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Robot follows:
8.4.2,
8.4.3(1)-(3),
8.4.4
8.5 (stirrups)
The bar capacity (maximal force in bar) looks like this:
In case the length of a hook is above 5 * bar diameter Robot applies 8.3(3) + 8.4.2 to limit the maximal force to Fbt assuming bent diameter according to the table 8.1N
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Artur Kosakowski
thanks for your reply.
The procedure seems right but I guess it is too simplified, (please note the help states that the figure you have shown applies to the Polish code and I was refering
to Eurocode).
I guess that it would be extremely useful to allow the user with the posibility to adjust the critical parameters for the development length, such as the mandrel diameter
and confinement by transverse reinforcement since these two may lower the length of the straight segment before the bend, providing that the rest of the parameters
(cover, spacing) are properly considered in the program once the rebar arrangement is done.
in particular, my concern is related to anchorage of longitundinal bars into the beam-column node. I did a very simple example consisting of a RC beam framing into
two columns (please find attached the model). All rebars are 16mm with hooks at one end only.
I noticed that there is basically no diference between bending capacities of the two ends (see the second image).
In both cases (left and right support) appears that the anchorage length is developed without considering the hook. This approach leads to
a great deal of additional reinforcement nearby supports which is not effective nor realistic.
The hook length is in direct relation with mandrel diameter by means of eq. 8.1 of EC2. which basically means that you can provide any hook if you are
properly taking care of mandrel diameter. Further on, Figure 8.1 (a) applies for any rebar shape measured along the centerline. Figures 8.1 (b,c,d)
are just shortcuts applicable to standard cases.
For instance, considering a C25/30 element, a 16mm rebar with a mandrel diameter of 7D(112mm)
and a spacing of 90mm the capacity of the bar Fbt before hook would be around 35kN (40% of its entire capacity). Assuming an anchorage length of
750mm, the straight segment would be 0.6*750=450mm, which means the bar is capable of developing almost its entire capacity at column face, which
is not the case Robot is showing. And I dont know how to control the program in order to get better results.
I noticed that there is basically no diference between bending capacities of the two ends (see the second image).
In both cases (left and right support) appears that the anchorage length is developed without considering the hook
I have made a quick test to check the influence of the hooks by reducing the anchorage lenghts of the rebars (I moved the suupport 20 cm outwards keeping the reinforcement as it had been defined by you) and I can see the difference.
Artur Kosakowski
The initial conclusion is that in some situations for calculations of anchorage capacity of a rebar Robot attributes a rebar in tension as being under compression. If this happens there is obviously no influence of the defined hook. The development team will work on this situation.
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Artur Kosakowski
thanks for the reply! 
I didn't specified there any seismic conditions, just to keep things as simple as possible. The top rebars nearby supports are subject to tension only.
So, this is not the case here.
As a side note, my humble opinion is that Eurocode 2 is wrong about longer compression vs tension anchorage, since BS,
ACI and other sound references indicates the contrary. So I guess it would be extremely useful if Robot could handle such
a delicate issue by means of providing the user with the posibility of adjusting the significant parameters (mandrel diameter,
etc..) in order to obtain a resonable rebar arrangement to real world problems.
This whole anchorage thing may pass as an unimportant issue, but to the knowledgeable R/C designer it makes the diference
between a good or a bad design. It's really worthwhile investing some time into this situation.
cheers!
The top rebars nearby supports are subject to tension only. So, this is not the case here.
Yes, it should not be but as i wrote "The initial conclusion is that in some situations for calculations of anchorage capacity of a rebar Robot attributes a rebar in tension as being under compression" it actually may be
We are still investigating this issue.
Artur Kosakowski
I didn't specified there any seismic conditions, just to keep things as simple as possible. The top rebars nearby supports are subject to tension only.
So, this is not the case here.
Yes, it should not be but as i wrote "The initial conclusion is that in some situations for calculations of anchorage capacity of a rebar Robot attributes a rebar in tension as being under compression" it actually may be
Edit: Can only be in the situation when cd is exactly equal to 3 fi (so indeed not a case here).
As a side note, my humble opinion is that Eurocode 2 is wrong about longer compression vs tension anchorage, since BS,
ACI and other sound references indicates the contrary. So I guess it would be extremely useful if Robot could handle such
a delicate issue by means of providing the user with the posibility of adjusting the significant parameters (mandrel diameter,
etc..) in order to obtain a resonable rebar arrangement to real world problems.
This whole anchorage thing may pass as an unimportant issue, but to the knowledgeable R/C designer it makes the diference
between a good or a bad design. It's really worthwhile investing some time into this situation.
After making the full check on your example:
EC requires longer design anchorage length in case of a bar with a hook then for a bar with no hook as due to larger alpha2 value:
It so happened in your example that the left side capacity (with hooks) looks almost the same as the right one (no hooks). If there is no hook then l_bd = 0.7304m and the full capacity is reached at x = 0.770m (see the blue line on the diagrams below):
with 90 deg hook with its straight length being 0.08m (5*fi16) then l_bd is 0.920 m and he full capacity is reached at x = 0.865m.
Due to the stress limit at the start of the bend Robot takes into account only straight length of a hook limited to 0.149m therefore setting its value as 0.3m doesn't change the results and the full capacity is reached at x = 0.796. which is similar for no hook situation (x = 0.770m)
For your suggestion - we are not planning to introduce a way of overwriting EC provisions .
From my point of view I'm considering this situation as explained
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Artur Kosakowski
Just got back from vacation. Thanks for putting so much effort into this matter.
and thanks! for all these clarifications, they're very useful.
Eurocode leads to some huge absurdities like the one we're getting here (longer development length in case of rebars with standard hooks
918mm, than simple straight bars 740mm). So, I guess it all comes down to everybody's own engineering judgment
when facing such situations. That's why the posibility to overide some constraints here and there may provide the advanced
user with the oportunity to come up with a realistic design from this very stage.
I don't want to push this discussion any more further but I would really like having few more parameters - and I'm talking
here just about legal ones (such as mandrel diameter) which could be very useful to take full advantage of the
reinforcement module since all of us have to deal with real world problems after all.
Hello,
I don't understand the results I'm getting from my models with RC beam design module. Capacity diagrams start to generate at the face of the support, like it's telling me I have an insufficient anchorage length even tough my hand calculations tell otherwise. If I increase supports width enough, capacity diagrams start to behave like I would expect them to do and rebars tension capacity start at the beginning of the bars - not at the face of the support! Here are results from a simplified case I manually calculated to be ok, but which from Robot I got warnings concerning insufficient capacity for moment and shear. I'm having this problem with most of my models. I'm confused is there something in the eurocode that I'm not aware of or is the problem in my models.. Help would be greatly appreciated! I also attached the model.
Based on the approach as above Robot calculates sigma_sd from the below formula:
which means it is equal to the design resistance of steel
Lbd_req = 97cm
and
Lbd-min = 29 cm which is larger than the support width minus the cover measured form its origin.
When you increase the number of calculation points from the default 11 you will see that the diagram starts about 3-4 cm from the support face.
My understanding is that you calculated sigma-sd using formula (6.18) at the face of the support:
whereas Robot uses the approach described in 9.2.1.3 instead:
Looking at your approach I'm wondering if actually you shouldn't calculate sigma_sd also in points along the first part of the span (beyond the support face) till the location where the steel if fully used and see what value of Lbd_min you get.
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Artur Kosakowski
@Artur.Kosakowski wrote:
Based on the approach as above Robot calculates sigma_sd from the below formula:
which means it is equal to the design resistance of steel
Lbd_req = 97cm
and
Lbd-min = 29 cm
I don't understand why I should use the design resistance of steel at that point, because "at the position where the anchorage is measured from" sigma-sd does not equal fyd. Besides wouldn't lbd req: ( 20mm / 4) * (434Mpa / 2,7Mpa) = 80,5cm and therefore lb,min = 24,1cm? So I would still have lb,min at the support of b=300mm?
Let me make another presentation of my calculations:
So using this method I see that I indeed would need to increase my anchorage length so I decide to try to increase support width to 380mm.
So besides calculating "the additional tensile force in the longitudinal reinforcement due to shear" 0,5Ved(cot theta / cot alpha), but actually taking also the tension force from the bending moment into account at the side of the support I start to get closer with Robot's results. I still wonder why bending moment capacity diagram reaches it's full capacity at 1,2 m.
It seems that you use C25/30 in your hand calcs but you set C20/25 in Robot.
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Artur Kosakowski
Somehow I had a feeling there was something like that going on.. I should have noticed that! However I managed to learn something in the process and I'm now satisfied with the results I'm getting from beam design module. Thank you for your help!
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