I Broke These Concrete Beams - Design Principles from Beam Failures
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- Опубликовано: 20 май 2024
- I constructed six reinforced concrete beams in the lab and then loaded them to failure. What can we learn about reinforced concrete behavior and good design principles from these tests?
Two of the beams exhibited flexural failures with lots of flexural cracking and ultimate crushing of the concrete in compression. Two beams had traditional shear failures, with a diagonal crack extending through the section. Finally, two beams had anchorage failures, where the ends of the bars pulled out at the support. Good beam design encourages flexural failures because of its ductility and predictability.
Chapters:
0:00 Beam Fabrication
0:49 Test Setup
1:23 Beam 1 Test
2:31 Beam 2 Test
3:28 Beam 3 Test
4:19 Beam 4 Test
5:31 Beam 5 Test
6:23 Beam 6 Test
7:03 Results
8:15 Lessons Learned 
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You will have to repeat #5 and 6 beams with rebars extended outside the beam to prevent anchorage failiure.
Yes I agree..or move the 2 supports closer.
@@geraldborja144 Indeed that was disappointing...
Rebars are not supposed to go outside though
To improve the anchorage, the end has to form an L shape pointing up in this case
Or as mentioned in the other comment, have the points of supports farther away than the end of the rebars ( either by moving the supports or by extending the beam)
Cog the bars
yes i think those hor. rebars are few short to the support area
I used to had a fear of buildings falling, I used to believe buildings just crack, but this video made me aware that buildings bend and they show signs before falling. Now i can rest well looking at concrete ceiling. Thank you so much
A well designed building will bend and show distress before failure. In practice we always try to avoid failures like we saw in beams 3 through 6. If the building will fail, we try to follow something like beams 1 or 2.
They can do both.
Building have alot of safety factors sprinkled in every step of the calculation
Appreciate this type of content. Keep up the good work sir 🙏🏾
This has to be the best video I've seen this year, so informative. Subscribing right away
This is very beautiful video. I work 15yaers like structural engineer but when you see it real it si much more better than read in books ! Thanks !
Glad you enjoyed it!
Wonderful beam test and analysis! Thanks for the helpful information.
Thank you! I'm glad you found it helpful.
I LOVED ur videos, especially this, it's kind of hard to find this kind of material with so much quality and professionalism, would like to see more domestic examples from other countries constructive processes and material, as in Mexico is used a pre fabricated steel elements called Armex, and see the pros and cons of confinement process, or called also load-bearing walls.
Greetings guys! Keep the quality
Appreciate your effort! Needs more such stuff!
More to come! I plan on doing new tests each semester, and I have a few other lab tests in my back pocket that might be interesting.
Wonderful video teacher, well explained. Thanks
Fantastic video, and a question, if you were to bend the rebar ends to allow the ends of the beam to be connected to the bottom of the beam in that way wouldn't that be able to prevent the ancor failure?
Hi Prof H, superb test. Very insightful. Instant sub. Thanks! ❤️
P.s: trying to find out info about placing a steel iBeam inside a concrete beam; think it’s called a compound/concealed beam. Are they advisable in a situation with high load? 💭
Thanks a lot for sharing prof.H
You're great explain led sir we need more video about structure design
Dang, I was excited for beam #6. Would love to see it’s potential after anchorage failure issue is addressed.
Working on it!
Hi, Dr.Broke.. it is amazing veido.. i want to know how you design these beams?? Are you following the ACI cold to determine the dimensions, main reinforced and stirupps ?? Can you let me know . I am interested to know about that. Thanks
shear and anchorage more important, we tends to put less emphasis on them. v good
Thanks very much Prof. Appreciate it.
This experiment more related to a pre-cast design otherwise development length plays a big role here. Top and bottom extra bars are also play a big role in construction. Also at the L2 should have stirrups closer than in the mid section.
Good stuff. It looks like bearing failures for those last two. A horizontal U-bar or two at each end at the bottom should do the trick
That's a good idea, Sam! We have a bunch of extra stirrups floating around our lab, so next time I'll drop a few of those horizontally into the end regions. That is an easier fix than making the beams longer (would need new forms) or moving the supports in (lab floor connections are where they are, and not much I can do about that without fabricating new supports).
I really wanted to see a compression-controlled flexural failure, but there is always next time. That would make a good follow-up video.
Dear Sir,
First of all, thank you so much for making such a practical video of structure behaviour after applying load.
Would you like to let me know in which software we can make the same animation of any beam and columns carrying load and showing us the results?
Thank you
Sir
Which is better to install
A girder or a concrte column and beam
I bet your classes would have been a blast. Unfortunately, i was finishing my masters when you started.
Thanks; please post concrete compressive strengths too.
For these beams, the concrete strength at testing was only about 3,000 psi (20 MPa). We did not test the rebar yield stress, which would have a greater impact than concrete strength for the flexural capacity.
@@StructuresProfH Thank you. 🙏
I just discovered your videos and I'm already a big fan. However, I am also an amateur to this. I have a couple of questions that I hope are not too naive. First, have you performed any of these tests with GFRP, a fiber reinforced resin based rebar? The second question is where the more experienced people on here might giggle at me. Would it be reasonable to build a metal truss within the cement? It seems that the amount of metal being used in some of your tests could be made to hold a lot more pressure if it were built in a truss pattern instead.
Glad you liked the video!
I haven’t tried any tests with GFRP bars yet, but that would be very interesting. In general, GFRP is higher strength than steel but it lacks ductility, so no more ductile yield plateau behavior. It also has lower stiffness, which can mean larger displacements.
As for the truss, the beam already kinda does that for you - the steel carries the tensile forces while the concrete carries the compression. So the concrete forms the top chord and the “diagonals” of the truss, while the rebar forms the bottom chord and the vertical members. Sure, you could layout the steel in a truss-like configuration, and it may have really great performance, but the standard rebar “cage” is much easier to build.
what does anchoring at its ends mean?
Brilliant dear sir!
Hello, sir! I have a question? How do you determine these failure results such displacement and maximum loading before beam's failure in ANSYS?
Also me , I want to know how you design these beams?? Are you following the ACI cold of beam design or how you determined the dimensions, the reinforced of longitude and shear?? I hope to let me know???
Thanks you sir
It was a great video
Prof H, I'd like to run through the same testing you did on our precast concrete products. What were you using for your displacement sensor, pressure sensor, signal conditioning, and your DAQ?
I'm not part of the team that did this, but I may be able to help as I have done these tests in the past (specifically the thirds tests done here). Deflection is usually measured by a laser displacement sensor below the beam or you could use an LVDT for more accurate measurement. For pressure, if youre referring to the force applied to the beam, the test rig (pretty much a specialised hydraulic press) usually has a force and deflection readout. You dont want a pressure readout in these conditions as the diameter of the hydraulic cylinder would then be a factor. The force applied by the press can be halved as shown in the video where P/2 is used to calculate the SFD and BMD for the beam at any point in the test.
Some machines can be set to increase the force applied over time, or deflection over time. For both methods, you will know your beam has failed when the force readout reaches a maximum and then starts to decrease consistently, not just briefly at the point where the steel yields (assuming you haven't over reinforced the beam).
Using the machine in deflection control is usually much safer. Because once the beam fails, the hydraulic cylinder will just coninue downwards at the same, usually very slow, speed. If you use force control, the machine will speed up after failure trying to apply a higher force. This cannot happen after failure due to the beam no longer being able to resist the ULS force which the machine is now trying to surpass.
If/when you conduct these tests, be aware of the very sudden nature in which the beams fail, it can be quite jolting or even quite violent if over reinforced. The over reinforced beams will fail without any warning.
I love structural engineering and CAD simulations, but you really can't beat a real world test!
What was the calculated load capacity of each beam compared to the observed load capacity at failure? In other words, how close do our calculations match reality?
For Beam 1, the flexural capacity was about 110 k-in., which should occur at a load of 9.2 kips. We observed about 12 kips of capacity. These calculations assumed nominal material properties for concrete strength of 4 ksi and steel yield of 60 ksi. Measured concrete strength was a bit lower (a bit over 3 ksi), but that barely affects the flexural capacity. We didn't test the rebar, but it was likely to have higher yield than 60 ksi, which would have a major impact on the capacity.
Repeating for Beam 2, the flexural capacity was about 162 k-in., which should occur at a load of 13.5 kips. We achieved nearly 16 kips in testing.
Beams 3 and 4 had a reinforcement ratio of about 2.94%, but did not have adequate shear reinforcement. Using ACI 318-19 Table 22.5.5.1(c), we would get Vc = 8.0 kips using the nominal concrete strength of 4 ksi. Assuming the stirrups provide no strength (which is reasonable, as they are spaced so far apart), this failure would occur at an applied load of 16 kips. Beams 3 and 4 saw about 14 or 15 kips of load, respectively. If we instead use the measured concrete strength of about 3 ksi, then Vc = 6.9 kips for a failure load of 13.8 kips - that's very close!
For Beams 5 and 6, this didn't look like a traditional "shear failure" because of the anchorage issue. However, the estimated shear capacity for these two beams, again neglecting the stirrups and assuming nominal concrete strength of 4 ksi, would be about Vc = 8.0 kips, meaning a failure load of 16 kips. That's pretty close for Beam 5 (failure load of just under 16 kips), but a little low for Beam 6 (failure load of about 18.5 kips). The stirrups maybe provide some help for Beam 6, if only to force the crack to take a steeper incline angle compared to the more traditional shear failures of Beams 3 and 4.
Your 10 minute-video is worth more than a few days of reading books for the same info. Thanks
Great content
What is the gap or inches of beam 2
It appeared that your supports were at the end of the bottom rebar or even somewhat outside of them. You need the steel to extend well beyond the supports.
Yep, I agree 100%. Best practice for simple supports is to terminate the steel at least 6 inches (75 mm) past the face of the support (ACI 318, section 9.7.3.8.1).
Though you can't see it, the same supports and bar termination was done for all the beams. This was done improperly here partially for ease of construction and partially to make a point.
Exactly my thoughts
I think in construction, concrete should leave up to 21 days to dry well
@@jpcraftconstruction.1828 It does get stronger with time, but the reason for failure of the stronger beams was the improper placement of the supports which is misleading in regards to which beams likely would have performed best. The weaker beams didn’t have sufficient capacity to overload the points above the supports, but the stronger beams did. This just confirms why the proper balance is important in design. Making one area stronger simply moves the failure to a different place.
@@StructuresProfH oh ok
Can you also explain rammed earth structure failures compared to concrete beams? Which would be the better option and why?
I’m no expert on rammed earth construction, so I’m afraid I can’t provide much insight. Sorry. That said, structurally it seems limited to walls and other systems that carry relatively low levels of compressive load and essentially no tension. If it’s stabilized with cement and has some rebar, I presume it behaves much like a low strength concrete or CMU wall?
Thank u prof❤
Thanks for sharing! Your tension bars do not appear to have been developed; need to hook them at the ends.
Yep. Our other idea, for next time, is to add some horizontal stirrups at the ends, mainly because we have a bunch sitting around the lab.
Why didnt you bentup the bars i think by doing that edges of beam wont show signs of failure if you add hoop reinforcement as well
Thank You
Changing the rebar iron with high tension cable will make more strength?
Possibly, yes. Moment capacity requires a balance of tension (in steel) and compression (in concrete). With low strength concrete, you might not get that much benefit from high strength steel, especially if you can’t yield the steel to use it to the fullest.
Plus, in practice, higher strength materials may run into serviceability or deflection issues - high strength does not always mean high stiffness (for example, all steels have roughly the same elastic modulus, regardless of strength). It may hold the load just fine, but have unacceptable deflections.
Architect here! I have a question does the type of hook provided in the stirrups matter? And how does it affect the Shear capacity of the beam?
As a general rule, a 135-degree hook is better than a 90-degree hook. However, we typically assume that the hooks in the stirrups don't matter for capacity, so long as they satisfy prespecified "standard" hook conditions. So for example, ACI Table 25.3.2 has standard hook geometry for stirrups - as long as we are following (or exceeding) that, we assume we get the full design capacity from the stirrups.
However, there are some conditions where the type of stirrup hook may be more important, like for seismic design (ACI Section 18.6.4) or for beams carrying torsion (ACI Section 25.7.1.6). Again, we treat these prescriptively - the stirrup hook geometry doesn't factor into our design calculations so long as our hooks meet the standard.
You should have bended the rebars at a 90 degree angle at both end of the beams
Load on beam ❤ beam test
❤thank you
i learned from this..
Firstly, Very Interesting Demonstration. Sir, I have a query. In case of beam 6, although the number of stirrups are sufficient and closely spaced enough why are we having shear failure despite having shear capacity to be on higher side? Also, why the main longitudinal bars are pulling out? are those not properly anchored and embedded in the concrete? and also, Why shear failure is coming in beam 6 even though we have enough shear capacity. I will be glad if you can please elaborate and resolve my query.
The longitudinal bars for beam 6 are definitely not anchored properly. They have a relatively large diameter, no hooks or anything to keep them in place, and the end of the bar was directly above the support. Ordinarily (and per ACI 318 code) you would need these bars to extend at least 6 inches past the face of the support. Once the bar started to pull out, it initiates into a shear-like failure. Though some of the simplified code equations for shear capacity neglect it, the longitudinal bars do play an integral role in developing shear capacity; in this case there was effectively no longitudinal steel area at the ends after bar pullout. I call it "shear-like" because unlike a traditional shear failure, which would be expected to follow more of a diagonal crack path, the crack extended at a very steep angle from the support. So I guess I'd call it more of a combined anchorage-bearing-shear failure, but initially caused by the poor anchorage.
@@StructuresProfH Sir, you meant at the support end stirrups were not there to anchor the longitudinal bars? Otherwise it would not have undergone pull-out. However, I meant enough shear capacity in the sense that stirrups provided were sufficient enough to provide shear capacity. Since, the bars pulled out at first place due to ineffective anchorage the stirrups didn't play any role. Am I correct?
You will have much more shear breaking in short beams and momentum breaking in long ones
Thanks......
The reason for the collapse of the beams in all cases is crushing in the concrete, right?
Brittle failure at the beam ends also
It seems to me like beam 5 needs to be longer. The weight can't break it at the center so it breaks it at the end since thats where the opposing force is
"For you folks in metric" literally everybody outside the US, and a bunch inside the US.
Shout out to the rest of the world, and thanks for watching!
I'll always have a place in my heart for English units though.
Would have liked to see tests 5 and 6 repeated so we can see actual results without the testing environment being the failure cause. Also, when the rebar was placed into the forms, I didn't notice any spacers/chairs on the rebar to prevent it being to close to surface. Were those intentionally left out, or were they just not visible on camera?
I agree. I actually did try these two tests again recently, with three horizontal stirrups/ties placed at the each end above the supports. Ideally, these should have bridged the split that formed at the beam ends. Long story short, it didn't work out, and I got the same failure mode again! Didn't publish a video on it, as I didn't think it added much value. Just goes to show that there is no substitute for good rebar anchorage at the ends.
As for chairs, yes they are there, but rather hard to see. Concrete cover is lower than would normally be acceptable for code (only 0.75 inches), on account of the beam's small scale and these not actually going into a building.
It kinda looked like the concrete failure didn't split the aggregate. Weak concrete? At What psi did the cylinders test / break?
It was indeed weak concrete. Cylinders broke at just over 3000 psi (~21 MPa).
no vibration????
How is it that nobody’s pointing out in the comment section that they use the a concrete truck for this little bit of concrete 😂😂
One cubic yard! Yeah, it’s a ridiculously small load, but our concrete supplier is really great about this kind of stuff. It’s certainly easier than mixing by hand (don’t have a drum mixer with this capacity).
Cool !
I agree :)
u need make negative iron for strong resist
جيد شكرا
Nice
Lớp trong sắt dài, chịu lực ... Mà chỉ có. Vài sợi dây Fe mỏng vài zem
7 days?😂
it is need a cranked bar to avoid bending the beam
Did you wait 28 days for it to cure fully?
Yes. To clarify, we did a wet cure in the forms for 7 days. After 7 days, we removed the beams from the forms, but they were not tested until just after 28 days.
To be fair though, many concrete structures in practice are loaded at ages much earlier than 28 days, despite that being the “standard” design strength. Strength gain is not linear. You gain most of the strength within the first week, but you can also gain strength (very slowly) for a year or more.
@@StructuresProfH Fantastic, thanks for sharing this video!
tie the iron link like a fool tie, and there are too many cavities in the beam
Need matches with. Theory?
Great
Thanks!
5 and 6 must be the strongest because the failure occur where the steel and bars end 🤔
Interesting 👍🏻🏗
I agree!
Can i use your videos to my channel to explain the same in my regional language
Sorry, I would prefer not. My videos are not licensed under Creative Commons.
And consider that these beams were otherwise constructed with care, out in the wild partiality skilled labor and / or poor quality materials further reduce the strength of the construction.
5 & 6 beams didn't fail but the testing equipment failed actually
7 days only?
Sorry, I realized I wasn’t clear on that in the video. We removed the beams from the forms at 7 days but tested at 28 days.
Brotha for accurate test you suppose to do a 7 day, 30 day and a 60 day. Those are thick so it’ll probably take a little longer to mature. 7 days is not a lot of time to mature.
Pushin P
Upload this in p df format
hay muchas cosas que decis que no estan bien teoricamente
Insufficient reenforcement that's why having a cracking concrete
beam 6 Failure was due tot he fact that Steel is not over the points stress and the Concrete Cracks.
Hey, can you make a video commenting these overspec foundations: ruclips.net/video/NlehbT8zFHg/видео.html
Do you think they have some wild expansive clay like in texas?
You need more study
Is a misleading video 😢. When you didn't extend and anchor your reinforcement accurately, what do you expect?
Was thinking same
Your Stirrup bending doesn't follow standards, and vertical bars should overlap columns bars. These are not actual stress conditions for beams.
🇦🇴🇦🇴🇦🇴🇦🇴🇦🇴🇦🇴🇦🇴🇦🇴It Is incredible ; test well done; work done sucessfull
❤❤
I dont know what is the logic of putting your support at a very edge of that beam where there is no steel. 🤨ofcourse it will crack in that position.
Use more steel. Simple
We definitely could have used more steel at the end of Beam 6 for sure! But it’s not always so simple. If Beam 6 had proper anchorage at the ends, it probably would still have had an undesirable failure mode - compression controlled flexural failure. Such a failure is brittle. The steel will not yield, meaning there is such a thing as too much steel.
❤🎉
Moments were not my thing st all.
❤❤❤❤❤❤❤❤❤❤❤❤❤❤
Boop
there should have been proper stirrup. and the #5 failed because the steel is short . it cracked at the very end where the steel ends. meaning the load and stress where only in the concrete . not the steel.
4 valid tests and a bunch of wasted time and material for 2 more tests
Cool test poorly done concrete tho.
Thanks for watching! But yeah, we are definitely not professional concrete masons.
How in the hell is this demo credible? Only a fool would build rebar in such a way where it does not tie into any supporting columns or rebar structure on the ends? I just don't understand why the rebar did not extend longer to the end of the form? This makes the demo dubious!
Improper anchorage is part of the point of this demo. You don’t really need any “structure” at the end of a simply supported beam. The rebar just needs to extend far enough past the support (6 inches per ACI-318), among other checks for development length that are difficult to satisfy for such large bars. Obviously, that wasn’t done here.
Repeat
After 7 days I can break ur beam with my bare hands. Ur beam didn't hv support of rings.
A C I 318- 19
L o a d 12 k i p s
I did not learn anything from this video
This was for fun purpose.
Yes bro ❤
The purpose seems to be simple visualization and way to engage with a real world beam.
😂😂😂😂
Thanks for wasting my time.
You are welcome. Always looking for a good time-waster.