ACI 318 - Calculation of minimum reinforcement (Slabs & RC- Walls)

[Refaat]

Dear colleagues

Since we calculate the required steel reinforcement for panels (horizontal "slabs" and vertical " RC-walls ") ,

Could I know how does ROBOT calculate the minimum reinforcement(slabs &walls) according to ACI318 ?

Thanks

Refaat

@Refaat IArtur Kosakowski edited the topic for better findability and marked posts as the solutions. If it did not work as an answer, please post your reply in this thread so that I can unmark the solutions. 

[Artur.Kosakowski]

In general for all supported codes Robot check the provisions for minimal reinforcement. The actual values may be governed by other factors e.g. max. allowable distance between rebars. As far as I can see you compare Arequired (map) with Amin (hand calcs). Foe Amin you should mark another check box in the reinforcement map dialog (below the one you marked on).

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[Refaat]

Dear Artur

Thanks for your quick  response , but when I used  the check box below that one I used before the results as appeared  in the attached capture .Please , your explanation.

Thanks a lot

Refaat

[Artur.Kosakowski]

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Refaat]

Dear Artur

You are right but I used  " minimum  reinforcement for whole panel " , It means ( Minimum reinf. = required reinf .)

But still I am not satisfied  with the results as you see in the attached capture

.

Thanks in advance for your cooperation.

Refaat

[Artur.Kosakowski]

Most likely it is another ACI provision that governs the minimal area of steel for your case. Please attach the file - I'll try to look at it next week.

[Refaat]

Dear Artur,

Thanks , the file is attached

http://www.mediafire.com/?6uwob8qmmomv0mr

Refaat

[Artur.Kosakowski]

In general for all supported codes Robot check the provisions for minimal reinforcement. The actual values may be governed by other factors e.g. max. allowable distance between rebars.

Point 10.5.4 - max spacing is 18 in

Point 13.3.2 - max. spacing is 2*h (critical sections)

The  displayed area of reinforcement is for selected 14mm bars in maximal allowed spacing. If you use smaller bars then your Amin become smaller too.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Refaat]

Dear Artur

Thanks for your response  , but this is in case if we have the bar spacing is more the code limitations ( as mentioned in your message "8 " ) . Actually , my case is  :

Amin(ACI318-8M , clause 7.12.2.1) = 0.002 bh =0.002 (100)(25)=5 cm2/m

selected bar ( 14 mm) =1.54 cm2

So, bars numbers / m = (5/1.54) =3.25  bars , say 4 bars ( Amin=4*1.54 =6.16 cm2)

Then spacing = 100/4 =25 cm

" S " is less than  " 45 cm and (2h =2*25=50cm) "

But in ROBOT ....Amin=8.85 cm2( as appeared in my capture " message 1) greater than  6.16 cm2

As I know the minimum  steel area for the footings , walls and slabs is governed  only by the gross concrete area not  affected by the steel bars diameter(Kindly , see the attached captures ).

Could I know the equation  by which ROBOT calculate the minimum steel area for slabs and walls?

Thanks for your fruitful cooperation

Refaat

[Artur.Kosakowski]

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Refaat]

Dear Artur

Sorry , but I would like to clarify this point (  how does ROBOT calculate  Amin  for panel ?) . As far as I understand :

1- This equation is only used to calculate the spacing for  distribution of flexure reinforcement Not to calculate Amin

2-This equation is only for(beams & one way slab ) Not applicable  for (two way slabs & RC-walls).

3- Even I a plicate this equation (S= 380 (280/fS) - 2.5 Cc) "metric " the result  is not the same obtained from ROBOT     as follow :

fS = 2/3(fy) = 2/3 (400) = 267 Mpa  , Cc = 45 mm

Then

S = 380(280/267)- (2.50x45) =286 mm

S  Not more  300(280/ fs)  = 300(280/267) =315 mm

but ROBOT gave me 174 mm

So, I think from my point of view to enable ROBOT to consider the Amin as follow

1-Slabs to apply clauses (7.12.2.1 &7.12.2.2&10.5.4and 13.3.2)

2-RC-walls to apply clauses (14.3.2 and 14.3.3)

3-Mat foundation to apply clause (15.10.4)

Your professional efforts will be appreciated.

Refaat  

[Artur.Kosakowski]

3- Even I a plicate this equation (S= 380 (280/fS) - 2.5 Cc) "metric " the result  is not the same obtained from ROBOT     as follow :

fS = 2/3(fy) = 2/3 (400) = 267 Mpa  , Cc = 45 mm

Then

S = 380(280/267)- (2.50x45) =286 mm

S  Not more  300(280/ fs)  = 300(280/267) =315 mm

but ROBOT gave me 174 mm

This is caused by the fact that accidentally the formula from ACI 318-99 is used instead of the one from ACI-318-08. I have asked the development team to correct this mistake.

2-This equation is only for(beams & one way slab ) Not applicable  for (two way slabs & RC-walls).

I have asked the development team to use it only when you mark the unidirectional reinforcement in the reinforcement parameters label.

1- This equation is only used to calculate the spacing for  distribution of flexure reinforcement Not to calculate Amin

Allprovisions of such nature are assumed as Amin ones as they are not determined by calculations of the cross section resistance. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Refaat]

Dear Artur

Thanks for your response ,

-I have asked the development team to use it only when you mark the unidirectional reinforcement in the reinforcement parameters label.

What s' about two way slabs or mat foundation (reinforcement two direction ) and  RC-walls ( vertical reinforcement & horizontal reinforcement ) ?

Are you going to use the ACI318 code clauses which I referred to them in my "message 11" ?

-All provisions of such nature are assumed as Amin ones as they are not determined by calculations of the cross section resistance. 

I am afraid if I understand you properly that ROBOT doesn't consider the cross section resistance when it calculates the Amin .

Actually , as appeared in my captures " message 11" that the minimum steel area is calculated for all situations (slabs , mat foundations and RC-walls ) based on the total gross concrete section(bh) . Pls ,confirm .

Thanks a lot

Refaat

[Artur.Kosakowski]

What s' about two way slabs

As I wrote I asked the development team to investigate this situation and use the formula we discussed only for panels with the unidirectional reinforcement selected. Currently two way slabs with small load and reinforcement governed by this formula may have larger reinforcement (such as for a one way slabs). This will not influence the reinforcement of slabs under larger load.

or mat foundation (reinforcement two direction ) and  RC-walls ( vertical reinforcement & horizontal reinforcement ) ?

Are you going to use the ACI318 code clauses which I referred to them in my "message 11" ?

The RC SLAB Required Reinforcement module is intended for slabs and it uses the provisions for Amin as for slabs. The RC walls are intended to be design in the RC Wall Design module.

I understand your point therefore I have created a wish list for considering the Amin provisions in the RC Slab Required Reinforcement module based on the selected type of the object (slab, wall, mat foundation).

 I am afraid if I understand you properly that ROBOT doesn't consider the cross section resistance when it calculates the Amin .

You have 3 options:

Amin set as none. In such case the required area of reinforcement is calculated based on the equilibrium of forces in the cross section (plus SLS conditions if included in the design)

Amin for elements where the theoretical area of reinforcement calculated described as above is larger than 0. In such case all provisions for Amin are checked and both values (As and Amin) are compared. The larger is displayed as the final one.

Amin for all panel. As above for all elements.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

 

[Refaat]

Dear Artur

Thanks  for your gentle responses . Could I know if possible this situation will be implemented in revision 2014 ?

Refaat

[Artur.Kosakowski]

The version 2014 has already been released and does not have any of the points we are discussing implemented.

[Artur.Kosakowski · Accepted solution]

3- Even I a plicate this equation (S= 380 (280/fS) - 2.5 Cc) "metric " the result  is not the same obtained from ROBOT     as follow :

fS = 2/3(fy) = 2/3 (400) = 267 Mpa  , Cc = 45 mm

Then

S = 380(280/267)- (2.50x45) =286 mm

S  Not more  300(280/ fs)  = 300(280/267) =315 mm

but ROBOT gave me 174 mm

This is caused by the fact that accidentally the formula from ACI 318-99 is used instead of the one from ACI-318-08. I have asked the development team to correct this mistake.

Corrected in Robot 2015.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski · Accepted solution]

2-This equation is only for(beams & one way slab ) Not applicable  for (two way slabs & RC-walls).

I have asked the development team to use it only when you mark the unidirectional reinforcement in the reinforcement parameters label.

Implemented in Robot 2015.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Refaat]

Thanks a lot  , Dear Artur for this citation

Refaat