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both of these documents still need some upgrading to cater for what are now standard 500 MPa N bars (400Y bars are superseded).’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.)’
