capacity check - shear links
capacity check - shear links
(OP)
Forgive me here but this is relevent to a British standard clause and I realise most here are US.
BS8110 (Concrete) states that all tension bars in a beam must be within 150mm of a vertical leg. It also gives other min dims. I don't know if you have similar clauses in US.
My question is this;
I am looking at an existing stucture for some increased loading, the check calls for links for the shear. The area is fine, however the link arrangment does not meet the detailing clause above. I am guessing that maybe in an older code version this was acceptable or the original design didn't need links so they put in the minimum area (albeit not to BS8110 detailing rules, but they could in theory be omitted totally).
To my mind the links are required to halt any shear crack propogating down to the main bars and reducing the capacity of the section be breaking the bond between the main steel and the concrete, Leading to failure. Hence the spacing only 150mm from the tension bar. Therefore in my assessment the non confoming detail will mean the beam in idadequate in shear due to the detail even though the area is above that required.
Any views?
BS8110 (Concrete) states that all tension bars in a beam must be within 150mm of a vertical leg. It also gives other min dims. I don't know if you have similar clauses in US.
My question is this;
I am looking at an existing stucture for some increased loading, the check calls for links for the shear. The area is fine, however the link arrangment does not meet the detailing clause above. I am guessing that maybe in an older code version this was acceptable or the original design didn't need links so they put in the minimum area (albeit not to BS8110 detailing rules, but they could in theory be omitted totally).
To my mind the links are required to halt any shear crack propogating down to the main bars and reducing the capacity of the section be breaking the bond between the main steel and the concrete, Leading to failure. Hence the spacing only 150mm from the tension bar. Therefore in my assessment the non confoming detail will mean the beam in idadequate in shear due to the detail even though the area is above that required.
Any views?






RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
stirrupus separation st
st ≤ 0,75 d (1+cotgα) ≤ 600 mm if Vrd≤Vu1/5
where α angle of the stirrup or shear bar with the axis of the member
Vrd design shear
Vu1 limit in shear of the section for compression of the web
st ≤ 0,60 d (1+cotgα) ≤ 450 mm if Vu1/5≤Vrd≤(2/3)Vu1
st ≤ 0,30 d (1+cotgα) ≤ 300 mm if Vrd>=2/3 Vu1
Shear is not a vertical section issue and I think I have read statements in a number of books where any shear reinforcement be thought to have shear containment effect on a length around where it is, that may be a wa of saying that an analogy of Mörsch truss scheme can adapt a bit to the actual position of a stirrup. Other thing is that following strictly the more recent codes one is more likely to get some reliable degree of assurance against unsightly cracks, starting for service level and beyond. But it is clear that typical cracks in shear maybe even more likely around 30 deg than 45 and so the crack will affect beyond d from the face, and any stirrup in such beyond d will have some effectiveness, specially for limit states.
By the way, in Spain the limits for separation of stirrups have been relaxed from 1998 to 2008. In 1998 were
st ≤ 0,80 d (1+cotgα) ≤ 300 mm if Vrd≤Vu1/5
st ≤ 0,60 d (1+cotgα) ≤ 300 mm if Vu1/5≤Vrd≤(2/3)Vu1
st ≤ 0,30 d (1+cotgα) ≤ 200 mm if Vrd>=2/3 Vu1
I really would agree more with this older statement than the new since trying to help to restrain a strut along some surface of the web, tighter spacings will be better.
Respect the BS code better one practicing there answer, I would have to make some reading.
RE: capacity check - shear links
Can you give a code clause that states that all tension bars in a beam must be within 150mm of a vertical leg.
I am not aware of this. I know that compression rebar in beams and columns needs to be within 150 from a link. This is to contain the compression rebar.
The detail that you post looks fine. I would include the links in your shear check calcs.
RE: capacity check - shear links
RE: capacity check - shear links
3.4.5.5 Spacing of links (see Table 3.7)
The spacing of links in the direction of the span should not exceed 0.75d. At right-angles to the span, the
horizontal spacing should be such that no longitudinal tension bar is more than 150 mm from a vertical leg;
this spacing should in any case not exceed d.
EC2 9.2.2 Shear Reinforcement
(8) The transverse spacing of the legs in a series of shear links should not exceed st,max:
Note: The value of st,max for use in a Country may be found in its National Annex. The recommended value is
given by Expression
st,max = 0,75d ≤ 600 mm
I am unsure how the values are derived but I would guess the code requirement is to ensure the shear can be adequately transferred to the links. A particularly wide member, that requires links for shear strength, would otherwise have to all off the load to the perimeter links through varying stiffness material.
It seems that EC2 follows the Australian Standard, so you might not meet the current BS standard you might meet the standard after 2010 once 8110 is withdrawn and replaced with the Eurocode.
RE: capacity check - shear links
RE: capacity check - shear links
If you dont meet the requirements as demonstrated by the EC2 testing, then although you have enough steel in certain locations, maybe you dont have it in the right places to distribute the load and prevent an 'internal' shear failure? With some digging you might be able to find out what testing was done.
If this is based on reliable test data, it may be that the BS8110 approach was just based on the PPI principle (put plenty in) so is very conservative. I dont have any 8110 references to hand. Alternatively it might be a throw back to CP110.
Does your 1500mm wide beam only have perimeter links?
RE: capacity check - shear links
I don't belive it even meets the requirement of CP110 which was less onerous. I am going to look into the handbook to 8110 when I can track one down.
I believe all shear equations come from test data rather than any exact theoretical principle and are very empirical.
One proposal given to me was to calculate the Vc (concrete shear capacity for US members) taking only into account the tension steel within the 150mm limit of a link, in my case only 4 bars. Then combine that capacity with the links. Not to sure of this approach?
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
1) your applied load and associated concrete shear stress means your require links.
2) your applied load is fixed
3) You have sufficient area of steel for shear, its just all out at the edge of 1.5m wide beam, with no transverse steel across the beam section
4) detailing as existing meets no code requirements
Problem. Demonstrate the section as existing has adequate shear capacity even though outwith the old, current and future design codes (and some foreign ones as well)???
Thoughts:
Can you strengthen the member?
Your load is fixed, can you reassess the load path or provide a different one by stiffening something else up?
Assuming no links and limiting shear stress to 0.5Vc is the capacity well under your applied (I realise this is for minor members)?
Ultimately you have a reinforcement arrangment which is outside the scope of the codes. Now as you point out the capacities and formulae in the codes are based on physical tests based on a set of parameters (cage arrangements) which you dont meet.
Do some tests yourself??? Not realistic.
Demonstrate capacity theoretically?? I'm not sure how.
Talk to some of the people who did the tests? wrote the design guides? or maybe try the concrete centre and the like?
Am i waffling? probably.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
But if you want to increase the loading on the beam, you may want to consider drilling through the beam and inserting through bolts. Is the flexural steel adequate to carry the additional loads?
BA
RE: capacity check - shear links
BAretired - the existing design loads require minumum links only but do not meet code in the arrangment.
The beam is 1000mm deep.
This is definatly a beam rather than a slab I would say. there are no alternative load distrubution paths. It is a beam sat on piles.
There is no other load path for what the client wants to do.
The new load requires design links and not minumum only.
there is a difficulty in doing any strengthening works due to ownership/liability issues.
think I'm stuck!!!
RE: capacity check - shear links
RE: capacity check - shear links
The logic behind the limit on the lateral spacing is less clear but experimental evidence suggests a reason why such a limit is valuable. One of the reasons of stirrup is to inhibit dowel failure of the tension steel....Clearly the effectiveness of the stirrup in achieving this will reduce with increasing distance of the vertical leg from the bar considered. Clearly, if a bar is placed further than 150mm from a stirrup leg, it can still be used to provide flexural strength but should be ignored in assessing Vc (for non UK this is concrete shear strength based on longitudinal reinforcement). The situation in slabs can be used to extend this interperation further. In slabs, stirrups are not required until V > Vc. It seems logical to argue from this that the requirement relating to the spacing of stirrup legs is to ensure that Vc can be maintained in circumatances where V > Vc. It therefore seems reasonable to conclude that the limitations on lateral spacing may be ignored where V < Vc.
RE: capacity check - shear links
the code for beams requires links if 0.5Vc < V < Vc + 04 for minumum links and designed if greater value.
Slabs it is simply V > Vc.
So to me the above interpretation doesn't add up for the following reasons;
1)Beams and slabs are treated differently the last sentance seems to go against the code equations above
2) If we have to ignore longitudinal bars in beams that are not within 150mm of stirrup how can we use these bars in slabs? Surely if beams and slabs are comaprable you would have no longitudinal steel contributing to shear strength since most slabs are detailed without shear reinforcement.
RE: capacity check - shear links
RE: capacity check - shear links
By AS3600, 0.5Vc doesn't apply for beams if D does not exceed the greater of 250mm and half the width of the web. The limit is Vc for those cases.
From the commentary to AS3600; "Concrete beams can possess onsiderable strength without shear reinforcement. However, this strength will be reduced if cracking occurs. Cracking resulting from restrained shrinkage and restrained thermal deformations have been responsible for a number of shear failures in members without shear reinforcement. Since shear failure can be quite sudden, the Standard adopts a conservative approach to the utilization of the strength of beams without shear reinforcement."
For some reason the same degree of conservatism wasn't necessary for slabs &shallow beams.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
The crane load given causes a problem as per the earlier posts.
RE: capacity check - shear links
Conversely, I don't see a way of acknowledging a better behaviour for the concrete respect shear because the load lasts, say, 1 day or part of it. If the load brings the member to its (code) shear strength plus safety factor (the load safety factor consumed), you bring the structure at a situation that is not resisted every 95% of the times, or so goes the limit states theory.
So the safety factor on loading is what separates the beam of that degree of safety, to be safe 95% of the times. And this level of safety is sought to be attained anywhere reliably in the structure by following the normative clauses. So for longterm structures you keep 1.5 or 1.6 times the required strength to be a sound structure 95% of the times, and for temporary works, 1.25 or 1.35 times such required strength (less, but still well over the required strength to be safe 95% of the times).
Shear is a fragile failure and the mere average shear stress is a good indicator of its closeness to failure. For ordinary concretes of say 20 MPa compressive strength of the past decades, if you showed to have over 3.5 MPa in shear in a section, you were looking at rubble in a steel cage.
Nawy puts the limit of shear stress of plain concrete at about 20% of the compressive strength (without concomitant compression). Essentially, we do NOT want the plain concrete to crack in shear since a fragile condition, so we normally project our sections to limit values that forfeit before than anything the shear failure of the plain concrete within. We are when reinforcing in shear, in a situation akin to when reinforcing glass with some mesh, or some normal (quite deep) footing with a mesh. In all the three cases, it is expected that the plain material will crack before the steel does what it can; for the glass case most of the times just keep the pieces together; for the footing case, it is unlikely you will have more strength from the mesh rebar than of the bare concrete section itself but at significant amounts of rebar unusual till recently mandated at some places; and for the shear case, if you let to go the average stress in shear (without concomitant compression over the section) to 20% of the actual compressive strength, you will have a ruined member.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
1 The transverse spacing limit is basically to prevent dowel failure of the tension reinforcement
2 Any bar > 150mm from a stirrup leg should be ignored in the calculation of vc, but ccan still be included for flexure.
3 The spacing limit can be ignored if vu < vc.
RE: capacity check - shear links
i.e. if you have your Vc based on the bars in 150mm of a leg, in my case the first two bars on the outside of the section. The Vu is greater than Vc can you include the links in the shear resistance? I only have vertical legs on the outside (side faces) of the beam. Probably explained better by my sketch attached in an earlier post.
RE: capacity check - shear links
But this can't make for the lack of compliance on transversal separation of vertical legs of shear reinforcement.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
A repeat of the worksheet with the same moment and fc at 10 MPa gives 85 tonne shear concurrent with 30 m·tonne (both factored), and with fc entry at 30 MPa the 30 m·tonne factored moment can be concurrent with 155 tonne factored shear. By tonne understand metric ton. It seems all quite linear growth in bearable shear with strength. In all three fc cases, at separation 15 cm you are over twice closer than required as a minimum (as the sheet was made). In all cases shear cracking and strut compression remains safely apart from limit values, so a failure of the strut and tie scheme seems unlikely.
Please look the worksheet and look specially for some area of bad input (input goes in blue), gladly may approach more another copy to your case.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
Estimates according to Rahal give even lower than by the previous evaluation.
One evaluation (on Gaetano@Puleri at ACI SJ) goes to 480 metric tons for fc=30 (without fi reduction). Maybe not as dissimilar as apparent, they surely are targeting the actual final value (put, as quoted above, by Nawy, about 20% of fc, here shown less percentage, depth effect likely) and so not comparable to code practice. As much difference we must expect between what we design and the limit strength, 3.5 for ordinary checks may be quite likely and over 4 for connections and risky checks, like fragility in shear. This is not a code check, but makes clear that one shouldn't expect THIS member fail quickly in shear.
A check as per ACI as I have it in a render of it would accept 190 tonne factored shear.
So you have a whole gamut of opinions about your member.
Hope this helps.
RE: capacity check - shear links
On another note, isn't there an exception in the code that allows no stirrups to be installed if Vu < 0.5*phi*Vc (See ACI 318 11.5.6.1). Also in ACI 318 sec 7.11, you should pay close attention to the wording which activates the tie spacing requirements for "compression reinforcement in beams." If you aren't using the longtudinal bars for compression, then you don't need to meet thetie spacing requirments.
If all else fails, propose strengthening (vertical epoxy dowels, fiber wrapping, ect...).
RE: capacity check - shear links
RE: capacity check - shear links
Field chack is the next step, the structure is only rented so any remedial work opens up a new can of worms
RE: capacity check - shear links
I recently downloaded the freely downloadable RESPONSE series of programs and may use your case to try its use.
RE: capacity check - shear links
RE: capacity check - shear links
RE: capacity check - shear links
http://www.ecf.utoronto.ca/~bentz/download.htm
In the first page note your section (forfeit the loads, these are not those later applied). Note that a 15M bar is accepted to have 200 mm2 section at the program.
RE: capacity check - shear links
RE: capacity check - shear links
In all, even when the program is for shear-field action and so not the best choice where strut and tie schemes are, note that by this MORE realistic assumption than a pure Mörsch scheme there is still no cracks at your section, and the implied deformation has not even started to pass stress to the stirrup. So this is another favorable opinion on that the section will behave well.
You see in this model I have counted the 9 bars atop and at bottom, and not included any skin steel. I plan to enter the 4 above 4 bottom plus some skin steel case later; if something negative develops I will let you know, otherwise give this free nice program a try, it takes 15 mins to get these results.
RE: capacity check - shear links
RE: capacity check - shear links
I am awaiting further loading information in order to proceed.
RE: capacity check - shear links
A few thoughts here - I think what hokie66 suggested above might be the direction I'd take - look at a reduced Vc based on what you DO have in terms of longitudinal bars contributing to shear strength.
In the US (ACI 318) I don't believe there is any provision similar to yours where stirrups must be spaced out across tension bars. Compression bars yes. I've always added additional links(stirrups) for wider beams though...just for feel good reasons.
The shear provisions for slabs vs. beams is based upon the idea that slabs can transfer load across slab widths and thus add to some redundancy. Beams are single, non-redundant elements so more conservatism required.
One "fix" idea: Could you drill vertically through the beam and install vertical bars in grout/epoxy/adhesive?...or perhaps plates top and bottom? Might be a lot of work and you might have struggles with hitting the longitudinal bars but since you said you were "stuck" this fix came to mind.
RE: capacity check - shear links
RE: capacity check - shear links
"If it is a wide, flat beam, you should be able to use the one way slab provisions rather than the beam provisions"
as using slab provision vs. beam provisions and maybe getting it to work....I was probably half asleep too.
RE: capacity check - shear links
RE: capacity check - shear links