Wind Brace Connection - Modified Workpoint

[abusementpark]

For typical diagonal wind brace connections into beam-column joints, I have often seen where people recommend for you to modify the workpoint so that it occurs at the intersection of the outside edges of the beam and column, instead of at the intersection of the beam and column centerlines. This is said to result in a more economical gusset plate and it also avoids having to design the beam to column connection for a transfer force due to the brace load. However, it introduces additional moments into the beam and column, which have to be accounted for in the member analysis.

Does anyone use this method? If so, how do you account for the additional moments in your analysis?

I use RAM Structural System and RAM Connection and haven't found a way to do it.

 

[Lion06]

I haven't done it, but I've had the fabricator's engineer do it in their calcs and just put a note for the EOR to confirm that the member can take the additional moment.

I'll be honest - that kind of pissed me off. Our documents clearly showed our intent with the workpoint at the intersection of the centroids of the members. Of course, though, we look like the uncooperative bad guys if we don't do the extra work to check it.

Anyway, what I did is get the interaction ratio for each member. Then I added in the Mu/phiMn into the interaction ratio and made sure I was under 0.95. I know, I know.................. it doesn't include second order effects from the additional moment, but that's why I limited the interaction to 0.95.

 

[ToadJones]

doesnt the Uniform Force Method cover this?

 

[abusementpark]

I haven't done it, but I've had the fabricator's engineer do it in their calcs and just put a note for the EOR to confirm that the member can take the additional moment.

I'll be honest - that kind of pissed me off. Our documents clearly showed our intent with the workpoint at the intersection of the centroids of the members. Of course, though, we look like the uncooperative bad guys if we don't do the extra work to check it.

I would be pissed off as well. It wouldn't be unreasonable to reject it if the intent was clearly stated on the drawings. He should have at least submitted an RFI beforehand.

Anyway, what I did is get the interaction ratio for each member. Then I added in the Mu/phiMn into the interaction ratio and made sure I was under 0.95. I know, I know.................. it doesn't include second order effects from the additional moment, but that's why I limited the interaction to 0.95.

Haha, I'm sure that is further than most engineers go. Some engineers use the modified workpoint, but neglect its effect on the members.

 

[abusementpark]

doesnt the Uniform Force Method cover this?

The steel manual lists it as a special case of the Uniform Force Method. However, the loads effects aren't easy to incorporate into a computer program as far as I know.

 

[ToadJones]

Abuse- I see your point. Special Case 2, is that what they call it?

I have used the modified work point many times for bracing wind columns together to shorten unbraced lengths in industrial buildings and only attached the gusset to the beam. This is on the gable wall and not part of the MWFRS.
I have also used it on braced bays of very long low rise buildings. The bracing design itself was sometimes a "KL/r" design b/c the loading was so low....usually the strut beam was designed for slenderness and there was was no other loading.
I'll admit, in these cases I did not check for additional loading.

I do have a bracing connection design spreadsheet that includes the additional loading and does a rough check similar to Lion's approach.

 

[ishvaaag]

B1 of 360-05

"The design of members and connections shall be consistent with the intended behavior of the framing system and the assumptions made in the structural analysis"

Not the contrary way ... the connection designer is overtaking you as the designer. Another point of friction, since what is thought by connection designers is ... etc.

 

[WillisV]

When I show the modified workpoint on the drawings, I check the columns and beams for the additional moments by hand similiarly to Lion (making sure the interaction ratio can handle the additional moments). If a connection engineer moved my workpoint, I would reject it unless there was a very good reason they did so, or unless they would like to pay me to recheck my members so as to save their fabricator the extra gusset money.

 

[Lion06]

Willis, can you do that? I thought that would be some kind of conflict of interest - getting paid by two separate parties on the same project?

 

[WillisV]

Lion06, I'm not sure if I can, but I have! Sometimes the steel fabricator/g.c. hires us to do erection shoring etc. for projects I am the EOR on, no real difference here.

 

[ToadJones]

When doing an additional check using the additional moment in the beam from the connection, and say the beam end connection was not a moment connection, adding in additional moment at the end where the connection is probably very conservative as the original interaction ratio was likely determined at a different location.

 

[Lion06]

Toad-
You're 100% correct, but that was getting way more into it than I cared to........... especially at that time.

 

[ToadJones]

Lion06-
I agree. I find myself "throwing the kitchen sink" at existing design checks all the time. I guess I was just justifying why I don't always check it.