High moment on base PL, anchor bolts

[akamike]

I have a base plate on a 54" sq pedestal with (2) 2-1/4 diameter x 27" anchor bolts on each of the four sides. There is a HUGE moment which puts the M/P eccentricity about 7 feet away from the center of the base plate. Assuming the base plate is thick enough that the failure is pullout or yield of anchor bolts, how do I calculate the compression stress block in the concrete pedestal? Thanks.

 

[aggman]

The procedure is pretty long to explain. Go to AISC's website and download Design Guide 1 (Column Base Plates). It will have the required procedure you are looking for. Also, Blodgett's Design of Welded Structures has some discussion on it in section 3.3.

 

[miecz]

Blodgett's Design of Welded Structures has a solution for "Base Plate for Column Loaded with Moment" under Column Related Design. Try the Basic Method (If Uplift).

 

[mrMikee]

Blodgett's long solution involves the solution of a cubic equation which is ok if you have Mathcad or Mathematica (or trial and error.) The basic method is good if you have a W shape with moment about the x-axis but for y-axis I would probably solve the general equation.

From what I remember the AISC is very comprehensive too.

Regards,
-Mike

 

[J1D]

A few years ago, I incidentally encountered some calculation inconsistency using this AISC design guide. The test is very simple, but the output is quite surprising.

Repeat Example 16 on page 23 (base plate with large eccentricity), simply change the Mu from 700 kip.in to 480 kip.in. The bending moment is reduced, but the base plate should be still part in tension, part in compression (large eccentricity) because: e = Mu/Pu = 5.45in > N/6 (=2.33in)

Then we have A = 4.04in and T = - 0.16 kip (compression)!

This is contradictory since T must be in tension with the loading.

Where, A = depth of compression zone; T = tension of bolts on the “tension” side.

Some one told me that they found the calculation “falling apart” as well. This phenomenon was verified with different inputs. The question is still outstanding up to now.