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Shear in slab desig need to be considered or not 2
- Thread starter tngv752
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rowingengineer
Structural
You should design for shear in the slab, be it punching or beam. I would not expect beam shear to be a problem in most slabs (especially two-way slabs) unless you have penetrations or heavily a loaded/reinforced slabs, in these situations it can be of significance.
When in doubt, just take the next small step.
When in doubt, just take the next small step.
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structuresguy
Structural
You absolutely must check shear for slabs. For concrete flat slabs supported on columns, shear failure is usually the critical failure mode. Even for wall one shear along say a supporting wall, shear can be a problem.
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apriley, you may be thinking of the minimum shear steel provisions.
All codes require a shear check (long. and punching if applicable). Most codes don't require minimum shear steel in some slabs under certain conditions. Whether there is shear steel or not you still have to check the shear strength. Recent research, and some catastrophic failures such as the Place de la Concorde overpass in Montreal, have shown that thicker slabs (say over a couple feet thick) should have minimum shear steel. This is more of an issue for bridges, unless you're looking at a heavy transfer slab or vehicle access slabs.
All codes require a shear check (long. and punching if applicable). Most codes don't require minimum shear steel in some slabs under certain conditions. Whether there is shear steel or not you still have to check the shear strength. Recent research, and some catastrophic failures such as the Place de la Concorde overpass in Montreal, have shown that thicker slabs (say over a couple feet thick) should have minimum shear steel. This is more of an issue for bridges, unless you're looking at a heavy transfer slab or vehicle access slabs.
Punching shear not only need to be checked especially in two way slabs, you also need to include the shear generated by the moment due to fixity between the column and the slab. In other words the punching shear stress distribution around the column is not uniform at the critical vertical stress plane.
If you have drop beams in a two way slab, the shear between the slab and beam is usually not be a problem since the final shear delivered to the column is through the beam which is already checked for shear. The same is true if you have drop beams in a one way slab. The same is also true if the slab is connected to a continuous wall.
The killer for two way slabs is almost always the punching shear (weakest link if I may), the solutions to the punching shear is always one of the following not in any particular order:
1) Nelson Studs or equal
2) Cross Shear head made of 2WXX sections moment welded together. A Cross Head is also sometimes made of top and bottom bars heavily reinforced with shear stirrups.
3) Drop panels
4) Column Capital aka Shear Head
If you have drop beams in a two way slab, the shear between the slab and beam is usually not be a problem since the final shear delivered to the column is through the beam which is already checked for shear. The same is true if you have drop beams in a one way slab. The same is also true if the slab is connected to a continuous wall.
The killer for two way slabs is almost always the punching shear (weakest link if I may), the solutions to the punching shear is always one of the following not in any particular order:
1) Nelson Studs or equal
2) Cross Shear head made of 2WXX sections moment welded together. A Cross Head is also sometimes made of top and bottom bars heavily reinforced with shear stirrups.
3) Drop panels
4) Column Capital aka Shear Head
I believe the OP was referencing one-way shear or beam action in the original question.
ACI 318-05 explicity states that one-way shear shall be investigated in Section 11.12.1 "The shear strength in slabs and footings in the vicinity of columns, concentrated loads, or reactions is governed by the more severe of two conditions: Beam action where each critical section to be investigated extends in a plane across the entire width or (two-way action)."
One-way shear should be checked.
ACI 318-05 explicity states that one-way shear shall be investigated in Section 11.12.1 "The shear strength in slabs and footings in the vicinity of columns, concentrated loads, or reactions is governed by the more severe of two conditions: Beam action where each critical section to be investigated extends in a plane across the entire width or (two-way action)."
One-way shear should be checked.
It is true that the program Slabs doesn't check beam shear in slabs.
However, the amount of load to get a slab to shear at a supporting wall is huge and usually causes bending failure first.
Punching shear is different altogether, but Slabs accounts for this.
However, the amount of load to get a slab to shear at a supporting wall is huge and usually causes bending failure first.
Punching shear is different altogether, but Slabs accounts for this.
rowingengineer
Structural
demayeng,
I make it a habit to not trust engineering programs full stop, I always do quick hand calcs. On top of that I don't trust a lot FEA programs for concrete design. The whole plastic vs. elastic situation, hotspots, stiffness criterion and most of all; relying on the factored up short-term deflections for long term deflection.
Thus you have a high level program relying on an old approximate answers to get deflections, why go to the extra effort when the program can’t even get you better approximations of deflection? Which to my knowledge is the most likely problem to occur onsite.
Thus it would not surprise me if Slabs didn't check for beams shear and I would also question whether it includes Mxy in it analysis of steel.
When in doubt, just take the next small step.
I make it a habit to not trust engineering programs full stop, I always do quick hand calcs. On top of that I don't trust a lot FEA programs for concrete design. The whole plastic vs. elastic situation, hotspots, stiffness criterion and most of all; relying on the factored up short-term deflections for long term deflection.
Thus you have a high level program relying on an old approximate answers to get deflections, why go to the extra effort when the program can’t even get you better approximations of deflection? Which to my knowledge is the most likely problem to occur onsite.
Thus it would not surprise me if Slabs didn't check for beams shear and I would also question whether it includes Mxy in it analysis of steel.
When in doubt, just take the next small step.
abusementpark
Structural
It seems to me like it would take a serious failure mechanism for a one-way shear failure to occur in slab. However, I am sure there are some exceptions and we should always keep in the back of our minds.
Anybody familiar with yield-line theory in reinforced concrete slabs??
Anybody familiar with yield-line theory in reinforced concrete slabs??
Be careful evaluating punching shear with engineering programs. I have not used SAP but I have used SAFE which is made by the same company and I have noticed that their punching shear values are not always correct especially at edge and corner columns.
I strongly recommend doing hand calcs.
I strongly recommend doing hand calcs.
rowingengineer,
Slabs appears to use all the same criteria out of AS3600 that we use in hand calcs, however it is able to work out all the parameters (moments, I effective, deflections) in 2 dimensions simultaneously...
Hand calcs seem to be a less accurate version of the same thing?
Although Slabs has a lot to be desired and could do with some proper support and further development, I find it very handy to use - once the model is built you can very easily tinker with slab and drop panel thicknesses without having to run numerous strips over and over. You do have to have a good understanding of how it works to use it accurately, i agree.
Haven't had any deflection problems so far, touch wood! or heard of any from the many other Slabs users..
Slabs appears to use all the same criteria out of AS3600 that we use in hand calcs, however it is able to work out all the parameters (moments, I effective, deflections) in 2 dimensions simultaneously...
Hand calcs seem to be a less accurate version of the same thing?
Although Slabs has a lot to be desired and could do with some proper support and further development, I find it very handy to use - once the model is built you can very easily tinker with slab and drop panel thicknesses without having to run numerous strips over and over. You do have to have a good understanding of how it works to use it accurately, i agree.
Haven't had any deflection problems so far, touch wood! or heard of any from the many other Slabs users..
I think this post has gone from whether a slab needs to be designed for one-way shear to acceptable use of computer packages for engineering design. If the engineer doesn't know what the computer package is doing, then they shouldn't be using the program. Personally I would favor my own spreadsheets for code calculations any day because I have tailored them to best suit my needs. I had a senior engineer in my office one day question the amount of shear reinforcement I had in a rc beam because of a model they built in a design package was telling them it needed more shear ties. What they didn't look at was the calculation where the effective width of the section they had modeled was completely wrong and giving an incorrect and lower concrete shear capacity. It is going to be the engineer who performs hand calcs that picks up those mistakes. The more you perform the hand calcs, the easier they are to check.
rowingengineer
Structural
tngv752,
My apologies for the change in topic, but I would like to see this one through. And everyone apologies for the long post.
demayeng,
Please don't get me wrong, engineering programs are tools of the trade and when in the hands of a capable engineer they can perform satisfactorily. But a few hand cals or spreadsheets will always catch those minor glitches in the system, as asixth has point out.
To use an old quote don’t know whom started it but it is a golden one “If a designer knows what results to expect from a computer program (within 5-10%), then he should be ok to use it, If not he is more than likely to get is wrong”. I use hand cals to get the (5-10%), some engineers may be able to do this with experience alone.
As for Slabs and deflection problems, I will speak only in general terms of FEA products, as I have only used Slab once before and it was before they really got serious.
The idea is generally to take FEA deflection and multiply it by 2 or 3 (Kcs) and call it a total long term deflection. Unless it has changed very recently (Slabs may work out Ieff for based on reo, If so it may be getting better than the last time i used it), cracking is defined by the designer as a factor of gross inertia for deflections (ie 0.2 Igross for slabs and 0.4 Igross for beams), it is not calculated based on the actual stresses at each cross-section.
The kcs factor for long-term effects for your comparison is not very good because kcs for beams and slabs is actually quite different because of the difference in the creep and shrinkage for the different section shapes.
Thus is you don’t include cracking in your analysis correctly and use Kcs to factor up your deflections the real deflections could easily be 2 to 5 times higher. I am currently sitting in a building where all the walls are cracked from settlement of the slab over time due to this issue (the person whom did the analysis didn’t include cracked section properties for his beams and slabs in his FE model), everything is strong enough but the deflections are L/100 and getting worse by the year.
You will get your best deflection estimates for RC and PT slabs from Calculations which include full cracking analysis and long term creep and shrinkage analysis.
As for the other issue I will steal a few notes from Rapt (hope you don’t mind rapt).
“An FEA programs generally allows for Mxy moments in the analysis and then ignores them in design then the difference will be at least 15% and possibly up to 25% UNDER estimation of the reinforcement requirements. This does not mean that you can use 15-25% less reinforcement, it means it has under designed by 15-25%.
It does not mean that FEA is wrong. FEA is reporting Mx, My and Mxy. It means that the design application tacked onto the end of the FEM is wrong because the developer does not understand design using FEM, or they are trying to cheat (hopefully it is the first but either is worrying).
This will become obvious below but we must distinguish between FEM analysis and software that produces reinforcement/prestress requirements based on FEM analysis. All FEM produces is a set of stresses which, when converted to moments, describe the moments on the floor system that have to be designed for.”
This is why I don’t trust FEA programs in general, While teh moment they give can be correct if you get the model correct (this would take to long to go through all the problems i have seen with people modeling in FEA programs), and just to show that i don't trust other 2D programs wither, I also have problems with 2D programs due to the fact that it they normally ignore torsion thus makes it very hard to do anaylsis with either compatibly or equilibrium torsions.
Sorry again for the lng post and I will get off my soap box now. now to eat some fish with my chips of my sholder.
When in doubt, just take the next small step.
My apologies for the change in topic, but I would like to see this one through. And everyone apologies for the long post.
demayeng,
Please don't get me wrong, engineering programs are tools of the trade and when in the hands of a capable engineer they can perform satisfactorily. But a few hand cals or spreadsheets will always catch those minor glitches in the system, as asixth has point out.
To use an old quote don’t know whom started it but it is a golden one “If a designer knows what results to expect from a computer program (within 5-10%), then he should be ok to use it, If not he is more than likely to get is wrong”. I use hand cals to get the (5-10%), some engineers may be able to do this with experience alone.
As for Slabs and deflection problems, I will speak only in general terms of FEA products, as I have only used Slab once before and it was before they really got serious.
The idea is generally to take FEA deflection and multiply it by 2 or 3 (Kcs) and call it a total long term deflection. Unless it has changed very recently (Slabs may work out Ieff for based on reo, If so it may be getting better than the last time i used it), cracking is defined by the designer as a factor of gross inertia for deflections (ie 0.2 Igross for slabs and 0.4 Igross for beams), it is not calculated based on the actual stresses at each cross-section.
The kcs factor for long-term effects for your comparison is not very good because kcs for beams and slabs is actually quite different because of the difference in the creep and shrinkage for the different section shapes.
Thus is you don’t include cracking in your analysis correctly and use Kcs to factor up your deflections the real deflections could easily be 2 to 5 times higher. I am currently sitting in a building where all the walls are cracked from settlement of the slab over time due to this issue (the person whom did the analysis didn’t include cracked section properties for his beams and slabs in his FE model), everything is strong enough but the deflections are L/100 and getting worse by the year.
You will get your best deflection estimates for RC and PT slabs from Calculations which include full cracking analysis and long term creep and shrinkage analysis.
As for the other issue I will steal a few notes from Rapt (hope you don’t mind rapt).
“An FEA programs generally allows for Mxy moments in the analysis and then ignores them in design then the difference will be at least 15% and possibly up to 25% UNDER estimation of the reinforcement requirements. This does not mean that you can use 15-25% less reinforcement, it means it has under designed by 15-25%.
It does not mean that FEA is wrong. FEA is reporting Mx, My and Mxy. It means that the design application tacked onto the end of the FEM is wrong because the developer does not understand design using FEM, or they are trying to cheat (hopefully it is the first but either is worrying).
This will become obvious below but we must distinguish between FEM analysis and software that produces reinforcement/prestress requirements based on FEM analysis. All FEM produces is a set of stresses which, when converted to moments, describe the moments on the floor system that have to be designed for.”
This is why I don’t trust FEA programs in general, While teh moment they give can be correct if you get the model correct (this would take to long to go through all the problems i have seen with people modeling in FEA programs), and just to show that i don't trust other 2D programs wither, I also have problems with 2D programs due to the fact that it they normally ignore torsion thus makes it very hard to do anaylsis with either compatibly or equilibrium torsions.
Sorry again for the lng post and I will get off my soap box now. now to eat some fish with my chips of my sholder.
When in doubt, just take the next small step.
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