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AASHTO LRFD Metric Equation

AASHTO LRFD Metric Equation

AASHTO LRFD Metric Equation

I have a question regarding the metric version of the 1998 AASHTO LRFD Bridge Design Specifications.  Equation shows Sxe=Sx*1.38/(Ag+0.63)<=2000 mm where Ag, Sx, and Sxe are all given in millimeters.  The English Units equation is Sxe=Sx*1.38/(Ag+0.63)<=80 where Ag, Sx, and Sxe are all given in inches.  I have been developing a spread sheet that will use these equations in calculating sheer in both English and SI units.  So far everything converts correctly, but this SI equation causes the Sxe value to drop dramatically when Ag is entered in SI units when compared with the English unit value.  In my opinion, the 1.38 and 0.63 numbers should be unit specific and not the same regardless of what unit system one is using.  I have not been able to find any errata regardgin this and was wondering if anyone has come across this problem and if so, what they have done to account for this.  

An example, if the above is not clear, is I have a Sx value in english units of 10.54in.  When the same model is done in equivalent SI units, I get an SI of 265.94mm, which is expected using a rough 25.4mm per in.  Using Ag=1.5in, Sxe is then calculated as 6.83in in English units.  In SI units, however, using Ag=40mm (the SI equivalent to 1.5in), an Sxe of 9.03 is calculated, instead of something closer to 173mm.  This causes problems mainly when using Table and seems to cause the sheer capacity to be less conservative.

If anyone has any insight, I'd appreciate it.  Thanks.

RE: AASHTO LRFD Metric Equation

Thanks a lot for this post.

Actualy you are right.

We need to send a info to AASHTO, as even my version (2003-SI) has this mistake writen inside.(page is marked as changed in year 2000)(my page is 5-65.1).

The coeficients should be 1.38*25,4=35.052 and 0.63*25.4=16.002 and the result in SI units will be exactly 173mm.
Thanks again


RE: AASHTO LRFD Metric Equation

Thanks for your response Zliazmei.  That's my assumption a well.  I have contacted AASHTO about this, but they have not respnded to my email yet.  If I here anything from them, I'll be sure to post it here.

RE: AASHTO LRFD Metric Equation

Hi bridgebuilder.

Did you recieve any responce from AASHTO?.


RE: AASHTO LRFD Metric Equation


AASHTO has corrected this error in the 2003 interm.

Sxe=sx*(35/(ag+16))<=2000mm   eq (

Sx,Sxe & ag in mm


Does anyone know what the proper equation for ( calculating the alternative development length (page 5-147 2001 interim).  This equation as written in the 2001 interim seems to be incorrect as it does not correlate to eq  Is there another conversion issue with this formula?

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