1. MÉTODO
1.1 Diseño
General
The property of a shear connector that is needed for design is a curve that relates longitudinal slip, δ, to shear force per connector, P, of the type shown in Fig. B.2. No reliable method has been found for deducing such curves from the results of tests on composite beams, mainly because bending resistance is insensitive to the degree of shear connection, as shown by curve CH in Fig. 6.4(a).
Clause B.2.2(1) Clause B.2.2(2)
Almost all the load-slip curves on which current practice is based were obtained from push tests, which were first standardized in the UK in 1965, in CP117: Part 1. A metricated version of this test is given in BS 5400: Part 5,82and referred to in clause 5.4.3 of BS 5950: Part 3.131 without comment on the need to modify it when profiled sheeting is present. This test has two variants, because the slab and reinforcement ‘should be either as given in [the code] ... or as in the beams for which the test is designed’. This distinction is maintained in EN 1994-1-1 (clause B.2.2(1)). A ‘standard’ specimen is specified in clause B.2.2(2) and Fig. B.1, and a specimen for ‘specific push tests’ is defined in more general terms in clause B.2.2(3). The principal differences between the ‘standard’ tests of EN 1994-1-1 and BS 5400 (the ‘BS test’) are summarized below, with reference to Fig. 11.1, and reasons for the changes are given.
The standard test is intended for use
where the shear connectors are used in T-beams with a concrete slab of uniform thickness, or with haunches complying with 6.6.5.4. … In other cases, specific push tests should be used.
It can be inferred from clause B.2.2(1) that separate ‘specific’ push tests should be done to determine the resistance of connectors in columns and in L-beams, which commonly occur at external walls of buildings and adjacent to large internal holes in floors. This is rarely, if ever, done, although a connector very close to a free edge of a slab is likely to be weaker and have less slip capacity than one in a T-beam.79 This problem can be avoided by appropriate detailing, and is the reason for the requirements of clauses 6.6.5.3 (on longitudinal splitting) and 6.6.5.4 (on the dimensions of haunches).
Push tests to clause B.2, compared with the BS test
Welded headed studs are the only type of shear connector for which large numbers of tests have been done in many countries, so all reported studies of push testing (e.g. see Johnson and Oehlers,67Stark and van Hove72and Oehlers120) are based on these tests. It has been
found that the results of the tests are widely scattered.76To obtain realistic characteristic values, it is necessary to separate inherent variability from that due to differences in the test specimens, the methods of casting and testing, and the ultimate tensile strength of the connectors.
The BS specimen was probably designed to give results at the lower edge of the band of uncertainty that existed 40 years ago, because it has very small slabs, prone to split longitudinally because the mild steel reinforcement is light and poorly anchored. It has connectors at only one level, which in effect prevents redistribution of load from one slab to the other92 and so gives the resistance of the weaker of the two pairs of connectors. The changes from this test are as follows:
(1) The slabs have the same thickness, but are larger (650 × 600 mm, cf. 460 × 300 mm).
This enables reinforcement to be better anchored, and so avoids low results due to splitting. The bond properties of the reinforcement are more important than the yield strength, which has little influence on the result. Limits are given in Fig. B.1.
(2) The transverse reinforcement is 10 high-yield ribbed bars per slab, instead of four mild steel bars of the same diameter, 10 mm, so the transverse stiffness provided by the bars is at least 2.5 times the previous value. In T-beams, the transverse restraint from the in-plane stiffness of the slab is greater than in a push specimen. The reinforcement is intended to simulate this restraint, not to reproduce the reinforcement provided in a beam.
(3) Shear connectors are placed at two levels in each slab. This enables redistribution of load to occur, so that the test gives the mean resistance of eight stud connectors, and better simulates the redistribution that occurs within the shear span of a beam.
(4) The flange of the steel section is wider (> 250 mm, cf. 146 mm), which enables wider block or angle connectors to be tested; and the lateral spacing of pairs of studs is standardized. The HE 260B section (Fig. B.1) is 260 × 260 mm, 93 kg/m.
Clause B.2.3(1) (5) Each concrete slab must be cast in the horizontal position, as it would be in practice
(clause B.2.3(1)). In the past, many specimens were cast with the slabs vertical, with the risk that the concrete just below the connectors would be poorly compacted.
Cover 15
150 150
35
35 Recess optional 30
150
150 100
150 260 150
200 200 200
180 180 180
Bedded in mortar or gypsum
Reinforcement: ribbed bars, ∆ 10 mm, resulting in a high bond with 450 £ fsk £ 550 N/mm2 Steel section: HE 260 B or 254 × 254 × 89 kg U.C.
250
250
600 100 P
Fig. 11.1. Test specimen for the standard push test (dimensions in mm)
Clause B.2.3(3)
Clause B.2.3(4) (6) Unlike the BS test, details of concrete curing are specified, in clauses B.2.3(3) and B.2.3(4).
(7) The strength of the concrete measured at the time the push test is done must satisfy
0.6 £ fcm/fck£ 0.8 (D11.1)
where fckis the specified strength in practice (clause B.2.3(5)). The corresponding rule for the BS test is
0.86 £ fcm/fcu£ 1.2
where fcuis ‘the cube strength of the concrete in the beams’. For both codes, the two strength tests must be done using the same type of specimen, cylinder or cube.
Condition (D11.1) is now explained. It is essentially fcm= 0.7fck
The resistance of a stud is usually found from equation (6.19):
PRd= 0.29αd2(fckEcm)0.5/γV
In the push test, fckis in effect replaced by 0.7fck. Then,
PRd= 0.29αd2(0.7fckEcm)0.5/γV= 0.29αd2(fckEcm)0.5/1.5 (D11.2) when γV= 1.25. This shows that the purpose of condition (D11.1) is to compensate for the use of a γVfactor of 1.25, lower than the value 1.5 normally used for concrete, and the likelihood that the quality of the concrete in the laboratory may be higher than on site.
Clause B.2.4(1) (8) The loading is cycled 25 times between 5 and 40% of the expected failure load (clause B.2.4(1)). The BS test does not require this. Stresses in concrete adjacent to shear connectors are so high that, even at 40% of the failure load, significant local cracking and inelastic behaviour could occur. This repeated loading ensures that if the connector tested is susceptible to progressive slip, this will become evident.
Clause B.2.4(3)
Clause B.2.4(4) (9) Longitudinal slip and transverse separation are measured (clauses B.2.4(3) and B.2.4(4)), to enable the characteristic slip and uplift to be determined, as explained below. The BS test does not require this.
Evaluation of results of push tests
Clause B.2.5(1)
Normally, three tests are conducted on nominally identical specimens to determine the characteristic resistance PRkfor concrete and connector material of specified strengths fck and fu, respectively. Let Pmbe the mean and Pminthe lowest of the three measured resistances per connector, and futbe the measured ultimate strength of the connector material. If all three results are within 10% of Pm, then, from clause B.2.5(1),
PRk= 0.9Pmin (D11.3)
Clause B.2.5(2) refers to Annex D of EN 1990 (Informative) for the procedure to be followed if the scatter of results exceeds the 10% limit.
A method to clause D.8 of EN 1990 for the deduction of a characteristic value from a small number of test results, which took no account of prior knowledge, would severely penalize a three-test series. It is necessary to rely also on the extensive past experience of push testing.
Clause D.8.4 is relevant. For three tests it sets the condition that all results must be within 10% of the mean, Pm. This appears in clause B.2.5(1). Clause D.8.4 then gives the characteristic resistance as a function of Pmand of Vr, ‘the maximum coefficient of variation observed in previous tests’, in which the ‘10% from the mean’ condition was satisfied.
Most of the previous results were from research programmes, with many different types of test specimen. The results for studs in profiled sheeting, for example, have been found to be samples from seven different statistical populations.76It has not been possible to establish the value of Vr. The method of clause B.2.5(1), of reducing the lowest of the three results by 10%, is mainly based on previous practice. It can be deduced from clause D.8.4 that for a set
of three results with the lowest 10% below the mean, the method of clause B.2 implies that Vr= 11%.
Clause B.2.5(1) gives a penalty that applies when fut> fu. This is appropriate where the resistance of a connector is governed by its own material, usually steel, but in practice the resistance of a connector can depend mainly on the strength of the concrete, especially where lightweight aggregate is used. The correction then seems over-conservative, because fuis limited to 500 N/mm2by clause 6.6.3.1(1), and the strength of the material can exceed 600 N/mm2for studs.
In the BS test, a ‘nominal’ strength Puis calculated from Pu= (fck/fc)Pmin
and, then, PRd= Pu/1.4
It so happens that 1.25/0.9 = 1.4, so from equation (D11.3) the two methods give a similar relationship between Pmin and PRd, except that the Eurocode result is corrected for the strength of the steel, and the BS result is corrected for the strength of the concrete. This is probably because the results of the BS test are rarely governed by the strength of the steel, as the slabs are so likely to split.
Clause B.2.5(3) Clause B.2.5(3) finds application for connectors such as blocks with hoops, where the
block resists most of the shear, and the hoop resists most of the uplift.
Clause B.2.5(4) The classification of a connector as ductile (clause 6.6.1.1(5)) depends on its characteristic
slip capacity, which is defined in clause B.2.5(4). From the definition of PRk(clause B.2.5(1)), all three test specimens will have reached a higher load, so the slips δuin Fig. B.2 are all taken from the falling branches of the load–slip curves. It follows that a push test should not be terminated as soon as the maximum load is reached.