Fabrication Dwgs

[SAIL3]

I have a question about the loads that a fabricator usually uses to design connections when member loads are not indicated on the engineering dwgs....in this case the fabricator said that they use the loads recommended by AISC...
my question are:
on compression braces...would this mean that they assume full capacity of the brace in tension instead of the capacity of the brace in compression which would be a much smaller load..
on beams...what load would they assume for the bm in calculating the shear in the connection..again is it full capacity, fully braced?
thanks..

 

[JedClampett]

This is the danger in just blindly letting the fabricator design the connections. I don't know anywhere where "loads" are recommended by AISC. They have some typical connections based on beam depths and these connections, in turn, have capacities, but a welded capacity is different than a bolted connection capacity, etc. In addition, there are seated connections, with different capacities. The good news is that fabricators are pretty reputable and the standard connections are pretty stout, so there's almost never an issue. A standard bolted connection will not control over the bending capacity of a beam, EXCEPT in very rare design cases.
I recommend either calling out typical connections that you're comfortable with the capacities of or at a minimum, saying "Connections should develop the capacity of an AISC Table 10-1 Connections with 3/4 inch A325 SC Bolts" or something like that, so there's no misunderstanding.
As far as braces, luckily almost any connection will develop the capacity of a brace. But it is an interesting case. Once again, a note or detail will work wonders.
I'm guessing that from your question, it's too late to correct the project you're asking about.

 

[SAIL3]

thanks Jed.....he is bidding off my engineering sketches that show only preliminary sizes and no loads....no time to wait for final design...fab and design has to overlap because of tite schedule...I understand what said about unusual type conn..these are all typical brace or bm conn....what I am trying to head-off is a conn that they would come up with that would be over-designed by 100 to 200%...

 

[connectegr]

The fabricator does not have a "standard" rule for determining connection design forces. The AISC Code of Standard Practice requires that the EOR provide adequate force information for the design of connections not provided complete. Ideally this information is the actual forces required, shear, moment, axial, and torsion. This information is easily obtained from the structural model.

But, in may cases general guidelines are given. A percentage of UDL for shear, often with a composite factor. Percentages of the moment or axial capacity of the member. The EOR must verify that this percentage is conservative enough to encompass the actual force. The fabricator can not verify any force information given. This method is typically very conservative and of little benefit to the overall project.

Refer to the Code of Standard Practice for industry guidelines on information required for connection design.

http://www.FerrellEngineering.com

 

[hokie66]

Our typical details, which can just be copied and included with preliminary plans, include all the information required for the standard connections. I think that is more important than worrying about a few connections that may turn out to be "overdesigned". For non-typical connections like struts which require knife plates to be stiffened, the fabricator just has to wait.

 

[nutte]

Jed, I would disagree with a couple of your statements.

A standard bolted connection will not control over the bending capacity of a beam, EXCEPT in very rare design cases.

Bolt bearing on the beam web, or bending on the coped section, often controls in the design of a bolted connection. The reaction resulting from the maximum bending capacity of the beam will often be impossible to achieve without adding reinforcing to the beam. A "standard" connection won't do it. This problem is exacerbated with shallow beams, beams with thin webs, and beams with short spans relative to their depth.

As far as braces, luckily almost any connection will develop the capacity of a brace.

Almost any connection _type_ can be sized to develop the member's strength. Tensile rupture on the net section will sometimes make it impossible to reach the member's strength without reinforcing.

As for the original post, and the fabricator saying they use AISC recommendations, there are no AISC recommendations. The requirements have to come from the engineer of record. It sounds like this fabricator is saying what he thinks you want to hear. Rather than wondering what loads he's designing for, you need to be providing the loads.

 

[JedClampett]

I stand by my statements. Maybe the capacity of the connection is not just the number of bolts times their shear capacity, but even if bearing or coped sections control, it's more than the bending capacity of the member. And that's if the full bending capacity is required.
As far as the braces, once again, the connection capacity can be reduced using tensile rupture or whatever. But in the range of most designs, two 3/4 inch bolts (or whatever the fabricator comes up with) will be sufficient.
My point was that fabricators, left on their own devices, will provide sufficient connections 90% or more of the time. But, we as design engineers, need to provide designs that work 100% of the time. And leaving all the connection work to fabricators is toom much risk in this regard.

 

[SAIL3]

more and more nowadys projects are on fast track..so all the phases of a normal schedule which would be staggered are overlapped and as a result the way we design radically changes...
..
in this type of environment, being conerned about the rupture capacity of one bold is not even in the mix...by intuition and experience you size the members..maybe some rough cal's..issue preliminary..then everybody runs with it..fabrication , construction etc..by the time you get around to doing some real design on it, some of it may be already fabricated, fdn's poured and maybe some erected....compare that with the trend of the codes today that are busey, busey coming up with more and extensive eguations and conditions on the premise that this may squeeze another 2 or 3% out of the weight of stl....somebody is not living in the real world..laughable , really..
anyway, thanks for all the input, I think I have enough info to influence the fabricator and quide him in a more reasable direction...

 

[JedClampett]

SAIL3, thanks for having my back.
In our case, we have a lot of light W8's and W10 roof framing, mostly controlled by deflection. Sophisticated connection design is not required. But we don't want shoddy connections either.

 

[Andries]

There a saying: "Anyone can design a structure but it takes an engineer to design a joint".
Joints are often where problems occur so one must be careful.

 

[ToadJones]

Jed-
I think many of your comments here must be with your typical light construction in mind.

On some of the large industrial projects I work on, "(2) 3/4" Bolts" in a brace might be what is required for the weight of the brace!