I have an edge beam founded directly on the ground with steel columns attached to it every 3m as the picture shows. I am struggling understand how to handle the eccentric load from the columns. A colleague told me that the moment from moving the load to center edge beam will be taken care of the top reinforcement in the slab and that I then can calculate the concrete beam as a normal one with a centered load. What I am not understanding is that he says it is enough that the top reinforcement in the slab just needs to take care of 20kNm/m (300kN/3m*0,2). I dont understand this and asking if he is correct? If I would have columns in each 1m with a load of 100kN I would agree since the edge beam is 600mm high but in this case I am not sure. Will it work as he describes it?
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Eccentric load concrete edge beam
- Thread starter Danne_83
- Start date
The approach that your colleague has recommended is quite common. From a strength perspective, it checks out. Your load causes a clockwise torsion on the beam. And the flexural resistance of the slab provides a counter clockwise torsion on the beam. So, prior to collapse, everything would balance.
All that said, like all structures, your system will obey the fundamental principle that stiffness attracts load. So how much load goes where will depend on the interplay between the torsional stiffness of your beam and the flexural stiffness of your slab. In my opinion, at typical proportions, you'd have to torsionally crack the heck out of your beam before you'd fully engage the flexural stiffness of your slab.
I think that the thing that saves these setups most of the time is that the support reactions (piles or soil stress) usually provide a rectifying torsion such that it is not really necessary to engage the flexural resistance of the slab. If the beams where truly pinned at the ends torsionally, like rolling pins, I think that we'd see a lot more torsional distress in our grade beams and would therefore pay a lot more attention to serviceability issues.
You wind up with a system where the loads impose concentrated torques at the columns and the slab imposes a uniform restoring torque along the length of the beam. As such, you'll still get some torsion waxing and waning along the the length of the beam but it probably won't amount to terribly much.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
All that said, like all structures, your system will obey the fundamental principle that stiffness attracts load. So how much load goes where will depend on the interplay between the torsional stiffness of your beam and the flexural stiffness of your slab. In my opinion, at typical proportions, you'd have to torsionally crack the heck out of your beam before you'd fully engage the flexural stiffness of your slab.
I think that the thing that saves these setups most of the time is that the support reactions (piles or soil stress) usually provide a rectifying torsion such that it is not really necessary to engage the flexural resistance of the slab. If the beams where truly pinned at the ends torsionally, like rolling pins, I think that we'd see a lot more torsional distress in our grade beams and would therefore pay a lot more attention to serviceability issues.
OP said:
You wind up with a system where the loads impose concentrated torques at the columns and the slab imposes a uniform restoring torque along the length of the beam. As such, you'll still get some torsion waxing and waning along the the length of the beam but it probably won't amount to terribly much.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
If the column could be connected to the beam to resist moment, it could participate with the slab in resisting eccentricity but unfortunately, the column is so close to the edge, the outer anchor bolts may be fouled by the beam reinforcement.
An embed plate or angle anchored to the beam and welded to the column could be used but, like your detail, there would be no provision for grout.
BA
An embed plate or angle anchored to the beam and welded to the column could be used but, like your detail, there would be no provision for grout.
BA
I've never liked ignoring it since it has an effect on the beam's strength in the interaction check of torsion and shear. Another example is in an elevated slab case where some engineers also ignore torsion coming from the slab side where the slab twists the beam. I would most likely also check it in the posted case if there was a void form under the grade beam.
- Thread starter
- #5
Thank you all for the answers. It seems like we agree that it is not that simple to just ignore this issue. I did discuss it with the colleague today again and he was speaking about the stiffness of the beam and could not even see a problem in this case. He has been working for more than 40 years so I guess he knows what he is talking about. Then that I dont understand it completely is annoying and also a problem since I will need to verify my calculations in a document.
Are you posting service loads or factored loads? I assume 300kN is a factored load.
A moment of 60kN-m is applied every 3m. In order to spread uniformly into the slab, the 600x600 beam would have to resist a torsion of 30kN-m each side of each column.
I don't know which code you are using, and I'm not current on any code, but I believe the cracking moment of a 600x600 beam with f'c of 30MPa is about Tcr = 71kN-m. Since the torsion required in your case exceeds Tcr/4, you would need to provide torsional reinforcement.
BA
A moment of 60kN-m is applied every 3m. In order to spread uniformly into the slab, the 600x600 beam would have to resist a torsion of 30kN-m each side of each column.
I don't know which code you are using, and I'm not current on any code, but I believe the cracking moment of a 600x600 beam with f'c of 30MPa is about Tcr = 71kN-m. Since the torsion required in your case exceeds Tcr/4, you would need to provide torsional reinforcement.
BA
Danne_83:
You should probably not be getting your ‘young engineer’ help from that particular colleague given the fact that he is so dismissive. Given his 40 years of experience, he doesn’t really give that problem a second thought, other than to put some top stl. in the slab and grade beam, and doesn’t seem to appreciate or care that you are trying to understand. Talk to your boss about who might be a real good mentor within or outside the company, so you do have someone who is willing and patient enough to spend some time to really be helpful. I’m all for helping young engineers learn the ropes and understand some of the things that were glossed over in school. I’m often accused of being grumpy here on E-Tips, I just don’t suffer fools or laziness lightly. You do have to do your own homework, a serious effort at learning what you need to know. You should not have to (or be allowed to) ask the same question too many times, without a kick in the pants. But, we (good mentors) should be patient enough and willing to help, if we care, so you do understand the basics of an issue before we put it to rest. You have to come with well thought out questions, showing that you have given things considerable thought, maybe several possible solutions. Then, we can go over the pros and cons of each, and how and why we would do it this way. These mentors are freely giving you their time and efforts, so every once in a while, you should buy the beer and pizza, or the box lunches, to show your appreciation. A good and dependable mentor is really invaluable.
You should probably not be getting your ‘young engineer’ help from that particular colleague given the fact that he is so dismissive. Given his 40 years of experience, he doesn’t really give that problem a second thought, other than to put some top stl. in the slab and grade beam, and doesn’t seem to appreciate or care that you are trying to understand. Talk to your boss about who might be a real good mentor within or outside the company, so you do have someone who is willing and patient enough to spend some time to really be helpful. I’m all for helping young engineers learn the ropes and understand some of the things that were glossed over in school. I’m often accused of being grumpy here on E-Tips, I just don’t suffer fools or laziness lightly. You do have to do your own homework, a serious effort at learning what you need to know. You should not have to (or be allowed to) ask the same question too many times, without a kick in the pants. But, we (good mentors) should be patient enough and willing to help, if we care, so you do understand the basics of an issue before we put it to rest. You have to come with well thought out questions, showing that you have given things considerable thought, maybe several possible solutions. Then, we can go over the pros and cons of each, and how and why we would do it this way. These mentors are freely giving you their time and efforts, so every once in a while, you should buy the beer and pizza, or the box lunches, to show your appreciation. A good and dependable mentor is really invaluable.
- Thread starter
- #8
Good advise dhengr, I will give it a thought. I recently found this forum and it is an amazing forum with a lot of experienced and helpful people. Keep up the good work all! Hopefully I will be helpful sooner or later.
Yes it is 300kN in ULS condition. I am using Eurocode. I attached a file showing how he is reasoning about spreading out the loads between the columns. He said that I just can ignore the beam to the left of the marked area and check the area directly below the column as a beam. I did ask if it is not a problem that I will just have one leg working in that area from the stirrup but it should be enough he said. If I do a calculation on this "thin" beam it says it requires about 100mm2/m for shear reinforcement so I guess he is correct about that? Is it still completely wrong to see it as he is doing?
He has taught me that I will need to add torsional reinforcement if we have columns at a grade beam with piles below and I do not really see the difference between these cases. If I am going to add torsional reinforcement to this beam I will need to add about 15Ø16 bars and the grade beam is approximately 250m long. Of course in some areas I will have less loads but anyway it is quite an amount of rebars needed.
Yes it is 300kN in ULS condition. I am using Eurocode. I attached a file showing how he is reasoning about spreading out the loads between the columns. He said that I just can ignore the beam to the left of the marked area and check the area directly below the column as a beam. I did ask if it is not a problem that I will just have one leg working in that area from the stirrup but it should be enough he said. If I do a calculation on this "thin" beam it says it requires about 100mm2/m for shear reinforcement so I guess he is correct about that? Is it still completely wrong to see it as he is doing?
He has taught me that I will need to add torsional reinforcement if we have columns at a grade beam with piles below and I do not really see the difference between these cases. If I am going to add torsional reinforcement to this beam I will need to add about 15Ø16 bars and the grade beam is approximately 250m long. Of course in some areas I will have less loads but anyway it is quite an amount of rebars needed.
OP said:
It isn't completely wrong nor was it ever completely wrong. While I may not entirely agree with your colleague's simplifications, they are quite common in the industry and quite defensible. As you progress in structural engineering, you will find many instances where the "right" answer is not easily teased out. You will also find that, on many subjects, considerable diversity of opinion exists, even amongst very experienced and knowledgable engineers. You'll see that in spades here on this forum if you stick around.
It's healthy to question things and to seek out alternate opinions. That said, don't be too quick to write off the help that you're getting from your real world mentor. My mentors taught me all manner of simplifications that seemed lazy and negligent at the time. Now I use and teach those same shortcuts in order to keep work moving out the door efficiently and to make decisions in situations of uncertainty.
One of our members here has a signature that goes something like "there's a reason that everybody does it that way". And, generally, there really is.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
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