Design and Detailing of Shear Studs in Composite Beams

INTRODUCTION

In many common applications of composite construction steel beams are made to act compositely with concrete slabs by means of shear connectors (see Figs 1, 2, and 3). In such applications, the concrete slab in the composite beams will be in compression, while the steel beam will be in tension. Thus, the advantages of concrete, which is strong in compression, and steel, which is better to resist tensile forces, are better utilized. Due to this, the required size of the steel section may be reduced when compared with a non-composite design. This saving in steel results in a considerable economy for bridges and high-rise buildings where composite construction is mainly used.

?In the early years of composite construction, many types of connectors were in use. Some of these include shear studs, channels (both considered flexible shear connectors and exhibit more flexibility and ductility before their failure), bars, angles, channels, and tee connectors with hooped bars for anchorage (considered rigid shear connectors). However, presently automatically-welded-headed studs are used extensively. These welding systems use a Capacitor Discharge (CD) Stud Welding process where an electric arc is established between the base of the weld stud and the workpiece creating a welded joint in a split second ( some examples are Studmaster?, Nelson?,?and Tucker??stud welding systems). These machines, operating at current settings of up to 2000 amps allow operators to weld nearly 1000 studs per day (ESDEP WG 10). The most advanced machines allow studs to be welded even through galvanized steel sheeting and eliminate the need to drill and make holes in the profiled steel sheeting (It has to be noted that the CD Stud Welding is generally used to weld smaller diameter studs (up to about 25 mm) to thin base metals(www.taylor-studwelding.com). A CD weld only penetrates about 0.1 mm, creating no burns or dimples on the reverse side).?This ability has enhanced the economic advantages of composite floor decks. An alternative to through deck welding is to punch holes in the steel deck and then weld the studs directly to the steel section. In this way, a more reliable weld may be obtained but this process is more complex and time-consuming.

?BEHAVIOUR OF SHEAR STUDS

In a simply supported composite beam (without any shear studs connecting the steel beam and concrete element), the steel and concrete elements will bend independently and the concrete fibers adjacent to the steel/concrete interface will tend to expand under flexure. However, the steel fibers adjacent to the interface would contract under flexure. If shear studs are provided at the interface, they will prevent vertical separation of the concrete and the steel element, and transmit longitudinal shear along the contact surface. The studs resist the longitudinal shear along the contact surface by acting as steel dowels embedded in concrete as shown in Fig.4. As shown in this figure, a slip occurs between the concrete component and steel component distorting the shear studs as shown in Fig.4, causing them to bear onto the concrete at the bottom portions of the stud. The head of the stud prevents the concrete component from separating from the steel component (Oehlers and Bradford, 1999). The transfer of the longitudinal shear by the dowel action of the stud shear connection exerts very high stresses onto the concrete surrounding the stud and to the steel flange of the beam, to which the stud is welded. For example, the concrete adjacent to the bearing zone has to resist bearing stresses in the range of 5.5 fck to 4.3 fck for concrete of strength M25 to M50 (Johnson, 2019, Oehlers and Bradford, 1999). This very high strength is possible only because the concrete bearing on the connector is restrained laterally by the surrounding concrete, its reinforcement, and the steel flange. The distributions of these stresses, which are local to the stud, are extremely complex. Hence, empirically derived detailing guidelines are prescribed in the design codes to ensure premature failure does not occur due to local stresses. Research has shown that the dowel strength is also dependent on the height of the weld collar, hwc, as the weld collar has a much larger cross-sectional area than the shank of the stud, and hence can resist a substantial amount of longitudinal shear (Oehlers and Bradford, 1999). The transition between shear failure and flexural failure will depend upon the ductility of the connectors. Beams formed with very brittle connectors will fail in shear even at very high degrees of connection.

Shear Connectors in Slabs Formed Using Profiled Steel Sheeting

The use of profiled steel sheeting in composite slabs has revolutionized the construction of office buildings, mainly due to the ease with which ‘Through Deck’ shear studs can be welded. However, when using this form of construction, there may be three possible causes for concern:

1. The quality of the weld. Modern welding equipment has been developed that can safely weld the stud through galvanized steel sheets of thicknesses up to 1.5mm.
2. The efficiency of the shear connector is reduced when the decking is oriented with ribs transverse to the beam. This is because the force transferred through the shear stud into the slab relies on a small localized area of concrete immediately in front of the slab. As shown in Fig. 2 and 3, for this orientation of the decking, this area of concrete is limited in size due to the profile of the sheeting. Hence, it is necessary to discount the area of concrete in the troughs when calculating section properties.

3.????The reduced volume of concrete around each shear connector also means that the connector is less well confined than when in a composite concrete slab. A reduced resistance and stiffness results. The reduction in resistance is dependent upon the size and shape of the profiled sheeting. As seen later in the paper, some reduction factor has been specified in codes, while calculating the stud resistance based on experimental research. It has to be noted that these formulae are generally empirical. Push-out tests which incorporate the particular geometry of profiled steel sheeting only provide a better estimation of the connector resistance in these situations.

DESIGN STRENGTH OF STUD SHEAR CONNECTORS

Ollgaard et al. (1971) proposed the first formula adopted by the AISC Manual 1993 to compute the shear strength of headed studs. Several research efforts have shown that the shear bearing capacity of studs depends on many factors, including the material and diameter of the stud and the properties of the surrounding concrete slab. Studs may reach their maximum load when the concrete surrounding them fails, but in stronger concrete, the studs may shear off (Johnson, 2019). These factors are considered and included in several design codes (e.g., EN 1994 1.1:2004, Draft IS 11384:2019, CSA S16:2009, AISC 360:2016, and NZS3404-1:2009).?

?Eurocode 4 Provisions

As per EN 1994 1.1:2004, and the draft IS 11384:2019, the design shear resistance is therefore given by the smaller of:

• Stud shear resistance (steel failure):

?????????????????Qrd = 0.8 fu Asc/Gamav????????????????????????????????????????????????????????????????????????????????(1)

Where, the shear resistance of the connector is related to the tensile strength of the steel fu (with fu ≤ 500), using a factor of 0.8;

• Concrete resistance (concrete failure):

??????????????Qrd =???0.29 Alpha d^2 Sq. root (fc Ec)/Gamav?????????????????????????????????????????????????????(2)

Where, Asc is the area of shear stud = (pi d^2/4), d is the diameter of the shank of the stud in mm (16 mm ≤ d ≥ 25 mm), fu?is the ultimate tensile strength of the stud material (≤ 500N/mm2), fc is the characteristic cylinder strength of concrete at the age considered (= 0.8 fck) with 20 ≤ fck ≤ 60, Ec?is the secant modulus of concrete.

The factor?a?is given by:

Alpha?= 0.2 [(hs /d) + 1]?????????????for 3?<hs/d?>?4???????????????????????????????????????????(2a)

Alpha= 1.0?????????????for hs/d > 4 ?????????????????????????????????????????????????????????????????????????????(2b)

Where, hs is the overall height of the stub in mm, and the partial safety factor?Gammav?is normally taken as 1.25.?

Composite Slabs Using Profiled Deck

As already discussed, when studs are welded in profiled sheeting with the ribs transverse to the supporting beams, the shear resistance is reduced. To account for this effect, the characteristic resistance, Qrs, is determined by multiplying the resistance of a stud embedded within a solid composite concrete slab by a reduction factor kt. It has to be noted that the draft Indian code IS 11384:2019 does not suggest any reduction factor but suggests that for composite slabs using a profiled deck, the strength of the shear connector should be established by experimental push-out tests.

In EN 1994 1.1:2004, different values of kt are given for profiled steel sheeting with ribs parallel to the supporting beam and for profiled sheeting with ribs transverse to the supporting beam. For profiled steel sheeting with ribs parallel to the supporting beam (see Fig. 2), the value of kt is given by

??????????kt =??0.6 (bo/hp) [(hs/hp)-1)]?????<1.0????????????????????????????????????????????????????????????????????????????(3)

Where bo is the mean width of a concrete rib (minimum width for re-entrant sheeting profiles). The overall height of the stud hs, should not be greater than hp + 75 mm.

For sheeting with ribs transverse to the supporting beam (see Fig. 3), the value of kt is given by

kt = c/(sq.rt.(nr)) (bo/hp) [(hs/hp)-1)]?????<1.0??????????????? (4)?????????????????????????????????????????????????????????????????????????????????

Where, where c is a calibration factor (in Eurocode 4 and NZS 3404.1, c = 0,7), nr is the number of stud connectors in one rib at the beam intersection, not to exceed two in calculation of the reduction factor kt, and of the longitudinal shear resistance of the connection. Eurocode 4 stipulates that the factor kt should not be taken greater than the appropriate value kt,max given in a Table in Eurocode 4.

Similar provisions are available in the 360:2016 (clause I8) and CSA 16-09 (Clause 17.7.2.2)?codes.

Instead of using these formulae, the designer may opt to obtain stud resistance values from tests. As full beam tests are expensive, a model test known as the "push-out" test is often used (see Johnson, 2019 for the details of this test).

DETAILING OF SHEAR STUDS

?Codes prescribe some rules for the detailing of shear studs. For example, EN 1994 1:1:2004 stipulates that the detailing of shear connectors should be such that concrete can be adequately compacted around the base of the connector. Other rules as per EN 1994 1:1:2004 and draft IS 11384 are given below (see also Figure 5):

1.?????For elements in tension and subjected to fatigue loading, the diameter of a welded stud should not exceed 1.5 times the thickness of the flange to which it is welded. This applies also to studs directly over the web. IS 11384 stipulates that the diameter should not exceed 2 times the flange plate thickness.

2.?????The overall height of a stud should be not less than 3d, where d is the diameter of the shank (as per IS 11384 it is 4d or 100 mm).

3.?????The head should have a diameter of not less than 1.5d and a depth of not than 0.4d.

4.?????The clear depth of concrete cover over the top of the shear stud should not be less than 25 mm.

5.?????The distance from the edge of the concrete flange to the center of the nearest stud should not be less than 6d, where d is the nominal diameter of the stud.?As per IS 11384, the horizontal clear concrete cover to any shear connector should not be less than 50 mm.

6.?????The slab thickness above the steel deck shall be not less than 50 mm.

Spacing of Shear Studs

1.?????Where a steel compression flange of steel beam, in spite of being semi-compact is assumed to be compact or plastic, based on the restraint?provided by the shear connectors, the center-to-center spacing of the shear connectors in the direction of compression should be not greater?than the following limits:

(i)????????????????where the slab is in contact over the full length (e.g., solid composite slab): ?21 tf Sq. rt(250/fy)

(ii)??????????????where the slab is not in contact over the ful1 length (e.g., slabs with ribs transverse to the beam): 15 tf Sq. rt(250/fy)

(iii)????????????In addition, the clear distance from the edge of a compression flange to the nearest line of shear connectors should be not greater than 9 tf Sq. rt(250/fy) ?or 50 mm whichever is less. where tf is the thickness of the flange; fy is the nominal yield strength of the flange in N/mm2

2.?????In buildings, the maximum longitudinal center-to-center spacing of shear connectors should be not greater than 6 times the total slab thickness nor 800 mm. (As per IS 11384, it is 3 times the total slab thickness nor 600 mm).

Note that uniform spacing of stud connectors makes detailing much easier but care needs to be taken if heavy concentrated loads are to be applied to the beam. In these cases, connectors should be spaced broadly in accordance with the shear flow along the beam. A minimum spacing of 600 mm for connectors is normally recommended to ensure that the shear flow along the beam is not too irregular.

3.?????The spacing of studs in the direction of the shear force should be not less than 5d (The minimum longitudinal stud spacing is specified as 6d by CSA S16:2009 as well as AISC 360:2016); the spacing in the direction transverse to the shear force should be not less than 2.5d in solid composite slabs and 4d in other cases. The minimum spacing specified by IS 11384 for shear studs: ≥ 75 mm.

4.?????Except when the studs are located directly over the web, the diameter of a welded stud should be not greater than 2.5 times the thickness of that part to which it is welded, unless proved by test results.

Headed Studs used in slabs with profiled steel sheeting in buildings (Eurocode 4)

(1)??The nominal height of a connector should extend not less than 2d above the top of the steel deck, where d is the diameter of the shank.

(2)??The minimum width of the troughs that are to be filled with concrete should be not less than 50 mm.

(3)??Where the sheeting is such that studs cannot be placed centrally within a trough, they should be placed alternately on the two sides of the trough, throughout the length of the span.

?Dimensions of the Steel Flange

?The dowel action of a stud shear connector imposes high-stress concentrations on the steel flange thickness, tf, also. Hence, the thickness of the steel plate or flange to which a connector is welded should be sufficient to allow proper welding and proper transfer of load from the connector to the plate without local failure or excessive deformation. Hence, codes recommend a flange thickness, tf ≥ 0.4 d and the distance between the edge of the connector and the edge of the flange Le ≥1.3 d, where d is the diameter of the stud (Eurocode) or 25 mm (IS 11384).

?SUMMARY AND CONCLUSIONS

Composite steel-concrete construction, in which the advantages of concrete and steel are utilized, will result in an economy. The shear studs perform an important role in such composite steel-concrete beams, by preventing vertical separation of the concrete and the steel element, and transmitting longitudinal shear along the contact surface. ?The behavior of the shear studs while resisting the shear flow is explained both with respect to composite slabs and composite slabs with profiled sheeting, which are often used for the advantage of eliminating the formwork. Although the draft IS 11384:2019 contains equations for the design of shear studs for composite slabs, these equations are not applicable to composite slabs using a profiled deck. Hence, the provisions available in other national codes are discussed. The rules for detailing shear studs based on empirical methods are also compared.

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References

1. ANSI/AISC 360-16 Specification for Structural Steel Buildings, American Institute for Steel Construction, Chicago, Illinois.
2. ?BS 5950-3.1: 1990. Structural Use of Steelwork in Buildings: Part 3: Section 3.1: Code of Practice for Design of Simple and Continuous Composite Beams, British Standards Institution: London, 1990
3. ?CSA S16-09 Design of Steel Structures, Canadian Standards Association, Ontario, Canada, Sept., 190 pp.
4. ?Draft IS 11384 :2019, Composite Construction in Structural Steel and Concrete-Code of Practice, Bureau of Indian Standards, New Delhi, 97 pp.
5. EN 1994-1-1:2004, Eurocode 4: Design of Composite Steel and Concrete Structures-Part 1-1: General Rules and Rules for Buildings, Comite Européen de Normalisation (CEN),. Brussels, Belgium.
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