Recommended Base Plate Thicknesses AISC Manual

[bear24]

I am evaluating an existing structure with W-shape columns for some proposed increased loading. Anchor bolts are located inside of the web and base plate only cantilevers an inch outside of the flanges. I have checked the columns themselves and concrete foundations are adequate for the increased loading. However, the additional load is 9% higher than the max load the base plates can have using the Recommended Base Plate Thickness equations 14-7 in the Steel Manual. Any recommendations for increasing the strength of base plates?

 

[r13]

Is the base plate fail in uplift, or...?

 

[DaveAtkins]

I would be OK with 9 percent overstress. Base plate failure is not something I hear about. Once the base plate begins to yield, even a little, the bearing pressure concentrates itself at other locations, under the flanges and web.

DaveAtkins

 

[KootK]

1) How thick is the existing base plate? With only 1" overhang, I'd struggle to see that as a true, Bernoulli bending situation unless the base plate is 1/4" thick. More like some steel version of strut and tie.

2) I'd suggest stiffeners but, with only a 1" cantilever, there's not much to work with for improving things.

3) At an over stress of only 9% over stress, I'd be trying pretty hard to make this a "do nothing" situation. Live load reduction, shave the loads down, use plastic steel section properties... whatever.

 

[bear24]

Base plate is only 3/4" thick. I have not done any analysis more detailed than the equations in 14-7 manual. The over-stress is not a result of uplift.

 

[KootK]

Yeah, at a shear span ratio of [1"/0.75" = 1.33], that's not even real bending in the Bernoulli sense that the AISC procedure is treating it. I wouldn't sweat this.

 

[Hokie93]

Are you operating under an "existing" building code, like IEBC? IEBC permits a 5% stress increase in existing structural elements before strengthening is required. Are you using ASD or LRFD? Depending on the live-to-dead load ratio, use of one (or the other) might result in a 4% capacity increase.

 

[JLNJ]

Those equations assume a uniform stress under a portion of the base plate, regardless of bearing stress on the concrete.

Figure the bearing area you need if the bearing stress is at the allowable max, then distribute this area under the column flanges and web. Then look at the base plate based on this localized stress distribution. Maybe you can make it work like this.

Sometimes plate designs get thicker when the plate gets larger. This always seems non-sensical to me. Base plates under bearing rarely actually fail in bending. Use the full bearing strength of the concrete - that’s likely the real load path. Load chooses a path of axial stiffness over flexural flexibility every time.

 

[Hokie93]

Out of curiosity, what governed between m, n, and λn'? Given the one-inch overhang, it may be that bending between the flanges is critical.

 

[r13]

Check concrete strength. If it is adequate, I won't worry the base plate been a little thinner.

 

[KootK]

This is basically the procedure that JLNJ described: Link

 

[bear24]

λn' governed. I got it to work with LL reduction calculation. thanks for the help.