Unfactored load_Steel Structures

[Veer007]

Hi guys, so far from the contract dwgs were denoted given loads/forces are unfactored. So, Do I need to use the ASD method?

And the reaction forces were not given, but the note says that connect all connections for 50% of UDL, Is this fine or Better to ask reaction force for each location as steel beam sizes are heavy.

Thanks in advance!!

 

[skeletron]

1. Most steel design is done with limit states (LSD or LRFD). Unfactored loads are likely provided so you can calculate service deflection but also design beams/connections for governing factored loads.

2. 50% UDL is usually a pretty heavy reaction force. It is equivalent to the end reaction that will resolve a UDL at the beam's full moment capacity. It would be wise to check back with the designer and get specific reaction forces.

 

[jayrod12]

Agreed 1/2 udl is usually conservative. Although we have that same typical note. However, if I have many long span beams knowing they're governed by deflection, I'll usually provide those loads.

 

[phamENG]

It may say the loads are unfactored, but dig a little deeper to see if they specifically mention ASD or LRFD (or LSD, if this is a Canadian job - I think I remember you saying your employer handles both). They provide it unfactored so you can run your own load combinations. Those combinations should, however, be the same ones they used to design the building. If the didn't specify, send an RFI. skeletron is right about LRFD/LSD - if you're in Canada or Europe. There are still a whole lot of engineers in the US that are holding on to ASD for dear life.

If they said 50% of UDL, that's what they get. I believe the code of standard practice specifically states that it's not your job to give them a peer review. That said, don't go through it with your eyes closed. If you come to a beam that has another beam framing into it really close to the end, give it some thought. I've seen a few "50%" beams that wound up with connections that were too weak because the load was concentrated to one end of the beam. The other thing to look for is really short beams. The UDL will max out to the shear capacity of the member if they get short enough, and designing a shear tab to meet the full shear capacity of a member is usually pretty silly.

 

[WeezerMike1]

If they are giving you "unfactored loads" that should mean that they're providing you with the actual values for D, L, Lr, E, W, H, etc. Then you plug those numbers into either the ASD or LRFD load combinations per the code.

 

[EDB9]

I am curious how the loads are being presented. Is it a listing giving you typical loads for DL, LL, Lr, etc? If you’re not getting reactions to design with, are you tasked to do the structural analysis or are they wanting you to just do the connection designs? If you’ve already got beam sizes, how did they size the members?! I feel like there’s a missing piece here.

Structural steel can be designed using either methods. ASD was quite the popular choice and is still in the 9th edition... LRFD is also good to design by as it is now more common. Either method works but I do notice more conservatism in LRFD. If you have unfactored loads, you can use the ASD load combos or you can choose to factor them using the LRFD load combos. You probably got unfactored loads so you can crunch deflections. Personally, I prefer to design steel in LRFD as that was how I learned it back in my college days with the 13th edition, but I use ASD more often. Mostly because I was working on facilities that had to keep their original code of record for licensing reason (power plants) and they were all ASD, either 6th or 7th edition. A lot has changed over those times!

Erica

 

[Veer007]

I was asking for connection design.. as phamENG stated 50% of UDL doesn't work at beam has concentrated load @ one end, moreover beam span is short will take larger shear capacity, it's not economical for connection design for the given load.. agreed..

From my understanding please confirm following are correct or wrong..

If beam designed for service loads, then loads are unfactored, connection design to be ASD method, also better to design connection as slip critical?..


If beam designed for ultimate loads, then loads are factored, connection design to be LRFD (or LSD per CISC) method, also okay to design connection as bearing type?..

I think per CISC standard they are using only factored loads, correct?





Thanks in advance!!

 

[EDB9]

You have to determine which limit state controls for the connection design. SC connections can be a bit overkill if it’s not controlling the design in my experience. While they will work, you could wind up installing twice as much hardware than truly needed.

Here’s a very short paper that gives some good suggestions.

https://www.aisc.org/globalassets/modern-steel/archives/2016/01/steelwise.pdf

Erica
Structural and Geotechnical Engineer (yes I know this isn’t a typical combo)

 

[phamENG]

Most of these questions should really be asked of the EOR. It's his/ her design and will have the ultimate say. I think the only way you can give us enough information to answer your questions specifically is to post the drawings, which I realize you can't do.

I think I see it a little more clearly, though. You're dealing with a pair of typical notes the EOR puts on all their steel drawings. They say loads shown are unfactored in case they show loads, but they don't. Then they give you the 50% criteria.

If the drawings don't say ASD or LRFD, send an RFI and ask which method the EOR used to size the beams.

ASD and LRFD have nothing to do with slip critical connections. If the drawings don't say slip critical, design them as bearing. If you have doubts, ask the EOR.

I don't know a single EOR who would complain about a call/ email from a fabricators engineer if it means being able to approve the calculations on the first pass.

 

[Veer007]

Guys, below are the table which I got from typical detail.

Thanks in advance!!

 

[Veer007]

And this is for the beam to beam connection detail.

Thanks in advance!!

 

[Veer007]

If, Loads are unfactored which does not mean ASD?

Thanks in advance!!

 

[phamENG]

If I had to assume, I would say yes.

Fortunately, we don't have to assume. Send an RFI! Call the EOR on the phone and say "Hey - I noticed you listed the reactions as "unfactored". That means you want the connections designed for ASD, right? Yes? Thanks. Thought so. A formal RFI is being routed your way to confirm. Thanks." Or, if they say No, ask them for a breakdown of unfactored loads OR worst load combination factored reactions. Once you have it, send it back to them in a formal RFI starting with "Per our phone conversation of <DATE>, beam connections are to be designed...."