Detailing of the shear connection and influence of execution

It is rarely possible to prove the general validity of application rules for detailing, because they apply to so great a variety of situations. They are based partly on previous practice. An adverse experience causes the relevant rule to be made more restrictive. In research, existing rules are often violated when test specimens are designed, in the hope that extensive good experience may enable existing rules to be relaxed.

Rules are often expressed in the form of limiting dimensions, even though most behaviour (excluding corrosion) is more influenced by ratios of dimensions than by a single value.

Minimum dimensions that would be appropriate for an unusually large structural member

could exceed those given in the code. Similarly, code maxima may be too large for use in a small member. Designers are unwise to follow detailing rules blindly, because no set of rules can be comprehensive.

Resistance to separation

Clause 6.6.5.1(1) The object of clause 6.6.5.1(1) on resistance to separation is to ensure that failure surfaces in

the concrete cannot pass above the connectors and below the reinforcement, intersecting neither. Tests have found that these surfaces may not be plane; the problem is three-dimensional. A longitudinal section through a possible failure surface ABC is shown in Fig.

6.17. The studs are at the maximum spacing allowed by clause 6.6.5.5(3).

Clause 6.6.5.1 defines only the highest level for the bottom reinforcement. Ideally, its longitudinal location relative to the studs should also be defined, because the objective is to prevent failure surfaces where the angle α (Fig. 6.17) is small. It is impracticable to specify a minimum angle α, or to link detailing rules for reinforcement with those for connectors.

The angle α obviously depends on the spacing of the bottom bars, assumed to be 800 mm in Fig. 6.17. What is the maximum permitted value for this spacing? The answer is quite complex.

Clause 6.6.6.3(1) refers to clause 9.2.2(5) in EN 1992-1-1, where a Note recommends a minimum reinforcement ratio. For fck= 30 N/mm²and fsk= 500 N/mm²this gives 0.088%, or 131 mm²/m for this 150 mm slab. However, if the slab is continuous across the beam, most of this could be in the top of the slab.

The minimum bottom reinforcement depends on whether the slab is continuous or not. If it is simply-supported on the beam, the bottom bars are ‘principal reinforcement’ to clause 9.3.1.1(3) of EN 1992-1-1, where a Note gives their maximum spacing as 400 mm for this example, and 450 mm for ‘secondary’ reinforcement. The rules of clause 6.6.5.3(2) on splitting may also apply.

If the slab is continuous over the beam, there may be no need for bottom flexural reinforcement to EN 1992-1-1. Let us assume that there is a single row of 19 mm studs with a design resistance, 91 kN per stud, which is possible in concrete of class C50/60 (Fig. 6.12).

The bottom transverse reinforcement will then be determined by the rules of clause 6.6.6.1 for shear surfaces of type b–b or c–c. These show that 12 mm bars at 750 mm spacing are sufficient. There appears to be no rule that requires closer spacing, but it would be prudent to treat these bars as ‘secondary flexural’ bars to EN 1992-1-1, not least because it increases the angle α. Using its maximum spacing, 450 mm, 10 mm bars are sufficient.

Cover to connectors

Clause 6.6.5.2 Shear connectors must project significantly above profiled steel sheeting, because of the

term (hsc/hp– 1) in equations (6.22) and (6.23) and the rule in clause 6.6.5.8(1) for projection of 2d above ‘the top of the steel deck’. The interpretation of this for profiles with a small additional top rib is not clear. If 2d is measured from this rib, and the rules of EN 1992-1-1 for cover are applied to the top of the connector, the resulting minimum thickness of a composite slab may govern its thickness. However, these slabs are normally used only in dry environments, where the concessions on minimum cover given in clause 6.6.5.2 are appropriate.

Fig. 6.17. Level of bottom transverse reinforcement (dimensions in mm)

Loading of shear connection during execution

Clause 6.6.5.2(4) As almost all relevant provisions on execution appear in standards for either steel or concrete structures, there is no section on execution in EN 1994-1-1. This is why clause 6.6.5.2(4) appears here.

The method of construction of composite beams and slabs (i.e. whether propped or unpropped), and also the sequence of concreting, affects stresses calculated by elastic theory, and the magnitude of deflections. It is significant, therefore, in verifications of resistance of cross-sections in Class 3 or 4, and in serviceability checks.

Where propped construction is used, it is usual to retain the props until the concrete has achieved a compressive strength of at least 75% of its design value. If this is not done, verifications at removal of props should be based on a reduced compressive strength.

Clause 6.6.5.2(4) gives a lower limit for this reduction in concrete strength. It also relates to the staged casting of a concrete flange for an unpropped composite beam, setting, in effect, a minimum time interval between successive stages of casting.

Local reinforcement in the slab Clause 6.6.5.3(1)

Clause 6.6.5.3(2)

Where shear connectors are close to a longitudinal edge of a concrete flange, use of U-bars is almost the only way of providing the full anchorage required by clause 6.6.5.3(1). The splitting referred to in clause 6.6.5.3(2) is a common mode of failure in push-test specimens with narrow slabs (e.g. 300 mm, which has long been the standard width in British codes). It was also found, in full-scale tests, to be the normal failure mode for composite L-beams constructed with precast slabs.⁷⁹Detailing rules are given in clause 6.6.5.3(2) for slabs where the edge distance e in Fig. 6.18 is less than 300 mm. The required area of bottom transverse reinforcement, Ab, per unit length of beam, should be found using clause 6.6.6. In the unhaunched slab shown in Fig. 6.18, failure surface b–b will be critical (unless the slab is very thick) because the shear on surface a–a is low in an L-beam with an asymmetrical concrete flange.

To ensure that the reinforcement is fully anchored to the left of the line a–a, it is recommended that U-bars be used. These can be in a horizontal plane or, where top reinforcement is needed, in a vertical plane.

No rules are given for the effectiveness as transverse reinforcement of profiled sheeting transverse to the supporting beams, with ribs that extend to a cantilever edge of the slab. The length needed to develop the full tensile resistance of the sheeting will be known from the design procedure for the composite slab. It is always greater than the minimum edge distance of 6d, and usually greater than 300 mm. Where it exceeds the length e available, bottom transverse reinforcement will be needed. The situation can be improved by placing all the connectors near the inner edge of the steel flange. Sheeting with ribs parallel to the free edge should not be assumed to resist longitudinal splitting of the slab.

b

b a

a

d e (≥ 6 d )

≥ 0.5 d

Fig. 6.18. Longitudinal shear reinforcement in an L-beam

Lying studs

There is also a risk of splitting where the shank of a stud (a ‘lying stud’) is parallel and close to a free surface of the slab, as shown, for example, in Fig. 6.19. The U-bars should then be in a vertical plane. Research has found⁷⁴that the ‘6d rule’ for edge distance in clause 6.6.5.3(2) and the provision of hoops may not be sufficient to ensure that the design shear resistance of the stud is reached. The height of the stud and the longitudinal bars shown are also important.

The slip capacity may be less than 6 mm, so the shear connection may not be ‘ductile’. The stud shown in Fig. 6.19(b) may be subjected to simultaneous axial tension and vertical and longitudinal shear, so details of this type should be avoided.

Lying studs are outside the scope of EN 1994-1-1. There is an informative annex in EN 1994-2.

Reinforcement at the end of a cantilever

Clause 6.6.5.3(3)P At the end of a composite cantilever, the force on the concrete from the connectors acts

towards the nearest edge of the slab. The effects of shrinkage and temperature can add further stresses³⁷that tend to cause splitting in region B in Fig. 6.20, so reinforcement in this region needs careful detailing. Clause 6.6.5.3(3)P can be satisfied by providing ‘herring-bone’

bottom reinforcement (ABC in Fig. 6.20) sufficient to anchor the force from the connectors into the slab, and to ensure that the longitudinal bars provided to resist that force are anchored beyond their intersection with ABC.

The situation where a column is supported at point B is particularly critical. Incompatibility between the vertical stiffnesses of the steel beam and the slab can cause local shear failure of the slab, even where the steel beam alone is locally strong enough to carry the column.⁸⁰ Haunches

Clause 6.6.5.4 The detailing rules of clause 6.6.5.4 are based on limited test evidence, but are

long-established.⁸¹ In regions of high longitudinal shear, deep haunches should be used with caution because there may be little warning of failure.

Haunches formed by profiled sheeting are considered under clause 6.6.4.1.

≥ 6d

≥ 6d

(a) (b)

Fig. 6.19. Examples of details susceptible to longitudinal splitting

Studs Steel beam

A

C

B

Transverse reinforcement not shown

Slab

Fig. 6.20. Reinforcement at the end of a cantilever

Maximum spacing of connectors

Situations where the stability of a concrete slab is ensured by its connection to a steel beam are unlikely to occur in buildings. The converse situation, stabilization of the steel flange, is of interest only where the steel compression flange is not already in Class 1, so it rarely arises where rolled I-sections are used.

Where the steel beam is a plate girder, there is unlikely to be any need for a top flange in Class 1. Its proportions can usually be chosen such that it is in Class 2, unless a wide thin flange is needed to avoid lateral buckling during construction.

Clause 6.6.5.5(2) Clause 6.6.5.5(2) is not restrictive in practice. As an example, a plate girder is considered, in steel with f_y= 355 N/mm², where the top flange has t_f= 20 mm, an overall breadth of 350 mm, and an outstand c of 165 mm. The ratio ε is 0.81 and the slenderness is

c/tfε = 165/(20 × 0.81) = 10.2

so, from Table 5.2 of EN 1993-1-1, the flange is in Class 3. From clause 6.6.5.5(2), it can be assumed to be in Class 1 if shear connectors are provided within 146 mm of each free edge, at a longitudinal spacing not exceeding 356 mm, for a solid slab.

The ratio 22 in this clause is based on the assumption that the steel flange cannot buckle towards the slab. Where there are transverse ribs (e.g. due to the use of profiled sheeting), the assumption may not be correct, so the ratio is reduced to 15. The maximum spacing in this example is then 243 mm.

The ratio 9 for edge distance, used in the formula 9t_f(235/f_y)^0.5 is the same as in BS 5400: Part 5,⁸² and the ratio 22 for longitudinal spacing is the same as the ratio for staggered rows given in BS 5400.

Clause 6.6.5.5(3) The maximum longitudinal spacing in buildings, given in clause 6.6.5.5(3), 6h_c, £ 800 mm, is more liberal than the general rule of BS 5950-3-1,³¹though that code permits a conditional increase to 8hc. The rule of EN 1994-1-1 is based on behaviour observed in tests, particularly those where composite slabs have studs in alternate ribs only. Some uplift then occurs at intermediate ribs. Spacing at 800 mm would in some situations allow shear connection only in every third rib, and there was a requirement in ENV 1994-1-1 for anchorage, but not necessarily shear connection, to be provided in every rib. It is expected that this requirement will be included in the forthcoming EN 1090 for execution of steel structures.

Detailing, for stud connectors Clause 6.6.5.6

Clause 6.6.5.7

The rules of clause 6.6.5.6 are intended to prevent local overstress of a steel flange near a shear connector and to avoid problems with stud welding. Application rules for minimum flange thickness are given in clause 6.6.5.7. Clauses 6.6.5.7(1) and 6.6.5.7(2) are concerned with resistance to uplift. Rules for resistance of studs, minimum cover and projection of studs above bottom reinforcement usually lead to the use of studs of height greater than 3d.

Clause 6.6.5.7(3) The limit 1.5 for the ratio d/t_fin clause 6.6.5.7(3) influences the design of shear connection for closed-top box girders in bridges. For buildings, the more liberal limit of clause 6.6.5.7(5) normally applies. It has appeared in several earlier codes.

Clause 6.6.5.7(4) In clause 6.6.5.7(4), the minimum lateral spacing of studs in ‘solid slabs’ has been reduced to 2.5d, compared with the 4d of BS 5950-3-1. Although connection to precast slabs is outside the scope of EN 1994-1-1, this facilitates the use of large precast slabs supported on the edges of the steel flanges, with projecting U-bars that loop over pairs of studs. There is much experience, validated by tests,⁸³of this form of construction, especially in multistorey car parks. Closely spaced pairs of studs must be well confined laterally. The term ‘solid slabs’

should therefore be understood to exclude haunches.

Clause 6.6.5.8 Further detailing rules for studs placed within troughs of profiled sheeting are given in clause 6.6.5.8. The first two concern resistance to uplift and compaction of concrete around studs.

Clause 6.6.5.8(3) Clause 6.6.5.8(3) relates to a common situation: where a small stiffening rib in sheeting

prevents the placing of studs centrally within a trough. It would be prudent to locate studs on the ‘favourable’ side (see Fig. 6.15(a)), which, for symmetrical loading on a simply-supported span, is the side nearest the nearer support. This was not given as an application rule because it is difficult to ensure that the ‘favourable’ side would be correctly chosen on site. Instead, alternate-side placing is recommended, with alternate studs on the ‘unfavourable’ side (see Fig. 6.15(c)). However, research has found⁷⁷that the mean strength of pairs of studs placed on the two sides of a trough is about 5% less than if both were central, with a greater reduction for sheeting less than 1 mm thick. Sheeting profiles with troughs wide enough to have off-centre stiffening ribs are more suitable for composite slabs than those with central ribs. This clause does not refer to layouts that require two studs per trough. These are discussed in comments on clause 6.6.4.2