75% UDL Capacity of Beam 2

[Veer007]

Hey guys, do anyone encounters the situation, I have a note from the contract dwgs which says "beams connection to be 3/4th of max UDL capacity of the beam" which means 75%. Why even we have to connect 75% which means 150% by both ends added up.
If a beam has 100% UDL load as per AISC, simply we can connect 50% at each end, Anyone gets me the way?

Thanks in advance!!

 

[DrZoidberWoop]

I run into EORs who require different percentages of UDL for connection design quite often. It's a pain in the butt and results in ridiculously conservative connections for short spans. However, if that's in your contract, either address it in the kickoff meeting or make some phone calls and write an RFI verifying it's what the EOR wants. You can't arbitrarily decide to use %50 instead of %75 because it makes you liable, and will likely be rejected when you submit for EOR approval.

 

[JAE]

Or is the EOR taking the maximum allowable design load from the table, times the beam length and then you determine the end shears from that, and design your end connections for 75% of those shears.

So the EOR is actually being less conservative. Perhaps they checked all their different end reactions and found that none were over that 75% derivation.

 

[phamENG]

Sounds like the EOR is trying to be conservative by accounting for some sort of "imbalance" in the max UDL. Take the full UDL, combine it into a point load, and shift it to the quarter point. The connection closest to it will see 75% of the UDL.

Since the UDL is given in kips, 75% would be 0.75*UDL (as listed in the manual). So I don't think they're talking about 75% of the UDL induced shears, JAE.

 

[phamENG]

To DrZ's point - this is REALLY conservative, especially if you're in a short span. When I was brand new I relied on a typical note like that (business as usually so reviewers didn't think anything of it) - unfortunately there were a couple of beams that had to be less than 10' long, and even a W8 that short can do a whole lot more than a 2 bolt shear tab can carry. Made for some really confused RFI's from the detailer and I learned a valuable lesson. It takes a bit more work to show them accurately, but it's better to show the actual reactions for design as the percentage cop out can have some expensive unintended consequences.

 

[Veer007]

Guys, if a beam has 90kips as total UDL load then 75% is 67.5 kips, if one end to be connected 67.5 kips and another end 67.5 kips, we have to connect 135 kips which is more than total UDL capacity of the Beam. Why we??????

Thanks in advance!!

 

[phamENG]

Because that's what the EOR decided. Remember - the UDL is the maximum amount of load that can be carried if it is distributed evenly. I could probably take that beam and and put two point loads on it, 67.5kips each, 8 inches from each end. Each end connection would see the 67.5kips, total load carried would exceed the table value, but an in depth analysis of the beam using the actual design spec would pass and the beam would be fully functional, serviceable, and strong.

If you think a connection is too weak, then absolutely say something. If the connection load makes the connection unreasonable, ask the question to verify and explain why. But if the connection can be done based on the what EOR stated - then do it.

 

[Veer007]

I could probably take that beam and and put two point loads on it, 67.5kips each, 8 inches from each end. Each end connection would see the 67.5kips, total load carried would exceed the table value

Ifso, the beam will take more load than Tabled as all load scheduled in AISC book is just UDL not the actual reaction of the beam, Right? This one Makes sense.

Thanks in advance!!

 

[phamENG]

Correct. That table is just a useful tool for designers and must be applied appropriately. You can use a screwdriver as a hammer, but it's not usually a good idea. Similarly, that table should only be used if uniform loading is being applied, or as a preliminary design tool to develop initial sizes. It's often used for connection design, though, and while it may work well enough in a lot of cases, it's generally better to provide actual reactions.

Now I'm going to contradict myself. Say the design is for a single story shopping center. This building is likely to see several tenant changes over its design life, and will probably be kept in service well beyond its intended design life. In this sort of a case, the EOR's 75% note can make sense. It's impossible to know where an RTU is going to be installed 30 years from now. Beams are pretty easy to modify for additional capacity - weld on some plates and angles and you're good to go. Connections can be harder to modify in place. So by cranking up the connection capacity (within reason), they can ensure maximum flexibility of use later for a minimum of cost now.

 

[skeletron]

Send an RFI and point out where the situation is a problem to the EOR. That's the best you can do in this situation while maintaining your liability.

I've only ever seen it as "one-half UDL" meaning the maximum total load from a UDL divided by two, at each end. So the 75% UDL seems like an extra 50% added capacity. Connection stronger than beam? Sure. Accounting for point loads or unbalances? Possibly. Laziness as liability protection in the structural design? Very likely.

 

[dik]

Veer007:
"Guys, if a beam has 90kips as total UDL load then 75% is 67.5 kips, if one end to be connected 67.5 kips and another end 67.5 kips, we have to connect 135 kips which is more than total UDL capacity of the Beam. Why we??????"

I would read that as 1/2 the 67.5K... as reactions not the total load.

Dik

 

[Hokie93]

Are the beams in question composite? Using 50% of the tabulated UDL can be unconservative for composite beams. I have seen engineers specify 60% of UDL for the shear connection at each end of composite beams.

 

[dik]

good point Hokie...

Dik

 

[IDS]

Why does being composite affect the end reaction?

The design UDL is a simplification of the actual maximum load, which is likely to be nowhere near uniform, so it is quite reasonable for the design load for a bolted connection to be more than 50% of the total design load on the beam.

If that note was part of the contract documents for a project I was working on, I would just follow the conservative interpretation of what it said.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JAE]

Why does being composite affect the end reaction?

Composite beams can take a lot more load than that shown in the AISC tables for non-composite beams.

 

[IDS]

Composite beams can take a lot more load than that shown in the AISC tables for non-composite beams.

Why wouldn't you design the connections for the actual design load then, rather than applying a small factor to the loads in a table intended for a different loading condition?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[dik]

IDS:
Most engineers don't put the reactions on their drawings.

Dik

 

[skeletron]

It's such a contentious issue. The designer issuing a loading table all of a sudden takes liability for providing exact reactions (or rounded reactions) that could have errors from typographical or transferring between model and drawing during updates. But in the connection designer's point of view, you have the model for the design, technology is so integrated that you have the ability to write scripts to automate the table, so why not and save some money. There's a big difference between an 5/16" weld and a 1/4" weld when it's repeated 100x.

 

[Veer007]

Guys if it is composite beam we have to connect 150% of udl totally for a single beam?? Still be a question??

Thanks in advance!!

 

[human909]

How common is it in the US for connections design to be off loaded to others like this? I find this surprising. (Though as we all know, there is significant variation in the way things are done depending on localities.)

For simple shear connections it relatively easy to tabulate plate/bolt details according to beam depth that will have you covered for 99% of connections. There is not much to be saved by penny pinching on most simple shear connections. And for moment connections there is often money to be saved by considering connections in the context of columns and beams being used.