Closed ties for edge beams 2

[asixth]

Hi guys,

I'm in the process of designing a two-way rc flat slab floor system with edge beams around the perimeter. The edge beams are 1200 x 550 (4' x 2' approx), I have designed my slab by equivalent frame assumming 100% of the design negative moment goes into the column strip at the edge columns. It was my intention to provide closed ties to account for any torsion the edge beam may encounter, however, a senior engineer has suggested that open ties be provided for construction purposes.

Is this wrong, should the edge beam be detailed with closed ties?

 

[jike]

I would use closed ties even though I have been practicing 37 years. I am an old dog but once in awhile I learn new tricks.

 

[frv]

In the US, torsion reinforcement MUST be by closed ties.

 

[dik]

Depending on the amount of torsion... you can use open ties as your engineer suggests with the edge dowels providing the closure required for torsional strength. For real torsion, this works less well <G>. Also, with torsion, you may require longitudinal 'face' bars.

Dik

 

[hokie66]

asixth,

Aren't you in Australia? AS3600, Section 8.3.4a is quite clear on where closed stirrups are required or not. I always insist on closed stirrups for spandrel beams. Builders will try you on for two part stirrups, but they should be used to being knocked back by now.

However, your 1200 x 550 beams are quite large, possibly for architectural reasons, and if the beams comply with 8.3.4(a), then you could use open stirrups.

 

[rapt]

asixth,

As long as your open ties and closers will fully develop around the section it is normally ok. This is defined in clause 8.3.8(b). But any laps would have to be a full tension lap in a tension zone. L bar closers would not work, they would have to be U bar closers.

Another way around it is to specify closed ties over a length of about .2 L either side of the column wnere the major torsion is and then open ties with closers elsewhere where torsion is low.

Torsion is significant in these beams so the maximum spacing should be the maximum torsion spacing, not normal shear reinforcement spacings limits.

Open ties are preferred by PT companies for tendon installation purposes but should not matter as mucch for RC beams.

 

[msquared48]

You know guys, it's nice to know that as Structural Engineers, if we ever get torqued, we can always tie one on...

Mike McCann
MMC Engineering

 

[hokie66]

Read also 8.2.12.4, and what Warner, R, H, & F say about that section. I don't think a full tension splice will necessarily comply, unless it is located in the right place.

 

[apsix]

As I understand it anchorage should be with hooks, cogs would only be acceptable if located within an adjacent slab depth. For an edge beam that's not possible on both sides of course.

I would take that as applicable to tension splices also.

For torsion ligs, an open tie would then only be acceptable if the U-bar closer legs are bent after placement to 135 deg. to form a hook.

 

[hokie66]

I don't think you can bend them after placement, but believe you are right about the requirement for hooks. And the hook would have to enclose a longitudinal bar. But fundamentally, you don't want to try to develop shear reinforcement near mid-depth of a rectangular beam, because that is where the shear stress is maximum.

I wonder about compliance with Clause 8.2.12.4(c), which requires the stirrup spacing to be multiplied by .8 where fitment hooks are in the tension zone, or negative moment areas. I think a lot of people are confused by these issues of anchorage of shear reinforcement.

 

[csd72]

If you are relying on the torsional capacity of the spanderel and it is above the threshold value then you should use closed ties.

 

[Lion06]

This seems more like compatibility torsion to me. I agree with CSD. If you want to take advantage of it for design of the slab, then you need closed ties. However, if you don't account for more restraint than is provided by the cracking torsion of the spandrel then it falls into compatibility torsion and you can probably get away with open ties.
That being said, we always provide closed ties for all beams, whether they are subjected to torsion or not.

 

[civilperson]

What is a "cog" with respect to rebar development?

 

[apsix]

From AS3600;
13.1.2.4 Development length of a bar with a standard hook
Where a bar ends in a standard hook complying with Clause 13.1.2.5, the tensile development length of that end of the bar, measured from the outside of the hook, shall be taken as 0.5Lsy.t or 0.5Lst as applicable.
13.1.2.5 Standard hooks
The standard hook referred to in Clause 13.1.2.4, shall be one of the following —
(a) a hook consisting of a 135° or 180° bend with a nominal internal diameter complying with Clause 19.2.3.2 plus a straight extension of 4db or 70 mm whichever is greater;
or
(b) a cog, consisting of a 90° bend with a nominal internal diameter complying with Clause 19.2.3.2 but not greater than 8db and having the same total length as required for a 180° hook of the same diameter bar.

 

[Artvegas]

I agree with hokie66. Anchorage of shear reinforcement as defined in AS 3600-2001 is confusing.

The previous AS 3600-1994 Clause 8.2.12.4 stated:
"A fitment hook which encloses a longitudinal bar shall be deemed to develop 0.67fsy.f in accordance with Clause 13.1.2.5."

This was revised in AS 3600-2001 to 1.0fsy.f by increasing the straight extension of fitment hooks by six bar diameters, from 4db to 10db, so that yield strength is developed in the bar at all points along the height of the beam stirrup.

For further details refer to page 51 from the 2008 ARC Reo Handbook, which can be downloaded at:

http://www.arcreo.com.au/content/news/docs/ARC_Reo_Handbook_08ed.pdf

 

[asixth]

Artvegas,

Thanks for the handbook, I haven't seen that one before. It is good to have more Australians on this forum.

 

[rapt]

My copy of AS3600 - 2001 says 50% development from either

a standard hook with a 135 or 180 degree bend about a nominal radius with a 4db straight dextension

or

a cog with a 90 degree bend and a 4db straight length

I acnnot find any reference to a 100% capacity or an extended straight length.

In fact, technically you cannot extend the straight length to increase the development length of a hook or cog unless you increase the diameter of the bend due to compression probelms in the bend, and this is not allowed as it reduces the bends developemnt capacity and the bar developemnt is then based on the straightened bar length with no bonus for the tight bend.

So your options are a standard cog/hook at 50% development or a large radius cog/hook with development calculated by normal bar development rules.

PS the arc reo handbook is NOT the code!

 

[Artvegas]

Rapt,

Yes 4db straight extension applies to "standard" hooks and cogs as per Cl 13.1.2.5. But anchorage of "beam" shear reinforcement is specially addressed in Cl 8.2.12.4 which:
1. calls for 10db straight extensions with 4db pins;
2. requires longitudinal bars to be placed in all bends with a diameter greater than the tie diameter; and
2. does not specify any minimum development lengths, or refer to Section 13.

This 10db straight extension requirement is addressed in technical literature produced by both main reo manufacturers in Australia (ie ARC and OneSteel). If you don't specify hook dimensions for beams, they will schedule 10db extensions.

To illustrate a potential problem with assuming 50% cog/hook development for ties, consider N12-200 beam stirrups in a 32MPa beam with say 25mm cover:
- Lsy.t = (1.0*2.4*500*110)/(((2*25)+12))*SQRT(32)) >= 25*k1*db gives you Lsy.t = 376mm and 0.5Lsy.t = 188mm.
- To develop fsy at mid-height of the beam would require a tie depth of 2*188 = 376mm.
- This would mean that the minimum total beam depth allowed would be 376+25+25 = 426mm.

So this would mean that N12 ties would not be effective in a 400mm deep band beam for example. R10 would be worse as Lsy.t = 40*10 = 400mm. You might run into the same problem if you tried to develop column ties to Section 13 clauses, not that I'm saying that column ties need to be fully developed to be effective.

But if you were allowed to use the 0.67fsy.f rule stated in the previous code, you would get a total beam depth of (0.67*376mm)+50mm = 252mm, which is more like it! Perhaps the longer beam shear hooks have more to do with earthquake requirements that bar development. But there must be some benefit to enclosing larger longitudinal bars within hooks/bends.

I note that OneSteel can produce closed ties for beams as small as 215mm x 215mm.

P.S. I never stated that the ARC handbook is the code. But at least they made some attempt to explain AS3600-2001. The authors didn't (no commentary).

 

[rapt]

ArtVegas,

OK, I was looking at the general clause.

I can only assume that (C) was added because (b) was unconservative when the hook is in the tension zone. THis is implied by the wording in (b). As I said I will follow up on this.

But you cannot use your logic that small beam depths are impossible to justify to then accept a certain rule. If a 12mm bar does not develop in a 215 deep beam, then it does not develop!

Also, shear reinforcement is not only required at mid-depth!

I will get back once I have found out the background.

PS and do not believe everything a manufacturer puts into print.