Long Term Deflection Using Safe Urgent Help

[mhdhamood]

Dear all;
I have a very very important issue that related to long term deflection (Cracking Analysis) Using SAFE.
I Argued with another Engineer High experienced.So please help me.
 
I am designing a slab 300 mm thick with 2.8 meter Cantilevers.

1.  CASE 1 :I want to see the results if I put only the mesh reinforcement T12@20cm.

                    I read the results from SAFE as in Figure 1, deflection = 46.7 mm. (I used the    
                    reinforcement source from finite element and minimum reinforcing ratios for tension and
                    compression 0.00256 i.e T12@20cm see figure 2).

2. CASE 2: Now I want to put additional reinforcement  to control the deflection.(HERE THE POINT
                  WHERE WE ARGUED ABOUT). I said that I have to put additional reinforcement in                             tension and compression =T16@10cm , that means I put in min. reinforcing ratios (in                           SAFE)   Tension and compression =0.0095 see Fig. 3 ) In this case the deflection become                   31.5 mm)  see Fig. 4.

3. CASE 3 : He said that we will put only additional steel T16@10cm in the tension face . But I
                    checked that on SAFE (I put min tension =0.0095 and min compression = 0.00256(the                         main mesh (see Fig.5) ). The deflection at the contrary increased to be 54.5 mm (Fig. 6)
Question To You Dears:
 What is the right procedure ? that what I did Or What the Engineer did?

 

[WR1]

If you will put on-site additional reinforcement only at bottom, then just do like what your engineer said (Tension=0.0095 and Compression=0.00256)

 

If you are going to put additional reinforcement on both sides then put 0.0095 on both sides in SAFE.

 

Having said that, you should know that compression (top) reinforcement helps reducing deflections. So put top reinforcement to control deflections.

[mhdhamood]

Dear ;
Thank you. But I am speaking about cantilever so the top reinforcement is tension, and the bottom is compression.!!!
Regards

[WR1]

Dear please make sure first that the minimum ratios you put are more than the source from FEM

 

See SAFE help:

 

• Minimum Reinforcing Ratio Used for Cracking Analysis edit boxes.

  • Minimum Reinforcing Ratio Used for Cracking Analysis edit box. Use this edit box to specify the minimum ratio to be applied to reinforcing during cracking analysis. This ratio applies regardless of the option selected for the source of the reinforcement specification (see three preceding bullet items). However, the program will use the largest minimum ratio. That is, if the minimum ratio specified here is lower than the ratio specified as part of any of the source options, the program will not use this value.

 

So first check what is the reinforcement you are getting at strength level from FEM. May be it is more than T12@20cm which you are applying for cracking analysis.

 

Please confirm this first!

[mhdhamood]

Dear;
I confirmed this. As I mentioned in CASE 1, I have chosen the reinforcement to be from F.E the deflection was 46.7 mm at the end of cantilever. here In this case the program will use not 0.00256(T12@20) instead it will use F.E results because it is bigger 0.00285(T14@20).
Now I want to solve this high deflection in CANTILEVER .But My question now is how to solve it by increasing steel?.
1. As far as I know if I put here a bigger min. ratio =0.0095  for the tension(TOP) and compression(BOTTOM) steel simultaneously,
    the deflection decreases to 31.5 mm .
2. As My Engineer Told me, he want only to increase the tension steel (TOP)[ i.e. I put in the min. tension steel box the ratio                =0.0095       and min. compression steel ratio = 0.00256 ], But I checked what he said and the result where 
     increasing deflection instead of decreasing. (deflection =54.5 mm )

Please provide me your way to decrease the deflection using steel in Cracking Analysis. I am sure from what I did because I checked it on many examples I know and on the Manual verification example of SAFE.
I will send you the SAFE FILE upon request .
Regards 

[WR1]

Dear;

 

If the deflection is excessive you need to increase tension steel (TOP) in cantilever.

 

But if the deflection is less excessive you can reduce it by adding additional compression reinforcement (BOTTOM) in cantilever.

 

What does it mean

 

1. Increasing compression reinforcement reduces long-term deflection (by small amounts) But it has very very minor effect on I-cracked (I-cracked almost stays same but Long Term Deflection decreases by little amount)

 

2. Increasing tension reinforcement reduces long-term deflection (by large amounts) But it has major impact on I-cracked (I-cracked increases and Long Term Deflection decreases by same amounts)

 

So it is very clear that long-term deflection can be reduced by increasing I-cracked or in other words tension steel (TOP) in cantilever.

 

But here arises one concern

 

that if you change tension steel (so it will also change I-cracked) which in turn will change analysis and moment attraction. So you have to repeat the calculations. (Thats why we say to add compression reinf to reduce long-term deflection without analysing again). This holds true for continuous structure. For single span, you dont need to re-analyze. If you change I-cracked equally in continuous system you also dont need to reanalyze again. (Very rare).

 

Conclusion

 

If the deflection is huge (which is, i think) then increase tension steel in SAFE. (You can increase compression steel too but it will have very much less effect than tension steel).

 

Hope that helps.

[mhdhamood]

Dear 
Sorry to disturb you too much. But this issue I am speaking about is so strange.
Really I agree you with all you have mentioned above. When we increase the Top steel in cantilever it will decrease the deflection in a good manner. and the compression steel will be reduced slightly compared with top steel addition.(I make sure by an example I did)
BUT THE IMPORTANT ISSUE that will make me CRAZY and need scientific interpretation is WHY in MY SLAB when I increase the steel at top of cantilever the deflection increased also, Is that related to the CRACKING MOMENT???
May be because the moments are small than cracking moment and in this case no meaning for increasing the tension(top) steel because already the whole section is still uncracked. please Help!!!!
But I noticed that when increasing the top and bottom steel simultaneously the deflection decreased .
 

[mhdhamood]

Can anyone share this issue to find the answer please!!!!

[mhdhamood]

ALL are Busy!!!   

[WR1]

I am sure SAFE is doing it properly but I cannot think an example which can clear your doubts. When you are specifiying additional reinforcement it is affecting the whole model not just the cantilever slab. So it is changing the I eff or I crack of whole model. (Reinf changing I so I affects M).

[mhdhamood]

Dear Ahhhhhhh so thanks to you for your comments............. I found the solution and I want to share you all for the benefits :
I am very happy now ... 
Related to LONG TERM DEFLECTION
What Mr Rana Waseem said is correct But applicable only if the moments in the slab are over the cracking moment i.e. cracked section is there. Here if we increase the tension steel that will lead to less deflection.

Now if the moments are less than the cracking moment then the whole section works i.e uncracked section. in this case no need for increasing the tension steel because already all the section works. 
Now we want to reduce the deflection by putting compression steel :
1. I tried to increase the compression steel only but that doesn't work.
2. I tried to increase the tension and compression steel simultaneously that worked successfully.

Now the reason for  that is (As I find it from a structural book) that:

 

1.When the tension reinforcement is little then the compression area of concrete section against it, is small in depth, also due to cover, that leads to case 1 and hence the compression steel may be located below  neutral axis (in neglected concrete area) so it will not work as compression steel resulting no reduction of deflection.

2. When increasing the tension steel the compression area of concrete is increased in depth then allow for the compression steel to be effective and it will work to reduce the deflection ( i.e CASE 2)

Summary :
In case of that the moments in slab is small (under cracking moments) then increasing the tension steel is not the solution and it will not reduce deflection. Instead of that you have to make use of the compression steel to reduce deflection but to make it work you have to increase tension and compression steel simultaneously.

 

 

[WR1]

Yeah you are right but I was doubtful abt your model if the moment is less than the cracking moment. But good finding.

[Luk]

But how can we know in CSI SAFE if the moments in slab are : under cracking moment or not ?

How can we find in CSI SAFE if the moments in slab are small or not ?

How can we find in CSI SAFE where it’s the limit between cracking moment and under cracking moment in slab ?

In other words how can we find in CSI SAFE in which moment it’s the slab moment ?

It’s cracked ?

Or under cracked ?

Because we have 2 cases :

a) If the moment in slab are cracked : than we can increase JUST the Tension stell ( TOP steel in Cantilever slab ) to reduce the deflection .

Without to change compression steel ratio !

b) If the moment in slab are under cracked moment ( small moment ) , than we need to increase together ( simultaneously ) Tension ( TOP steel ) + Compression ( BOTTOM steel ) in Cantilever slab , to reduce ( decrease ) the deflection .

We need to know in CSI SAFE in which moment are the slab : cracked or uncracked moment .

But how can we find this ?

 

Edited November 3, 2018 by Luk