RC design - analysis of results

[Anonymous]

Hi;

I have some questions about RC results :

1) How can I change no. of current combination or ponderations from "plates theoretical reinforcement" to see maps of corresponding internal forces, displacements ?

When I am in the option "plates theoretical reinforcement" the current load selection window is inactive.

2) Working with scales is a "nightmare" .

- first of all, it's hard to define your scale

- if you do it you have to read it all the time because the Robot mixes the units 

- why Robot does not remember the appropriate defined scales with units, e.g. (reinforcement cm ^ 2 / m, forces kNm / m, kN / m, cracks width mm)

Here are some examples:

Maybe I'm doing something wrong and it's my fault ?
Please tell me how to define user scales and assign them to specific quantities so that Robot reads them well ?
Or maybe something has to be changed in the configuration files (*.cfg etc..) and set it there manually?

[Anonymous]

One more question:

Constant errors/warnings from calculations :

1) lack of supports while elastic support is defined

2) error in assigning to the floor - I purposely used floors to group selected elements of the structure

  for better viewing

[Krzysztof_Wasik]

Hi @Anonymous 

Regarding load case selection for moments presentation.

Use Edit >Select then choose Case selection. When this window will be opened you will be able to switch between load cases. You are using layout for reinforcement maps presentation so not all options are visible (layouts are designed for particular purposes).

All results you can call  from Geometry layout by selecting single option.  If you are using layouts some options have to be opened from menu.

Regarding scales .

I do not know tool for assigning user scales to view. Usually when I am changing scale, I am changing scale range then change scale to User defined and Apply. Default scale can be restored by Normalization (after switching back to linear scale). Sometimes it is necessary to switch between load cases to refresh scale ranges.

Elastic supports and no supports warning.

Elastic support is not defined using Support option. Program detects that there is no support type objects. This is warning only and can be ignored. For me (service purposes) this information is useful, but generally I agree this warning is not necessary.

Story definition.

Warning is reported when members exceed story limits.

For instance there is not divided wall defined across more than one story, or when member vertical coordinates are defined not precisely (column top node is higher than story level)   

[Anonymous]

Hi support Staff;

Another issue with RC check for foundation slab:

Md_uls=-628.2 kNm/m

Mchar=-449.8 kNm/m

Mchar_quasi_permanent=-414.8 kNm/m

Mchar_short_term=-449.8-414.8 = ~35 kNm/m

Md=-628.2 kNm/m

Mchar=-449.8 kNm/m

Mchar_quasi_perm=-414.8 kNm/m

Mchar_short_term=-35 kNm/m

Your EXPERTcalculator  (C30/37, h=60cm, a1_bottom=90mm, a2_top=60mm, env. class "XA1" , w_crack=0.2mm, c.bott=50mm+10mm, c.top=30+10mm, d.bar.bottom=25mm, d.bar.top=20mm)

Robot results (panels with bending only)

How am I supposed to understand 58cm2/m in relation to 49cm2/m  (Expert result) - if the foundation slab is onedirectionally bent in general ?

[Krzysztof_Wasik]

Hi @Anonymous 

As far as I remember calculator uses old PN 82 code. I do not see code used for Robot calculation but combination names suggest calculation according to Eurocode.

If all settings are the same (PN in Robot and Calculator) results should be similar. If you are using PN in both programs problem should be verified. Please report it using Autodesk Account as official service case. 

[Anonymous · Accepted solution]

Hello Krzysztof;

Thanks for explanations - please consider some improvements in the next versions of Robot

Regarding RC design for bending:

Some facts:

Expert RC design is based on PN/B-03264:2002 (this one is based on pre Eurocode ENV-1999).

Rules of bending design are the same as in PN-EN 1992-1-1 and both codes give similar results.

Older code was PN-84/B-03264.

PN-82 is *.rgl file in Robot for old polish load combinations.

I checked a section with my own algorithm for bending and results from Robot surprised me so much I checked

it again with Expert. Expert gave similar results - for capacity check also.

Nevertheless you can check it yourself.  😊

Please check this with your internal algorithms.

My client requires an economical design from me,  58cm2/m instead of 49cm2/m  makes a big difference.

I wonder what other users get using given moments values according to other RC codes...???

[Krzysztof_Wasik]

Hi @Anonymous 

You are right I have missed correct code number it should be PN/B-03264:2002. Sorry for my mistake, but anyway you have compared results for two different codes.

Regarding reinforcement area calculation according PN-EN-1992

I have followed setting from your screenshot (internal algorithm) and tried to calculate similar model. I used simply 2x1m plate loaded with constant linear moment on edge (1 kN*m/m)

Then created the following combinations to reflect your loads 

I gave uniform moment distribution near slab edge.

for SGN

for SGU

I have the following remarks regarding slab reinforcement parameters

- you have defined creep factor 1.245 (default value in Robot is 2 so I have modified it to 1.245)

- I have activated non linear creep option (it is not activated by default)

- in your example cover is calculated to reinforcement center (In Robot it is calculated to bar edge) so for cover 9 cm and bar size 25 (described in previous mail) I have defined in Robot 7.75 cm as bottom cover value.

- Robot takes into account (by default) cover deviation 1.0 cm (I have changed it to 0 to neglect deviation).

After modifications Robot calculated  reinforcement area 49.47 cm2/m. It is quite similar to 49.087 cm2/m from your example

[Anonymous]

Thanks Krzysztof;

I set also nonlinear creep as you did(a creep factor value 1.24 resuls from eq. B.1 annex B Fi(t,to) for t=180days , long term Fi >2 when t-> +infinity). Now results look better and are close to manual calculations.

I would like to ask about minimal reinforcement - how is it done in Robot acc. to EC2 7.3.2 eq. 7.1. ?

Do we have any theory manual with any example.

Please compare a difference - between Asx_min and Asy_min (parameters as above, foundation slab t=60cm)

I would like to know where the values 19.63cm2 (Ax_min )vs 15.09/12.57cm (Ay_min) come from ?

Asx_min (-)

Asy_min (-)

Asx_min (+)

Asy_min (+)

total As_y (-)

total As_y (-)

[Krzysztof_Wasik]

HI @Anonymous 

Regarding minimal reinforcement

19.63 cm2   - fi 25 every 25 cm

12.57 cm2  - fi 20 every 25 cm

15.09*2 =30.18 cm2 (top+ bottom) is calculated from 7.1 (7.3.2) for sigmas= 200 and cracking= 0.3 

[Anonymous]

This is a theory manual for minimal reinforcement in Robot ?

Do I undestand correctly - 4 bars / per 1m ???

Please be so kind and provide the values of each variable in this formula

[Krzysztof_Wasik]

Hi @Anonymous 

I do not have any special manual for minimal reinforcement calculation.

If other values do not apply Robot distributes 4 bars for 1 m length as minimal reinforcement

Coefficient values in 7.1 used for minimal reinforcement estimation.

[Anonymous]

Hi;

Thanks !

OK now it is clear what has been implemented in Robot

k(h) = 0.79 - it's OK from interpolation

A.ct = 0.6*1.0 m2 - OK

sigmaS=200 MPa - OK as mean value

(from table interpolation  for d.bar=25mm, w.lim=0.2mm => 160MPa // d.bar=20mm, w.lim=0.3mm => 215MPa)

f.ct.eff=f.ctm=2.9MPa (C30/37)

k.c should be kc=1.0 (tension during shrinkage)

                         kc=0.4 (for bending only)

with kc=1.0 this formula gives absurd results => 0.5*A.smin = 0.5*68.73 = 34.36cm^2 /per side  !!!

with kc=0.4 this formula gives acceptable results  => 0.5*A.smin = 0.5*27.49 = 13.75cm^2 /per side  !!! (with f.ct.eff=2.9 MPa)

That's why  most of us take values for bending (k.c=0.4) which are similar to those obtained from DIN-EN ...but this is a topic for different story...😃

I don't understand only one thing - if the minimum reinforcement is calculated from formula [7.1] as 15.09cm2/m

why Robot displays a higher value 19.63cm2/m  (4#25mm/m)  for As.x.min?

In my opinion 15.09cm2/m should be also displayed...

[Krzysztof_Wasik · Accepted solution]

Hi @Anonymous 

 For bottom layer you have defined fi 25 in X and fi 20 for Y direction (the same for top layer).

Program takes minimal value from formula 7. 1, or if it generates bar distance bigger than 25 cm, this distance is used as maximal bar spacing.

For Y direction calculated area 15.09 is bigger than fi 20 with distance 25 cm.  This is the reason why spacing criteria is not used. Distance for fi 20 set for Y direction will be smaller than 25 cm.

For x direction using 15.09 will give distance bigger that 25 cm for fi 25. So program uses 25 cm as maximal fi 25 spacing.  

[Anonymous]

OK - I will change, for whole bottom mesh #25 and top #20

[Anonymous]

Hi Krzysztof;

There is a bit of confusion with this minimal reinforcement - I do not know why it is so complicated ?

Reading help I found something like this:

For me minimum reinforcement for bending Amin (per side)

 should be calculated as below:

max { 0.26*f.ctm/f.yk *b*d  ;  0.13% *b*d   ;  0.5 k.c*k(h)*f.ct.eff*A.ct/ sigma.s.lim)

                                                                             |

                                                                          per side

k.c=0.4 (if Robot uses this value)

A.ct = b*h

Yesterday was but today it is not logical for me what the program calculates.

Displaying values for different thicknesses I get sometimes three values for a panel (see results for t=70cm).

This is probably due to the complexity of this algorithm quoted from Robot help.

[Anonymous]

In other words - the minimum reinforcement in Robot is not the minimum structural reinforcement in the sense of the design code

[Krzysztof_Wasik]

Hi @Anonymous 

See below my comments

- except three minimal reinforcement area criterions (you have mentioned), Robot uses the fourth one -bar distance spacing (not smaller than 25 cm). It is activated by default in reinforcement parameters, but cannot be switched of on Reinforcement tab (Disable spacing conditions). 

- In your example (where 3 different values are displayed)

- Two different values 16.04 cm2/m and 13.15 cm 2/ is caused by different crackling width defined for top and bottom layer. Both values are correct and are calculated from (7.1)

- The last value 12.57 cm2/m refers to fi 20 by 25 cm. It is displayed on the regions boundary and is probably defined by point indication option - bigger font is used. This is not value displayed for FE centers.

I was not able to obtain such effect.  Maybe it is glitch in Robot display refreshing procedure. I agree that this value should not be presented. I cannot verify this issue without Robot model.

- Description from Robot manual does not refer to minimal reinforcement defined by code. This procedure describes scaling procedure for calculated (not minimal ) reinforcement in respect to minimal reinforcement requirements

In applies (in my opinion)  in very rare situation - when for instance

- calculated top reinforcement is bigger than minimal top reinforcement (0.5 minimal reinforcement from 7.1)

- calculated bottom reinforcement is smaller than minimal top reinforcement (0.5 minimal reinforcement from 7.1)

and sum of calculated top and bottom reinforcement is smaller than minimal reinforcement for whole section (from 7.1)

In that case calculated reinforcement is scaled to obtain minimal reinforcement area from 7.1. Probably it could be written in more clear way.

So far in your example I do not see discrepancy with code (except probably glitch with results presentation for points indicated graphically, this value - 12 -, should not appear in results). 

[Anonymous]

Hi;

Citation "...Two different values 16.04 cm2/m and 13.15 cm 2/ is caused by different crackling width defined for top and bottom layer. Both values are correct and are calculated from (7.1).."

Are you sure what you say ???

Compare settings AY_min (-) is selected only, which means it is for bottom secondary layer of reinforcement, where w.cr.lim = 0.2mm condition is still valid.

I want to get a minimal reinforcement for the top or bottom surface of the slab and both independent directions of reinforcement (main and secondary). It should be one value resulting from mentioned 3 or 4 conditions - but only ONE for top or bottom reinforcement !

Please notice that crack width is hidden under sigma.s.lim (if we are using  table 7.2N neglecting some limitations marked in red) . All other values in these formulas don't depend on crack width but on geometry conditions only !

[Anonymous]

Some other examples after today calculations:

Bending moments myy:

Minimal reinforcement Ay_min (spacing condition disabled)

Minimal reinforcement Ay_min ( [7.3.2] conditions disabled)

control check for slab 70cm thick :

There is probably a bug in the alghoritm checking conditions of 7.3.2.

As a result, it should only be ONE value - as in the last picture.  

[Krzysztof_Wasik]

Hi @Anonymous 

Regarding different values for minimal reinforcement calculated according 7.1 and depending on moment sign value.

According to 7,3,1 (1)  

...If crack control is required, a minimum amount of bonded reinforcement is required to
control cracking in areas where tension is expected....

According to this recommendation Robot takes sigma s according to reinforcement bar diameter and crack width, defined for slab layer which is under tension. It depends on calculated bending moment sign.

-When top layer (only) is under tension - reinforcement bars diameter and defined crack width is regarded as defined for top layer.

-When bottom layer only is under tension - reinforcement bars diameter and defined crack width is regarded as defined for bottom layer.

It results in different minimal reinforcement values for bottom (or top)  layer, for areas with different calculated moment sign and different crack width defined.

Since 7.3.2 does not define minimal reinforcement separately, for top and bottom layer, but defines minimal reinforcement for whole section, those (different values) follows code recommendation.

When