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Shear in slab desig need to be considered or not
2

Shear in slab desig need to be considered or not

Shear in slab desig need to be considered or not

(OP)
I use SAP2000 to design slab, I can shear result in slab.
 But I use another Slab software, the output results don't have shear in slab, just moment and reaction.
 Someone tell me that slab never fail by shear ?
 Seem to be right ?

RE: Shear in slab desig need to be considered or not

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.
 

RE: Shear in slab desig need to be considered or not

Obviously as a general tool for designing slabs, something that is not accounting for shear is a deficient one. Check the manual, it may be giving a warning on this. Whilst use the other.

RE: Shear in slab desig need to be considered or not

It is a very simple check to do by hand.

RE: Shear in slab desig need to be considered or not

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.

RE: Shear in slab desig need to be considered or not

There's a statement in bridge codes that says slabs designed for moment do not need to be checked for shear. Check your code - guidance should be there.

RE: Shear in slab desig need to be considered or not

For heavily loaded slabs (>100psf/5kPa) I have seen slabs designed with shear reinforcement at the face of columns where beam shear is going to be the highest. Punching shear checks always need to be performed.

RE: Shear in slab desig need to be considered or not

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.
 

RE: Shear in slab desig need to be considered or not

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

RE: Shear in slab desig need to be considered or not

I suggest designing a slab by hand, at least once, before relying on the software.  Then, you'd know the answer to the question.

tg

RE: Shear in slab desig need to be considered or not

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.

RE: Shear in slab desig need to be considered or not

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.

 

RE: Shear in slab desig need to be considered or not

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.
 

RE: Shear in slab desig need to be considered or not

what programs don't check for beam shear?  i usually check with quick hand calcs, too.  particularly punching shear.

RE: Shear in slab desig need to be considered or not

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??

RE: Shear in slab desig need to be considered or not

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.  

RE: Shear in slab desig need to be considered or not

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..

RE: Shear in slab desig need to be considered or not

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.

RE: Shear in slab desig need to be considered or not

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.
 

RE: Shear in slab desig need to be considered or not

tngv52,

You should check everything, until you have the experience and confidence to decide what is not critical.

 

RE: Shear in slab desig need to be considered or not

csd72,

That is one of the truest statements about engineering I've ever seen.

Talk about hitting the nail on the head - Star for you.

tg

RE: Shear in slab desig need to be considered or not

I think even though the original post was about shear in slabs, the change to discussing FEA in concrete design is relevant.
I agree with everyone saying you need to know how to check it by hand - it would be foolish to use these tools without knowing how they work. I believe that to increase their use and trust in them, FEA programs should be transparent in their documentation to show what calculation process they use (and if they don't, why they don't consider shear in slabs!!!).

It sounds like people trust their hand calcs over FEA because of accuracy errors in the programs, which is fair enough. But what if these developer errors were fixed and your checks validated the results?

ps, rowingengineer:  Slabs now has the option of using the Branson formula to work out Ieff for every finite element (plus you can set different shrinkage strains for either slabs or beams), as well as the approximate method you mentioned..
Thanks for the rapt note - i'll ask the makers of slabs what they say about that!

RE: Shear in slab desig need to be considered or not

Demayeng,
Sounds like Slabs is starting to make moves in the world. How dose Slabs handle wall/slab connections. Does slabs allow for I eff of the wall stiffness and columns?  This would be really starting to get close to the solution. How does it handle the hot spots? I may just have to take anothe look at it.

As for the question, If the developers got it to a point where there were no holes in the program, and it did pattern live loading thus that you didn't need like 20 different load cases. Then I would maybe consider trusting it on a more regular basis. But I should mention that my distrust also comes from the fact that one error in these programs can make a huge difference, And that error can be me. Hand calcs are simple and hard to get wrong.

And one really good thing about building your own calc set is that you get to know things that wouldn'teasly be known if you just followed a program manual.


  

When in doubt, just take the next small step.
 

RE: Shear in slab desig need to be considered or not

rowingengineer:
Wall slab connections can be pinned or fixed, but they recommend using pinned as it is hard to achieve fixed in real life. Plus they don't have specific wall and column stiffness - probably embedded as an approximate method though. Do you allow for wall stiffness in your hand calcs?

Hot spots - Slabs tells you they are there, but doesn't tell you how to deal with them. Even though stress concentrations do exist (hand calcs generally won't pick them up) concrete seems to behave in a manner that you get local relaxation and they don't often cause major problems (another post topic).

Slabs has covered some of these issues in their documentation, but not all which is pretty frustrating. I picked up a bug in the program early on - they fixed it and sent me a new copy.. another user told me this is common. Seems pretty slapdash.. they need to spend some money on development and make it more professional..

You can do pattern live loading

RE: Shear in slab desig need to be considered or not

Demayeng,
I normally like to reinforce the edges of slabs running into walls with some reo to stop cracking and ensure the load arrives at the centre of the wall, in my hand clacs I normally allow for some stiffness of wall, generally I make this about 50% of the wall strength. However I would expect most programs would advise that you don't design for moment in the walls.

 does the program average hotspots over an area, or just ignore? While I agree hotspots are not all that important to designing a slab because of relaxation ect, They could be fairly important if ignored in a beam design for secondary bending, which I must now ask, how does slabs handle the reinforcement of beams that are large and wide. As i would expect that these would be modelled as plates, not beams, hence you would have compatibility torsion and My? Which brings me to my next question, does slabs handle compatibility torque? Like an edge beam? if so does it iterate it's answer when cracked and uncracked conditions for beams?

I think i could spend many hours trying to find more holes in the program but I guess the truth of the matter is that a program no matter how good can't replace an engineer.   

When in doubt, just take the next small step.
 

RE: Shear in slab desig need to be considered or not

check out the Slabs manual online..

specifically here is the page on wide beams http://inducta.com.au/help/000068.htm

I don't have an answer for the rest of your questions - at uni and work i haven't learnt about compatibility torsion. However I can't see anything in the program about torsion stiffness..

RE: Shear in slab desig need to be considered or not

demayeng,
Thanks for the info, have enjoyed the discussion, I will get the boys at inducta to send trial copy, and give the program a working out.
I would recommend taking a look at compatibility and equilibrium torsion in Warner rangan et al. Ch13.1, always good to know things like this for modelling purposes.  

When in doubt, just take the next small step.
 

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