topic Re: minimum reinforcement in slabs again in Robot Structural Analysis Forum

minimum reinforcement in slabs again

Jummybear — Mon, 09 Feb 2015 11:14:41 GMT

I cant understand the robot values of min reinforcement, probably I am doing something wrong..

Input: Concrete slab t=400mm, C35/45, reinforcment bars A500HW (bars, predefined in robot diam. 12mm). Upper layer is XD3, what means concrete cover of 45mm+10mm, bottom layer is XC2 with 25+10mm cover.

Output in Robot: if i uncheck the cracking reinforcement adjust then I get similar results (but not exact) with hand calculation ~4,6cm2 (Robot) vs 5,64cm2 (hand calc). If i put it in (reinforcement adjust), then Robot show me 8,5cm2 for 12mm bar and ~10cm2 for 16mm bar!!!

My hand calculation:

EC 1992-1-1 p.9.2.1.1 (1) As,min=0,0013*bt*d=0,0013*1000*339=441mm2 or 4,4cm2.

EC 1992-1-1 p.9.2.1.1 (1) As,min=0,26*fctm/fyk*bt*d=0,26*3,2/500*1000*339=564mm2 or 5,64cm2.

EC 1992-1-1 p7.3.2. (7.1) As, min=kc*k*fct,eff*Act/fyk=0,4*1*3,2*200*1000/500=512mm2 or 5,12cm2.

Firstly, it looks that Robot doesnt take As,min=0,26*fctm/fyk*bt*d, but only takes As,min=0,0013*bt*d (if reinforcement adjust is unchecked), but is first expression gives bigger value it should go in.

Secondly, As, min=kc*k*fct,eff*Act/fyk doesnt go with hand calculation.

Can anybode comment this?

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Mon, 09 Feb 2015 15:10:17 GMT

EC 1992-1-1 p7.3.2. (7.1) As, min=kc*k*fct,eff*Act/fyk=0,4*1*3,2*200*1000/500=512mm2 or 5,12cm2.

assuming bending only. For bending + tension/compression kc may increase up to 1.0. Is this the answer to your question?

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Re: minimum reinforcement in slabs again

Jummybear — Tue, 10 Feb 2015 13:13:17 GMT

Yes, assuming bending only and I have bending only reinforcement type (simple bending). Kc coef. in robot for bending only should be also 0,4. So the problem is in some other place. I will upload my model and provide here link to download it. By the way I supposed to use ARSA 2013 at work, probably something was fixed in newer versions (2014,2015).

Link to Robot file LINK

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Thu, 12 Feb 2015 09:37:11 GMT

The 'additional' area is due to

Personally I would consider upgrade to the latest version of Robot so that you have the options to switch use of some code provisions off when you decide to disregard them

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Re: minimum reinforcement in slabs again

Jummybear — Thu, 12 Feb 2015 09:51:12 GMT

Due to what? SLS condition? I agree with that becouse i checked twice with reinforcement adjust and without it. But my second question is still unanswered, my hand calculation of As,min by 7.3.2 (2) dont go with Robot ones.

Agree with Robot upgrading, we will take it in near future 🙂

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Thu, 12 Feb 2015 10:02:24 GMT

My hand calculation:

EC 1992-1-1 p.9.2.1.1 (1) As,min=0,0013*bt*d=0,0013*1000*339=441mm2 or 4,4cm2.

EC 1992-1-1 p.9.2.1.1 (1) As,min=0,26*fctm/fyk*bt*d=0,26*3,2/500*1000*339=564mm2 or 5,64cm2.

EC 1992-1-1 p7.3.2. (7.1) As, min=kc*k*fct,eff*Act/fyk=0,4*1*3,2*200*1000/500=512mm2 or 5,12cm2.

Secondly, As, min=kc*k*fct,eff*Act/fyk doesnt go with hand calculation.

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Re: minimum reinforcement in slabs again

Artur.Kosakowski — Thu, 12 Feb 2015 10:12:10 GMT

But my second question is still unanswered, my hand calculation of As,min by 7.3.2 (2) dont go with Robot ones.

Agree with Robot upgrading, we will take it in near future 🙂

Set the concrete grade from as in the model to C35/40. As far as I can see the change of some parameters in the label reset it from as in the model to C20/25 which then changes fctm in the formulas you referred to. We will investigate why this happens.

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Re: minimum reinforcement in slabs again

Jummybear — Thu, 12 Feb 2015 10:42:10 GMT

We spoke in different languages 🙂
I agree that with this setting Robot calculates minimum amount of reinforcement right.
But if you take SLS conditions into account the result should be (by my hand calculation) 5,12cm2 by 7.3.2 (2) or 5,64cm2 by 9.2.1.1 (if Robot pick bigger value into account) and in my file with SLS condition As,min is 8,5 cm2. I cant understand why its not around 5,12...5,64? My hand calculation are wrong or something wrong in my ARSA2013

PS I found that problem also with switching concrete class if you change something in reinforcement dialog box.

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Thu, 12 Feb 2015 11:51:04 GMT

EC 1992-1-1 p7.3.2. (7.1) As, min=kc*k*fct,eff*Act/fyk=0,4*1*3,2*200*1000/500=512mm2 or 5,12cm2.

σs is the absolute value of the maximum stress permitted in the reinforcement

immediately after formation of the crack. This may be taken as the yield strength

of the reinforcement, fyk. A lower value may, however, be needed to satisfy the

crack width limits according to the maximum bar size or spacing (see 7.3.3 (2))

According to the table from this point Robot assumes 280 instead of 500.

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Re: minimum reinforcement in slabs again

Jummybear — Thu, 12 Feb 2015 12:34:06 GMT

Thanks Artur, now its clear.

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Fri, 17 Apr 2015 14:55:27 GMT

But my second question is still unanswered, my hand calculation of As,min by 7.3.2 (2) dont go with Robot ones.

Agree with Robot upgrading, we will take it in near future

Corrected in SP1 for Robot 2016.

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Re: minimum reinforcement in slabs again

ByrnePM — Thu, 30 Apr 2015 16:24:16 GMT

Hi Artur/Jummy,

This is a really great thread, very informative and it has helped me clear up some of the queries I was having with the SLS design for cracking performed in Robot.

However, is there anymore information you can give me on how Robot deals with Eq7.1 in Eurocode clause 7.3.2, i.e.

Any information on how robot deals with the Kc factor for envelopes of different combinations and Is there a way to control the age of the concrete at the time the cracks occur, specifically is it possible to control the "fct,eff" value.

I presume Robot calculates this value as "fct,eff = fctm" i.e. the strength at 28 days (a function of the concrete class).

Any info would be much appreciated,

Thanks

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Mon, 04 May 2015 07:27:40 GMT

For simple bending Robot uses the envelopes of each of combination types separately. For axial force plus bending each combination is checked separately.

Design is based on all SLS combinations unless the minimal reinforcement for 7.3.2 has been switched off. Calculations are not done for X0 and XC classes.

Kc is calculated exactly as in the code for each of SLS combinations separately (or envelopes when applicable).

There is no change in concrete resistance or other parameters to set the time below 28 days.

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Re: minimum reinforcement in slabs again

Anonymous — Thu, 10 Nov 2016 13:38:52 GMT

Hi Arthur. I'm picking this item back up again because I have just downloaded Autodesk Robot Structural Analysis 2017 and am having the same issue regarding min. areas of steel. I have a 300mm thk. slab (C30/37) in simple bending with 30mm cover top and bottom and H12 bars assumed throughout. By my calculations, 0.26(fctm)(bt)(d) / 500 = 398mm2/m and 0.0013(bt)(d) = 343mm2/m. The min. steel I get from Robot for the bottom layer is 330mm2/m. I have all of the reinforcement adjust boxes unticked for cracking and deflections so these should not be influencing anything. Based on this, the issue still seems to exist.

If I change the plate /reinforcement properties by manually changing the concrete the concrete grade instead of 'as in structural model', the issue corrects itself. The problem I have with this method is that the materials are designated as British in stead of European so I can only pick cube strengths. For example, If I want to choose C30/37 concrete, the nearest I can get is C35 cube strength.

Can you please look into this for me?

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Thu, 10 Nov 2016 13:50:24 GMT

My assumption is that you have materials in the model defined according to one code and your reinforcement design code is different. Try to set materials as Eurocode for a model and EC2 (BS EN) for reinforcement of British for model and BS for the design code. Mind to make sure the panel thickness definition and the reinforcement type assigned to the panel are with these sets of materials before running calculations.

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Re: minimum reinforcement in slabs again

Anonymous — Thu, 10 Nov 2016 14:11:58 GMT

Thanks Arthur. I have now changed the setting to the following:

Materials = 'Eurocode'

Design Code = 'BS EN 1992-2-2:2004 NA:2005

Reinforcement Bar Database = BS 8666_2005

Is this ok?

The min. steel seems to be working better now. It's still a little off from what I calculated by hand. The values I get are based on the specified cover + 10mm. Any reason why 10mm is added to the cover in the calculation?

Re: minimum reinforcement in slabs again

ByrnePM — Thu, 10 Nov 2016 14:21:53 GMT

This 10mm "additional cover" is an allowance in design for deviation DeltaCdev

Eurocode recommended value is 10mm

This is added to Cmin in order to calculate Cnom

Maybe make sure not to be entering Cnom into Cmin box in ROBOT parameters if this deviation is still set as 10mm

Re: minimum reinforcement in slabs again

HenkaCarlsson — Mon, 12 Dec 2016 14:40:21 GMT

The tables under 7.3.3(2) is based on a few conditions according to note a).

Question 1: If there is some other cover or fcteff for instance does Robot concider that in anyway?

As I understand it Robot also does the crackcontrol according to 7.3.4. This should be a more exact calculation of the cracks than using 7.3.2+7.3.3 which is rather rough.

Question 2: If I choose to do the cracking Control under "SLS parameters" should I check the "Disable the SLS conditions 7.3.2(2)" under "Reinforcement" then?

Best regards

Henrik C

Re: minimum reinforcement in slabs again

Artur.Kosakowski — Tue, 13 Dec 2016 10:27:22 GMT

The tables under 7.3.3(2) is based on a few conditions according to note a).

Question 1: If there is some other cover or fcteff for instance does Robot concider that in anyway?

I'm not sure if i understand you correctly but fct,eff is used for calculations of minimal reinforcement according to formula (7.1)

As I understand it Robot also does the crackcontrol according to 7.3.4.

As Robot follows 7.3.4 the paragraph 7.3.3 is not 'used'.

Question 2: If I choose to do the cracking Control under "SLS parameters" should I check the "Disable the SLS conditions 7.3.2(2)" under "Reinforcement" then?

I don't think so. If you do the formula (7.1) will not be applied. This is to be used only when for some reason you don't want to include the minimal reinforcement provisions at all.

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.

.

Best regards

Henrik C

Re: minimum reinforcement in slabs again

HenkaCarlsson — Tue, 13 Dec 2016 15:25:22 GMT

Clarification of Question1: Look at note 1 in the picture below which specifies the assumptions, for the table that Robot uses, is based upon.

Let's say I choose concrete with some other value for fcteff or some other value for cover than 25 mm. Does this affect Robot's results or is the results only valid if my slab fits with these assumptions ?