# RC beam analysis - too low stress in rebar **URL:** \ **Category:** Uncategorized\ **Created:** [February 10, 2025, 2:30pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585 "2025-02-10T14:30:45Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 10, 2025, 2:30pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/1 "2025-02-10T14:30:45Z")
Hi, I’m analyzing an RC (reinforced concrete) beam with a rectangular cross-section and two steel bars in its lower part (no interactions/constraints - all parts are merged into a compound). The beam is subjected to pure bending. I used compression-only material for concrete. The maximum stresses in concrete are close to the expected value but the stresses in rebar are way too low: ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/b/b52eb69797a5d94101414979bcba2d8b85a94482.jpeg) It should be 170 MPa in each bar: ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/8/85f887a54f8f4734fb433092f03ba3a57771ae90.jpeg) [![](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/b/bd723ef96ab3bb2e6a25aa8774b088c2776aa8f2.jpeg "How to calculate stresses in reinforced concrete beams | Worked Example") ](https://www.youtube.com/watch?v=idwCMXAwYFs&t=407) Here’s the input file: [Dropbox](https://www.dropbox.com/scl/fi/rqku6z8zkp9icow4uozjc/RC_beam_test.inp?rlkey=xesstqny3tvs0wsi97r1u6jl2&st=fmtkeo6x&dl=0) Any ideas what I am missing ? Mesh refinement doesn’t seem to help much. At some point it even led to non-convergence. I wanted to try it in Abaqus as well but it doesn’t converge there with the compression-only material (Abaqus doesn’t have that option to allow for some small tension to aid convergence) even when embedded beam elements are used instead of solids for rebar. ---
**Author:** ![JuanP74](https://avatars.discourse-cdn.com/v4/letter/j/b5a626/32.png) [@JuanP74](https://calculix.discourse.group/u/JuanP74)\ **Post date:** [February 10, 2025, 6:02pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/2 "2025-02-10T18:02:20Z")
In frd file you have stresses averaged. If you want to avoid it, tied constraints between separated solids can be used so ccx doesn’t average the stresses in the output. ---
**Author:** ![MichaelPE](https://avatars.discourse-cdn.com/v4/letter/m/9de0a6/32.png) [@MichaelPE](https://calculix.discourse.group/u/MichaelPE)\ **Post date:** [February 10, 2025, 7:32pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/3 "2025-02-10T19:32:32Z")
The compression only card is not normally compression only, but allows a tension component. Looks like you have allowed tension in your concrete ---
**Author:** ![MichaelPE](https://avatars.discourse-cdn.com/v4/letter/m/9de0a6/32.png) [@MichaelPE](https://calculix.discourse.group/u/MichaelPE)\ **Post date:** [February 10, 2025, 7:58pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/4 "2025-02-10T19:58:20Z")
Also looks like it might habe been tensioned by different temperatures for the differing materials. ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 1:19pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/5 "2025-02-11T13:19:45Z")
> [@JuanP74](#): > > In frd file you have stresses averaged. If you want to avoid it, tied constraints between separated solids can be used so ccx doesn’t average the stresses in the output. I’ve tried with tie constraint but the stress is still too low. Maybe I should try refining the mesh (and using hex elements) but again - it may lead to non-convergence. ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/2/270a43f9ec6220c7d164b9ea3b7e7cb5a5bbd63a.jpeg) > [@MichaelPE](#): > > The compression only card is not normally compression only, but allows a tension component. Looks like you have allowed tension in your concrete Yes, I have 1 MPa of maximum allowed tension but this is just to make it converge (in Abaqus, it doesn’t converge at all because even a very small tension is not allowed). > [@MichaelPE](#): > > Also looks like it might habe been tensioned by different temperatures for the differing materials. I don’t take temperatures into account here. The analysis is only meant to provide results agreeing with those shown in the YouTube video (it’s a simple exercise from Mechanics of materials book). ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 1:36pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/6 "2025-02-11T13:36:40Z")
With tie constraints and linear hex and wedge elements (C3D8/C3D6): ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/5/5707041aee1e0bf01a8867bbeaf169b07a5e4f2a.jpeg) ---
**Author:** ![xyont](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/xyont/32/3553_2.png) [@xyont](https://calculix.discourse.group/u/xyont)\ **Post date:** [February 11, 2025, 2:00pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/7 "2025-02-11T14:00:49Z")
stress about half only from expected, can sketch problem and boundary condition (support and load symbol) provided? another possible is related to size effect and uniaxial response as in video links. ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 2:18pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/8 "2025-02-11T14:18:16Z")
Sure, here are the BC and load symbols: ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/0/0ff183f6983b26bd4a1dd7a71f6331e2804dfca0.png) So there are rigid body constraints attached to both end faces (yellow). Their reference points have only UR1 unconstrained (the other DOFs are fixed) and opposing moments are applied in that DOF on both ends (-60000000 Nmm and +60000000 Nmm). All this is defined to simulate pure bending (the exercise assumes only the bending moment acting on a section of the beam) while avoiding convergence issues. ---
**Author:** ![xyont](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/xyont/32/3553_2.png) [@xyont](https://calculix.discourse.group/u/xyont)\ **Post date:** [February 11, 2025, 2:46pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/9 "2025-02-11T14:46:29Z")
beam length and load assignment seems it was normal and properly set. > [@Calc\_em](#): > > The maximum stresses in concrete are close to the expected value but the stresses in rebar are way too low: if the condition is true then i will trust FE result first since in balanced condition and equilibrium, is satisfied, about half result of stress in rebars may indicated some erroneous is analytical calculation. i’m only quick in view of video links, this approach is an old and known as working stress in early reinforced concrete design. It seems inconsistent notation, in previous minute use total rebar areas but later is each rebars, maybe it counts double. ---
**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\ **Post date:** [February 11, 2025, 2:54pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/10 "2025-02-11T14:54:12Z")
I guess you need a simply suported beam for the moment to properly develop across the beam. That is unconstraining Uz at one side. Anyway I would measure the bending moment at the midsection to be sure you are reaching the required 60KNm ---
**Author:** ![MichaelPE](https://avatars.discourse-cdn.com/v4/letter/m/9de0a6/32.png) [@MichaelPE](https://calculix.discourse.group/u/MichaelPE)\ **Post date:** [February 11, 2025, 3:09pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/11 "2025-02-11T15:09:47Z")
The program doesn’t know the 1mpa is just for convergence. It may be 1 mpa, but it is as much as 50 times the area of the rebar. The input for the steel and concrete materials has temperatures. Make sure they are the same for all materials. Change the max tension some and notice how large a change you get in the bar strasses. Yes it is difficult to converge with compression only materials. Note also many problems end up with internal arching not accounted for with classical methods. ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 3:11pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/12 "2025-02-11T15:11:04Z")
> [@xyont](#): > > It seems inconsistent notation, in previous minute use total rebar areas but later is each rebars, maybe it counts double. I’ll have a closer look at the exercise solution in the book (the video is only following it). > [@Disla](#): > > I guess you need a simply suported beam for the moment to properly develop across the beam. > That is unconstraining Uz at one side. > Anyway I would measure the bending moment at the midsection to be sure you are reaching the required 60KNm I tried `*SECTION PRINT` in the middle of the beam and got 5.978354E+07 Nmm for the current support scheme and 5.978097E+07 Nmm for the modified support scheme (U3 released at one end). ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 3:17pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/13 "2025-02-11T15:17:48Z")
I’ve tried with 0.01 MPa instead. It converged without issues but the stress results didn’t improve. ---
**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\ **Post date:** [February 11, 2025, 3:33pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/14 "2025-02-11T15:33:10Z")
I would remove Poisson Ratio effect (=0). Not sure if it is foreseen in the formulas. ---
**Author:** ![JuanP74](https://avatars.discourse-cdn.com/v4/letter/j/b5a626/32.png) [@JuanP74](https://calculix.discourse.group/u/JuanP74)\ **Post date:** [February 11, 2025, 4:33pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/15 "2025-02-11T16:33:53Z")
are deflections according to hand calculations? How the user material manages the shear stresses? loads have to be transferred by shear, I guess. ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 7:25pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/16 "2025-02-11T19:25:52Z")
> [@Disla](#): > > I would remove Poisson Ratio effect (=0). Not sure if it is foreseen in the formulas. Unfortunately, this made the stress even lower (but the distribution is better). With PR=0: ![zero_pr](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/d/de043672f76743f4059609c50fd390ffebda90d1.png) Without PR=0: ![non-zero_pr](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/4/495f0c2d9e60ebdcaa283e74a02fca0c8a977812.png) The formulas don’t take PR into account. > [@JuanP74](#): > > are deflections according to hand calculations? I don’t have analytical results for deflection, only for stresses. I use a ready solution from Hibbeler’s book. Maybe there’s a mistake in it but I’m not really familiar with civil engineering calculations: ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/8/8b3b93fedd2cc0d0d5646054a78c722a784f1dcf.png) ---
**Author:** ![JuanP74](https://avatars.discourse-cdn.com/v4/letter/j/b5a626/32.png) [@JuanP74](https://calculix.discourse.group/u/JuanP74)\ **Post date:** [February 11, 2025, 9:09pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/17 "2025-02-11T21:09:43Z")
maximum deflection for a simply supported beam under pure bending is d=M L^2/(8 E I) = 6e7 x 3000^2 / (8 x 25000 x 788.52e6) = 3.42 mm ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 11, 2025, 9:37pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/18 "2025-02-11T21:37:30Z")
Yeah, but that’s assuming no rebar, only concrete. ---
**Author:** ![JuanP74](https://avatars.discourse-cdn.com/v4/letter/j/b5a626/32.png) [@JuanP74](https://calculix.discourse.group/u/JuanP74)\ **Post date:** [February 11, 2025, 9:41pm UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/19 "2025-02-11T21:41:14Z")
Rebar is accounted through the use of equivalent inertia provided in Hibbeler’s book. Is the usual analysis for multi material beams ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [February 12, 2025, 8:18am UTC](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585/20 "2025-02-12T08:18:29Z")
Ah, right, I see. The maximum deflection I get with PR=0 is 1.967 mm. ![image](https://global.discourse-cdn.com/free1/uploads/calculix/original/2X/f/fe49689527d95a7a33dc4c536fc91763cf00ffce.png) [Next page](https://calculix.discourse.group/t/rc-beam-analysis-too-low-stress-in-rebar/2585.md?page=2)