rc beam design (Eurocode)

[monchie]

Hello,

I'm designing a simply supported rc beam(450x750dp) with effective span of 16.30 m.(pls. see attached loading and shear&moment diagram). If I used the criteria of "allowable span to effective depth ratio" it is failing. But when I consider the actual deflection = 27mm < L/250, it is OK. Would I go ahead since it is passing in the deflection criteria?

Also, Mrd = 0.167fckbd^2 =.167x40x450x670^2 = 1349.40kN.m > 801 kN.m(OK), it is passing with the Moment capacity against the design moment.

Any ideas will be greatly appreciated.

Regards,

 

[KootK]

You would go ahead so long as you're confident that your deflection limit is appropriate and you've accounted for all relevant sources of deflection (creep, elastic, etc). Span to depth ratios are very useful but rarely are they expressed as hard limits. They can usually be overruled but detailed engineering evaluation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bkal]

It is quite likely that I have misunderstood something (as your link does not seem to work) but your moment looks quite small for such a long span, and it seems that you are calculating the beam capacity based on a rectangular section (450x750) with a material strength being equal to the compressive strength of concrete - is that right? Are there any partial factors for load or strengths included anywhere?

I agree with KootK's comments, too.

 

[IDS]

The bending resistance of a rc beam depends on the steel content, and has to be factored down by the factors given in the code.

How did you calculate the deflection? If it is based on the 2nd moment of area of the concrete section, and the short term concrete E value, the actual long term deflection will be several times higher, due to concrete cracking, creep and shrinkage.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[monchie]

thanks Guys,

I've modelled the beam and got this short term deflection(DL+LL) =27.8 mm, but considering long term deflection, I would only consider due to live load, short term deflection(LL) =3, so, long term deflection,ie is about 12(four times. approx). I've run it in TEDDS with 4nos.-H40 reinforcement, and it works with the span/depth ratio.

Also, why I cannot attached any pdf or jpeg on my comment,something wrong with it?

Anyway, the description of the loads for the rc beam(450x750dp) and L = 16.35

DL = 4.54kN/m, LL = 0.95 kN/m (from 0 to 6.45 m, L to R)

DL1 = 9.33 kN/m, LL1 = 1.48 kN/m(from 6.45 to 16.35)

Regards,

 

[rapt]

Monchie,

I do not believe your deflections. Short term is very low for that span length for an RC beam.

And long term deflection is not based on only LL deflection.
- Visual long term deflection is based on all of the loading, not just LL.
- Incremental deflection effect on attached elements is based on the creep and shrinkage effects on dead load plus any live load effects.

Has your software considered cracking, and also long term effects correctly. It does not look like it.

 

[IDS]

I suggest you get someone to review your input loads and calculations. The unfactored self weight of the beam is 8.4 kN/m, so how can the dead load be only 4.5 kN/m? Also the live loads look very small. What is the application?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[monchie]

Thanks IDS,the SW of the RC beam is actually included in the model(rc beam model),this rc beam(16.35 m long) is an edge beam of a car park,where all the hollow core slabs spanning "parallel" to it, its side actually "skewed" a bit that's why it pick up loads,normally, when the hollow core slabs perfectly parallel to it, no loads will be transferred to rc beam except its own
weight.

Regards,

 

[rapt]

Monchie,

The initial deflection of a 450w * 750d RC beam spanning 16.35m under it own weight only (no added DL or LL of any sort), would be about
- with bottom reinforcing only = 57mm and long term = 102mm
- with equal top and bottom steel = 55mm and long term = 85mm (bottom steel is amount required for ultimate strength)

If you do not allow for cracking, the initial deflection is 24mm under beam SW only.

Concrete cracks! Especially when the tensile stress is over 6MPa as it is in this case! Find some software that allows for it!

 

[monchie]

Hello Rapt,

If you try C40/50 concrete, the short term deflection for a 450x750dp rc beam(self-weight), L =16.35, is only 19 mm. What software are you using?

 

[zakmuh]

Hi

I think your beam depth is inadequate.

Based on Basic Span/Depth Ratio Table, for simply supported beam, with As/bd>1.5%, the ratio is 14.
Span/14 + cover = beam depth
=> 16300/14 = 1164 + 30 cover = 1194. Use 1200mm deep beam

When it comes to deflection, 'I' governs it. I - moment of inertia is BD^3/12. So the deeper the beam, the reduced deflection it is.

Please correct me if I'm wrong with the above.

Cheers

 

[Klitor]

Sorry monchie, but RATP is right. My Eurocode2 self made spreadsheet for deflection estimates your beam to deflect long term - 110mm (C40/50,using average creep/shrinkage data, ULS bottom rebar and about 10cm2 top rebar)

I could look up my "world famous" prestressed beam spredsheet and see could prestressing help you?

 

[monchie]

@rapt, you're right and double checking the long-term deflection due to self-weight alone is 76.86mm!(short-term is 16.23mm)
now the only option is prestressed/posttensioned, Im hoping that "klitor" will give me his ideas regarding this, my experienced in prestressed is limited, I suggested a steel I-beam but they want an all concrete solution to the car park, its to late to request additional column in the midspan.

@klitor,in your spreadsheet, can you predict the long term deflection of the rc beam

"Anyway, the description of the loads for the rc beam(450x750dp) and L = 16.35

DL = 4.54kN/m, LL = 0.95 kN/m (from 0 to 6.45 m, L to R)

DL1 = 9.33 kN/m, LL1 = 1.48 kN/m(from 6.45 to 16.35)"

Regards,

 

[rapt]

Monchie,

I am using RAPT software which does analysis and design for RC and PT 2D members.

I would suggest that your previous calculations did not allow for cracking or long term effects. Cracking will normally multiply the deflection by 2-2.5 and long term by about 3 on top of that!

At those spans I cannot understand why the whole design would not be PT (and bonded PT preferably!).

 

[IDS]

Thanks IDS,the SW of the RC beam is actually included in the model(rc beam model),this rc beam(16.35 m long) is an edge beam of a car park,where all the hollow core slabs spanning "parallel" to it, its side actually "skewed" a bit that's why it pick up loads,normally, when the hollow core slabs perfectly parallel to it, no loads will be transferred to rc beam except its own
weight.

I'm not sure what you mean by the self-weight of the beam being "included in the model". What model are you talking about? You have just given us some loads, which over part of the length are less than the beam self-weight.

Also how have you calculated the loads distributed from the "slightly skewed" hollow core slabs? A plan showing the layout and load distribution would be useful. The loads seem very low for a car park (presumably a commercial car park, with that span).

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hetgen]

Monchie,

750mm deep RC beam will not be able to span 16.35m! don't even bother checking.

How about building the upstand on top of the hollow core slab?

You should also allow for horizontal vehicle impact load, if this beam is to be used as vehicle barrier.

 

[Klitor]

@rapt, you're right and double checking the long-term deflection due to self-weight alone is 76.86mm!(short-term is 16.23mm)
now the only option is prestressed/posttensioned, Im hoping that "klitor" will give me his ideas regarding this, my experienced in prestressed is limited, I suggested a steel I-beam but they want an all concrete solution to the car park, its to late to request additional column in the midspan.

@klitor,in your spreadsheet, can you predict the long term deflection of the rc beam

Sure.
By the parameters you gave, the beam will pass if you put 8 wires (1.5cm2 each) and prestress them to 175kN each (total 12cm2 of prestressing steel), debond 5 of them near the supports.
It will remain uncracked for frequent load combination (as per EC0)
Its no big deal,but you should hire someone to provide you with a detailed calculation if you are not into it.

 

[monchie]

@IDS, what I mean, is that,normally when you model the rc beam, only the superimposed DL /LL will be inputted and most basic general purpose software will automatically included the self-weight of the rc beam.The loads comes from triangular shaped hollow core slab(quite small),that's why the loads are not too great but still the deflection is quite high(long-term). That's why it is being designed now as "pre-stressed" rc beam.

@klitor, is it possible to have your spreadsheet(pre-stressed0?

 

[Klitor]

I can help you out,calculate anything you need, but cant send you spreadsheed - I use it to make money

 

[monchie]

@klitor,

Thats OK, my apology.