SZ 2-4 (moderate to high risk)UBC Secs 1921.4.4.8/1921.7.4...and for masonry columns designed by either strength design or working stress design there is a similar provisions applicable to all seismic zones..UBC 2106.3.7.

The UBC and ACI provisions for reinforced concrete do not state where the first tie starts, however, in the case of masonry, the UBC provision states that the uppermost tie is to be within 2 inches of the top of the column and that's what a lot engrs out here use for RC. Bear in mind that there will be about 2 inches of non-shrink grout or dry pack directly on top of the column..sandwiched between top of column and bottom of base plate; this will increase protection for uppermost tie.

The second sentence in commentary to Sec 7.10.5.6 implies columns and piers are treated the same in this regard.

I have no problem with the though and usefulness of ties at the top of the column, the 5 inch zone and specified no. of bar requirements just seems overly conservative.

WillisV, this provision like many others are included in the code as a result of structural failure due to natural hazard..in this case I believe it was the 1971 San Fernando Earthquake...where steel and wood griders slid off the top of columns and pedestals.

To the best of my knowledge the provision was first introduced in the 1976 UBC...and other model codes followed suite.

The performance of concrete col-steel(or wood grider) bm connections designed in accordance with this new provision in subsequent earthquakes must have been good or the provisions would have been modified...no?

The provision was just placed in the ACI code in the 2002 code cycle. The express reason for its placement had nothing to do with seismic performance as indicated by the commentary which states "confinement improves load transfer from the anchor bolts to the column for situations where the concrete cracks in the vicinity o fthe bolts. Such cracking can occur due to unanticipated forces caused by temperature, restained shrinkage, and similar effects."

A few more points:

1. The first edition of the IBC (2000) modified the seismic design provisions of ACI 318-99 with inter-alia Sec 1910.4.3 Anchor bolts in the top of columns...this section is basically the same language you find in the seismic provisions of the UBC-97

2. ACI 318-99 did not have this provision and that edition was the referenced standard in IBC-00

3. IBC 2003 dropped this provision and presto..it now appears in ACI 318-02...satisfying all SDCs and CA's SZ 3 and 4. Why do you think this happened?

4. The language of the ACI 318-02 provision is basically the same as in IBC-00, UBC 76 thru 97 etc

So I submit to you, that the new provision in ACI 318 for ties engaging bolts/bars does have its genesis in the seismic requirements of the UBC..and while the commentary does not state so (not enough space to provide a complete historical account I guess), items 1-4 is clear evidence that the provision evolved from and still takes into account seismic considerations.

This is an issue I had looked into in the past as I'm often curious about the reasons for certain code provisions. If I can find a link which has an article outlining the background to this provision I shall post it.