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!!

 

[JAE]

Here's a link to an article in Modern Steel Construction magazine that outlines the three methods specified in the AISC Code of Standard Practice for connection designs.

https://www.aisc.org/globalassets/modern-steel/archives/2009/05/2009v05_connection_design.pdf

Veer007's original post suggests that the EOR on their project was using Option 2 as described in the article.

Veer007, if your beams are composite, then they are indeed capable of resisting, sometimes, far more load than the standard WF beam tables in the AISC Manual.
If composite, you have two choices:
1. You can assume that the EOR has gone through all their end reactions and has assured themselves that all the project beam reactions are less than the value of the end reaction (75% of the total beam capacity (i.e. both ends can carry 150% of the tabulated capacity).....or
2. You assume that the EOR screwed up and pasted their "standard" note on the drawings not remembering that composite beams can carry much more load.

I would tend to assume the worst - choice 2 - to be safe and avoid a serious connection problem (failure) later - and call the EOR to verify as others here have suggested.

 

[Veer007]

1. You can assume that the EOR has gone through all their end reactions and has assured themselves that all the project beam reactions are less than the value of the end reaction (75% of the total beam capacity (i.e. both ends can carry 150% of the tabulated capacity).....or

This one makes sense. But one thing, if a beam is composite than load will be distributed through uniformly, why we can't take Max UDL shown in AISC? By the way, we have to add such a factor to strengthen the connection, Is it okay the capacity needs to be connected to more than what the beam can max carrying the load?

AlSo then non-composite beams need to be connected 50% of UDL. Right?

Guys, please post your concerns rather asking EOR, I have seen this many of the contract dwgs, I want to know whats the theory actually behind this...

Thanks in advance!!

 

[mtu1972]

Most of my career involved industrial buildings. We typically did a general note regarding connection loads with a UNO on drawings. Special reactions were noted somewhere on the plans or beam schedules. We only detailed those connections that we had specific needs that were required.

Because the uses within those facilities can change over time, we’d often find existing connections that would no longer work for the new conditions. Because of that, my last employer had a note that was conservative (similar to what you’ve described) so as to minimize future problems.

gjc

 

[human909]

AlSo then non-composite beams need to be connected 50% of UDL. Right?

No. That is a dangerous assumption to make. A composite beam might have a significantly greater shear than 50% of UDL!

 

[Veer007]

No. That is a dangerous assumption to make. A composite beam might have a significantly greater shear than 50% of UDL!

Are non-composite beams need more than 50% of UDL at each end? (100% total), am wondering.

Thanks in advance!!

 

[r13]

No. If you are confident in that the beam will be loaded in the same manner as the beam table indicated - an uniform load with intensity equal to, or less then the value given by the table.

 

[phamENG]

Veer007 - yes, sometimes a beam will be loaded such that the end reaction exceeds 50% of what the table indicates. That table is the maximum load a beam can carry if it is spread uniformly along the beam's length. It is a tool to assist designers in selecting a beam size, nothing more. In all but the simplest of structures, the beam should be verified using the applicable sections of the specification (the design tools provided in the steel construction manual are based on the specification, but are not a material part of it).

It may be a moving load, so sometimes one end will see most of the load and sometimes the other. So, in that case, both end connections would need to be designed for much more than 50% of the max UDL. They won't both be loaded to capacity at the same time, but they both need to have that higher capacity at some time during the service life of the structure.

As I mentioned before, it may be a non-uniform load. If you concentrate most of the load toward the ends, the moment at mid-span will be much lower for the same total load. Therefore, the end reactions will exceed what the table shows.

In short, that table shows information for an ideal situation that, though frequently designed for out of expediency, almost never occurs in a real structure.

 

[cliff234]

I may be repeating what was already said, but this is my 2 cents...

This is a common bad practice with many engineers. The reason for requiring connections to be designed to support a reaction resulting from a UTL = 150% of the AISC Manual Table 3-6 value (reaction = 0.75 of Table 3-6 load) is that composite construction will usually increase the load capacity of beams, on average by “about” 50%. (The Table 3-6 loads are for non-composite beams.) But this is a very inaccurate value. The actual increase in load capacity can vary from 20% to 100% depending on whether a beam is partially composite or fully composite. I always scratch my head when I see engineers compute wind pressures to two decimal places (i.e., 25.37 psf versus 25 psf, and seismic accelerations to three decimal places (can we really predict seismic accelerations to three decimal places???), but when you ask some engineers for beam reactions (due to REAL gravity loads) they’ll just say, “Oh, give me a connection good for supporting a reaction from 150% of the Table 3-6 beam load capacity.” This practice can also be dangerous and lead to seriously understrength connections. A common example is when heavy concentrated loads occur near the end a span. Another common situation (previously alluded to) is when a beam is loaded with studs and it working at 100% composite capacity. The best practice is to show the reactions on the framing plans. Another benefit of showing reactions on framing plans is that the EOR can more easily see the flow of the load through the floor framing if the reactions are shown. When I look at a framing plan with no reactions, I feel like I’m flying blind. I don’t have a feel for how hard the framing is working. Another benefit of showing the reactions is that fabricators will be able to design more cost efficient connections, the bids will be more competitive and your projects will be more economical. Solution: Always show the reactions on your framing plans! It will make your structures safer and your life easier!

 

[Veer007]

Solution: Always show the reactions on your framing plans! It will make your structures safer and your life easier!

If EOR doesn't address properly it might be an issue but its a good idea.

Thanks in advance!!

 

[Veer007]

Understood very well, thanks for the info dude...

Thanks in advance!!

 

[Veer007]

Below snap is from one more contract dwg which says 80% of UDL for beam connections for composite beams. Which means a composite beam to be in min moment and max reaction force. Right?

Thanks in advance!!

 

[phamENG]

Essentially. I'm not sure how familiar you are with composite design, so my apologies if I'm stating something obvious to you.

In a composite beam, you consider both the steel and some width of the concrete slab above it as the flexural member. The concrete is in the compression zone, and the steel is in the tension zone. Your neutral axis will be somewhere near the top flange of the beam. In some cases, the NA is in the concrete. In this case, you can consider the beam to effectively be in pure flexural tension, while the concrete is in compression. If it's below, then MOST of the beam is in tension, a little in compression, and the concrete entirely in compression. In either case, you get a drastically higher section modulus and much higher flexural capacity than just the steel beam. For a demonstration, look at Table 3-19, "Composite W-Shapes, Available Strength in Flexure." It gives the moment capacity of the beam, and then a table of the possible composite capacities.

But....all that load has to pass back through the beam in shear. So the shear connections are still designed to take the whole reaction. Designing using the composite capacity table, you'll end up with between 130% and 150% of the steel beam flexural capacity (acting alone). In the sense of a uniform load, the moment varies linearly with length for a constant total load (the UDL will decrease as length increases to maintain the total value constant). That means, for a given length, the composite beam can carry between 130 and 150% of the uniform load in Table 3-6. That means each end would see a reaction equal to between 65% to 75% of the total load in Table 3-6. By saying you need to design the capacity of the connections for 80% of the table 3-6 value for composite beams, you have at least about a 5% buffer for the vast majority of composite sections designed using the design table.

I'm not advocating this approach. The way the note is worded it may just be a CYA note in case a drawing slips through QC without a proper reaction schedule for the connection detailer.

 

[r13]

I prefer to provide table that lists design loads. However, I'll give the benefit of doubt for the percentage given by the designer, as he knew the support system better, and what he is doing.

 

[Veer007]

thanks for the info, Now I just clear.
thank you..

Thanks in advance!!