capacity check - shear links 2

[oneintheeye]

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?

 

[ishvaaag]

A scheme maybe would help ... I think you refer to stirrups or ties as links, and the stated minimum separation for some case, but would love to ascertain if such is the case.

 

[oneintheeye]

sorry another difference in terminology, yes a link is what you would call a stirrup. Vertical or inclined bars to resist shear. I have attached a sketch. Basically the area of stirrup steel is ok, but the positioning does not meet code. I am trying to assertain the problems associated with this.

 

[hokie66]

I can't help with the British Standard requirement. The Australian Standard AS3600 specifies a maximum transverse spacing of stirrup legs to be the lesser of 600 mm and D, which is a less restrictive clause than yours.

 

[oneintheeye]

so if you were asseesing a structure and it had greater spacing would you take the links as uneffective and neglect all the links in your analysis?

 

[hokie66]

No, I wouldn't neglect them. I would try to research the impact the variation from the code provision has on the capacity of the member, just as you are doing. I just can't tell you why your code has that provision.

 

[oneintheeye]

how does your code approach it if the detail is as described?

 

[ishvaaag]

by spanish code EHE 2008
stirrupus separation st

st ? 0,75 d (1+cotg?) ? 600 mm if Vrd?Vu1/5

where ? 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+cotg?) ? 450 mm if Vu1/5?Vrd?(2/3)Vu1

st ? 0,30 d (1+cotg?) ? 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+cotg?) ? 300 mm if Vrd?Vu1/5

st ? 0,60 d (1+cotg?) ? 300 mm if Vu1/5?Vrd?(2/3)Vu1

st ? 0,30 d (1+cotg?) ? 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.

 

[patswfc]

Herewego,

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.

 

[oneintheeye]

BS 8110-1 clause 3.4.5.5

 

[Ussuri]

BS8110

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.

 

[oneintheeye]

nice try ussuri but I've already gone down that road. It doesn't meet the eurocode criteria or BS5400 (bridges). The beam is 1500mm wide. The links must have some affect but if I follow BD44 which is an assessment guide for bridge decks it says they are ineffective, but I take that as a conservative code cop out.

 

[Ussuri]

having a look through some of design guides, one of which states the limits in EC2 come from test data. The spacing has been chosen to ensure that a shear failure plane cannot be formed between two adjacent sets of shear reinforcement (designers guide to eurocode 2 by Hendy and Smith, p279).

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?

 

[oneintheeye]

yes it does only have perimeter links, so 2 legs on each edge of the section.

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?

 

[Ussuri]

I assume you mean the shear capacity is dictated by two thin sections (150mm+50mm?) either side of a 1500mm wide beam. How would you demonstrate any load carried on the 'middle bit' is transferred to the perimeter where the shear strength is? Maybe unless your arrangement was such you could argue that structurally you had two beams and the concrete in between was just fill? I dunno.

 

[oneintheeye]

No I wasn't convinced either to be honest. seemed like a fudge at best. The problem I've got is due to what is going on the structure will definatly be loaded as I am analysing for. Its not a theoretical live load like is applied to normal floors and beams.

 

[Ussuri]

So:

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.

 

[ishvaaag]

Anyway a width of 1.5 m without inner shear reinforcement seems too much. So your intention of looking at it as not acceptable sounds right. The analogy of Mörsch is better understood applied regularly on the mass of the beam. Otherwise you will need to ensure that through transverse strut and tie action (because if I would put deep-beam action reinforcement along the web -here inner reinforcement- is normally also understood to be required) where the tie is the inferior horizontal leg of the stirrup the tributary load is properly transferred to the stirrups proper etc. Also, it might be a strut and tie model at the sloped 45 or 30 deg of the compression strut, or whatever some truss analyses may say is adequate. It might work and still not meet the code.

 

[hokie66]

How deep is your 1500 wide beam? If it is a wide, flat beam, you should be able to use the one way slab provisions rather than the beam provisions. We often use "band beams" which require little shear reinforcement, and the ligs provided are only for support of the top steel.

 

[BAretired]

If the beam is satisfactory for current loading with only one stirrup, then the shear stress must be very low, almost to the point where you do not require stirrups.

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