AS3600 Compression Laps

[OzEng80]

AS3600 states the 20db is required to develop a bar in compression. It is also states that to achieve a lapped splice in compression (0.125fsy-22)db = 40db (for fsy=500) (32db if you have ligs) is required.

What is the basis of this difference? The commentary gives some history but not reasoning.

Also what are the scenarios where a bar is required to be developed in compression?

Do column starters bars need to be developed in compression and the vertical bars lapped accordingly (32db)? Or is this only required if fixity is assumed in the connections?

Thanks!

 

[sdz]

My understanding is that the starter bars would require 20xdia development length for compression but 40xdia for a lap splice.

Starter bars might require more if tension can develop.

 

[OzEng80]

sdz
Can you explain why the development length is different from the lap length? It seems illogical that you can have two bars side by side, fully developed (in compression) yet not be able to justify continuity (a lap). I would have thought that the concrete matrix that transfers the bar force would be more conducive to compressive transfer than tensile transfer and shorter laps would result...this is not the case (with fy=500MPa)

When do you require a compression lap? I have treated columns as 'fixed' with tensile laps of starters on the basis that the concrete takes the compression and the starters take the tension. Given that column design accounts for compression in the reinforcing, does that imply that fixed column connection requires a compression lap through the connection? This takes a fair bit of 'meat' given hooks/cogs do not contribute....

Thanks for your assistance.

 

[hokie66]

Whether a bar is in compression or tension is up to the designer to determine. Bars in tension require longer laps because the stress development can be adversely affected by flexural/tension cracking.

 

[csd72]

I believe he was referring to compression in both cases.

A bar in compression can transfer the compression to the concrete in all directions whereas when you have a lap you only have the bar on one side and therefore the stress transfer occurs over a smaller volume of concrete.

I hope this answers your question.

 

[hokie66]

Yes, sorry, I should have read the post more thoroughly. I agree that the intent of 13.1.3 and 13.2.4 are not well explained. As I understand it, the 20db applies to things like column starters in footings where there is no splice, and 10.7.4 requires lapped splices to be in accordance with 13.2.4. Probably for the reason given by csd72.

 

[OzEng80]

I need to restate that AS3600 generally results in compression ‘splices’ that are ‘longer’than tensile splices. The development length in compression is always less 20db<25k1db but not the ‘splice length'.

Say:

N16, N32, 40 cover
Lyst = 400mm (governed by the 25k1db minimum)
For the same bar
Lysc = (0.125x500 - 22)db = 648mm, even if ligs are present it is still 648x0.8 = 520mm

I am confused by:

- The fact that the compression development length does not equal the splice length (like in tension) – maybe compression development utilizes end bearing?

- The mechanism of load transfer between splice bars – isn’t it the same as for tension; or even better due to the concrete being ‘prestressed’ by the compression?

- When is a compression splice required?
It would make sense that if you were undertaking a splice mid-height of a column (member carrying compressive load in the reinforcing) you would provide the longer compression splice length to ensure ‘continuity’. So does that imply that to achieve a fixed/’continuous’ connection at the top or bottom of a column the longer compressive laps are required?

Your thoughts are appreciated.

Cheers

 

[hokie66]

OzEng80,

You have me confused now. I would suggest you go to the Steel Reinforcement Institute of Australia site and make an enquiry. Hopefully, they will put you straight, and if so, would appreciate if you would report back.

 

[csd72]

If you require the compression force to be transferred from one bar to the other then use the splice length, if you only require it to transer to the concrete then the development length may be appropriate.

 

[asixth]

OzEng80,

The development length of a bar in compression is shorter than the development length of a bar in tension because it does account for force transfer by end-bearing as you where indicating. It also accounts for the absence of cracking due to compressive stresses for the bond to transfer force more efficiently.

The minimum length of a lap for a compression splice is indeed approximately twice that of the development length of a bar in compression.

Why is the minimum length of a lap in a compression splice greater than the development length of a bar in compression - I don't know.

AS3600 may be accounting for the fact that the compression splice may indeed be required to transfer tensile forces under some load case.

It is an interesting scenario, I don't have my Warner or code commentaries with me right now, only my Nilson (written to ACI) which also indicates that the compression splice lap legnth is indeed greater than the development length in compression but does not give a reason why.

Also, I have always used a 600 tension lap length for a N16 bar.

 

[OzEng80]

Thanks asixth

The fact that the development length in compression utilizes end bearing must be the reason the 'splice length' is longer. End bearing forces obviously can't transfer though adjacent bars, so the transfer mechanism in a compression splice is limited to the bond action along the length. It also follows that given the compression splice is approx (2 x the comp. development length) the majority (if not all) of the load in the bar must be taken by end bearing.

When do you think a compression splice is required? I am particularly interested in your thoughts as to whether a compressive splice is required between starters and verticals for a column with a fixed base.

Cheers

 

[apsix]

I'm not an expert but I'll comment anyway:

"the majority (if not all) of the load in the bar must be taken by end bearing";
As the concrete stress is limited to 0.85f'c, compared to reinforcement yield of 500MPa, I can't see that end bearing capacity is that significant.

csd72's explanation of 17 Apr 08 seems plausible.

As to the requirement for compressive splices to the column starters it must depend on the level of stress in the bars I would think. If the reinforcement is not much more than nominal a shorter splice should suffice. I haven't checked whether AS3600 would allow a shorter splice.

 

[OzEng80]

Aspix

I agree that end bearing seems an inappropriate assumption for carrying the majority of load (the code would have to specify minimum cover requirements to prevent the bar ‘punching out’). I just can’t get my head around a tension splice < compression splice. Having all the compression load applied at the bottom of a bar was a convenient way to explain a lap length = 2 x development length (and also the ineffectiveness of hooks in compression).

It is interesting that there is no f’c parameter in the compression formulas.

As to the compression splice for columns question, I am referring to a hypothetical case when a column’s reinforcing and base fixity is required for strength. Do you provide a compression lap (> tension lap) for your starters/vertical bars?

Thanks for your assistance!

 

[asixth]

Apsix,

I don't think the level of stress in the reinforcement will allow for a shorter splice length. As a matter of fact, I'm reading through the reinforcment detailing handbook published by the CIA (Concrete Institute of Australia) and it states quite emphatically that all splices must be designed to carry the full design strength of the steel (fsy) in either tension or compression.

OzEng80,

I am always indifferent about modeling a pad footing as a fixed restraint because I cannot see how the pad footing would be able to prevent rotation.

Nevertheless in your situation I would employ a 40db tension splice because I am sure there is some lateral load case (wind or robustness) that would create a tensile stress at the base of the columns.

It is an interesting point you make about bars 'punching out' of the footing, I believe this is something that engineers should take into account when going through their calculations.

 

[OzEng80]

asixth

Cl 13.1.2.2 provides development lengths to develop less than yield strength???

The column fixity example is hypothetical - to provide a scenario similar to a splice location mid height of column where a compression splice would be required. I was after insight as to whether engineers provide compression splices (which are longer than tension splices) for column connections.

Note that 40db is the typical ‘upperbound’ for a compression splice, tension splices are almost always less than this.

Thanks for your response.

 

[hokie66]

OzEng80,

The large office I work for uses the 40db for column bar laps. I note that Smorgon Steel's reinforcing handbook rounds 40.5 up to 41db. As to whether the compressive laps we specify exceed the tension laps, that doesn't seem to be consistent on our drawings, and is something I am looking into. Sometimes they are specified longer than 40db, but I am not sure why at this stage, but my guess at the moment is to allow for inaccuracy in placement in horizontal elements, which would not be much of an issue in columns.

Would be interested in hearing back if you can get a definitive answer to your query.

 

[OzEng80]

Aussies,

Below is the remarkably detailed and insightful SRIA response to my question (thanks for the tip hokie - had no idea SRIA could provide such a great response!). The quoted attachment wasn't provided - i will pass it on when i recieve it.



I have some code related questions regarding compressive development/splice
lengths and their application.

‘The commentary to AS 3600–2001, i.e. AS 3600 Supplement 1–1994, contains some brief background material about the code clauses, noting that some clause numbers have changed. You should use these documents along with their latest amendments, although as explained below, both of these documents still need some upgrading to cater for what are now standard 500 MPa N bars (400Y bars are superseded).’

AS3600 generally results in compression 'splices' that are longer than
tensile splices. The development length in compression is always less
20db<25k1db but not the 'splice length'.

Eg:

N16, N32, 40 cover

Lyst = 400mm (governed by the 25k1db minimum)

‘Some changes to the equation in Clause 13.1.2.1(a) for calculating Lsy.t have been recommended, as explained in the ASEC 2008 Conference paper attached. The absolute minimum for Lsy.t should be increased from 25k1db to 29k1db on account of fsy=500 instead of 400 MPa.

The value you have calculated depends on the values of k1, k2 and 2a. Assuming say bottom bars in a beam (so k1=1.0 and k2=2.2), with a clear distance between adjacent bars (anchored or spliced, and all assumed to be developing stress) of at least 2xcover=80 mm, then Lsy.t=min(405, 29x1.0x16)=464 say 470 mm.

Based on these assumptions your calculation is correct, except that the minimum has been increased to 29k1db.

This value is a tensile development length provided the clear distance between adjacent anchoring bars is at least 80 mm, or a tensile splice length provided the clear distance between adjacent spliced bars is at least 80 mm, i.e. if both of these conditions apply, then the tensile development and splice lengths are the same value.

HOWEVER, if the cover were less, or the clear distance between adjacent bars developing stress were less, then much longer tensile development lengths can be calculated using AS 3600. The example you have chosen to consider provides for the minimum permitted length (which is currently wrongly specified in AS 3600, thus compounding the apparent anomaly you have raised). So you are not correct that AS3600 “generally” results in compressive splices that are longer than tensile splices, although this can occur in practice.’

For the same bar

Lysc = (0.125x500 - 22)db = 648mm, even if ligs are present it is still
648x0.8 = 520mm

‘As you know, this formula for Lsy.c is used for calculating the compressive splice length, which as you correctly calculate for 500 MPa N bars equals max (40.5db, 300 mm), or max( 32.4db, 300 mm) in compression members with suitable ties, thus exceeding the minimum compressive development length (also Lsy.c) of 20db. (Obviously the 40.5 db and 32.4db values would exceed 300 mm, for all practical bar diameters db (10 mm minimum made)).

In Warner, Rangan, Hall & Faulkes (see p.393 Bars in compression) it is stated that “The value of Lsy.c is less than that for tension, since the adverse effects of flexural cracks is absent.” This provides one explanation for why bars in compression can possibly be anchored more rapidly.

In McGregor and Wight (Reinforced Concrete Mechanics and Design, 4th Ed. – see p. 341 Compression-Development Lengths) it is stated that “Compression-development lengths are considerably shorter than tension-development lengths, because (as you suggest below) some force is transferred to the concrete by the bearing at the end of the bar and because there are no cracks in such an anchorage region (like Warner et al. have reasoned)”.

However, even for the case you have cited, the minimum tensile splice length of 470 mm is now only slightly less than the value of 520 mm, appropriate to a normal beam or column with stirrups or ties.’


I am confused by:



- The fact that the compression development length does not equal the splice
length (like in tension) - does compression development utilizes end
bearing?

‘So, the answer to your question is Yes. ‘


- The mechanism of load transfer between splice bars - isn't it the same as
for tension; or even better due to the concrete being 'prestressed' by the
compression?

‘A wide range of failure mechanisms can occur that are too difficult to theoretically predict, so have been determined from testing. Some examples are shown in the paper attached for tensile bars being anchored or spliced. You could refer to some of the extensive literature on the subject, including finite element modelling, if you want to understand some of the complex (non-linear) longitudinal and local stress distributions that can arise, and the relative influence between end bearing and longitudinal bond for bars in compression. The American Concrete Institute web site is easily accessed for leading technical papers on the subject. Several good references are also found in McGregor & Wight.’


- When is a compression splice required?

It would make sense that if you were undertaking a splice mid-height of a
column (member carrying compressive load in the reinforcing) you would
provide the longer compression splice length to ensure 'continuity'. So does
that imply that to achieve a fixed/'continuous' connection at the top or
bottom of a column the longer compressive laps are required?

‘A useful discourse on this topic can be found on p. 507 of McGregor & Wight under Reinforcement Splices. They explain that “The requirements for lap splices vary to suit the state of stress in the bar at the ultimate load. In columns subjected to combined axial load and bending, tensile stresses may occur on one face of the column....”

Importantly, getting to the point you have raised, they go on to explain on p.508 that “Column-splice details are important to the designer ......, a compression lap splice will automatically be provided by the reinforcement detailer unless a different lap length is specified by the designer. Hence, if the bar stress at ultimate is tensile, compression lap splices may be inadequate, and the designer should compute and show laps required on the drawings.....”

This would not present the problem portrayed for the case you have examined. However, it could if the cover were less, the bars were closer together, the concrete compressive strength were less, etc. as these can all give rise to situations when tensile splice length Lsy.t exceeds compressive splice length Lsy.c (potentially by a large amount). (Please note in the paper attached that a lower bound has been placed on the term 2a to avoid excessively large values of Lsy.t from being computed using the formula in AS 3600-2001 when longitudinal bars (like can occur in heavily-reinforced columns) are closely spaced together.)’

 

[hokie66]

I thought SRIA would respond. And like we thought, it is complicated. I think that is why most offices conservatively specify splice lengths.

 

[OzEng80]

Paper attached - i think.

I would be interested to hear your thoughts on it...

 

[rapt]

OzEng80

There are changes to the tension splice lengths in the next version of AS3600 to make the splice length greater than the development length. This was also in earlier Australian codes and in most international codes.

Depending on who you talk to, there was supposed to be a difference between these in the current AS3600 also but people have been interpreting it differently to what was intended.

SRIA is opposing these changes but it looks like they will be over-ruled and the cha=nges get through and AS3600 will again be consistent with overseas codes. The paper you have attached is attempting to justify the status quo. SRIA are about the only people who agree with this.