## Braced Frame Connections

## Braced Frame Connections

(OP)

In the AISC Seismic Design Manual design examples for SCBF I am not understanding the f peak and f avg on the welds. At the chevron gusset plate, figure 3-13, the weld has shear,axial and bending. In determing the average, the axial and bending are conbined. The bending has a + or - sign associated with it, but because the axial is very small and the quantity fa +/- fb is squared, the signs are lost and the average is equal to the peak. Later when they are checking the gusset to beam or column connection in figure 3-16, they state that because there is no bending, the peak is the average. This seems to be incorrect at the chevron connection in figure 3-13. Can anyone enlighten me on this seemingly discrepancy?

## RE: Braced Frame Connections

I am not sure which of those you disagree with - please clarify.

## RE: Braced Frame Connections

Have you looked at the Steel Construction Manual Part 13 on brace connections? Also, there are brace example problems for the Steel Construction Manual that can be downloaded from AISC.

## RE: Braced Frame Connections

I am not sure that the AISC Seismic provision example the Struct1007 is talking about uses the Uniform Force method. When I rechecked the example there are indeed moments at the connections - which is correct. There are areas of the connection that don't have moments - like between the gusset and column, but this is because the forces here are coupled and can resolve the moment in the column without it impacting the gusset. There are moments between the gusset and beam due to unbalanced forces, which would be non-existent if the uniform force method was being used.

Struct1007 - please clarify your issue when you get time.

## RE: Braced Frame Connections

## RE: Braced Frame Connections

The example you are referring to does in fact come up with different values for both the Favg and Fpeak, it's is just the values are so similar that the answer was rounded to 16.1kips/in.

If you check the calculations by hand you will find that...

Fpeak = 16.1158 kips/in

and

Favg = 16.06 kips/in.

You see that they aren't exactly the same, but very close. This doesn't happen unless you Faxial stress is very low.

That make sense?

You pretty much nailed it by noticing that axial stress is so low - and that is your answer why the avg and peak are so similar in value. It is purely a mathematical issue.

## RE: Braced Frame Connections

## RE: Braced Frame Connections

As far as I understand it the elastic method for determining weld stresses isn't exact. It is taking the extreme fiber bending stress of the weld and assuming it is constant over the full weld - as you noticed.