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Composite floor slab with channel shear connectors

Composite floor slab with channel shear connectors

Composite floor slab with spiral shear connectors beff

FIGURE 4.38 Composite floor slabs.

where fC = compressive strength of concrete Ac = effective area of the concrete slab = tcbeff tc = thickness of the concrete slab beff = effective width of the concrete slab = min (L/4, s) for an interior beam

= min (L/8 + distance from beam centerline to edge of slab, s/2 + distance from beam centerline to edge of slab) for an exterior beam L = beam span measured from center-to-center of supports s = spacing between centerline of adjacent beams As = cross-sectional area of the steel beam Fy = yield stress of the steel beam

Ar = area of reinforcing steel within the effective area of the concrete slab c

Fyr = yield stress of the reinforcing steel XlQn = sum of nominal shear strengths of the shear connectors

The nominal shear strength of a shear connector (used without a formed steel deck) is given by

For a stud shear connector:

For a channel shear connector:

where Asc is the cross-sectional area of the shear stud (in.2), /.' is the compressive strength of concrete (ksi), Ec is the modulus of elasticity of concrete (ksi), Fu is the minimum specified tensile strength of the stud shear connector (ksi), tf is the flange thickness of the channel shear connector (in.), tw is the web thickness of the channel shear connector (in.), and Lc is the length of the channel shear connector (in.).

If a formed steel deck is used, Qn must be reduced by a reduction factor. The reduction factor depends on whether the deck ribs are perpendicular or parallel to the steel beam. For deck ribs perpendicular to steel beam:

TNNTV h

When only a single stud is present in a rib perpendicular to the steel beam, the reduction factor expressed in Equation 4.143 shall not exceed 0.75.

For deck ribs parallel to steel beam:

The reduction factor expressed in Equation 4.144 is applicable only if (wr/hr) < 1.5. In the above equations, Nr is the number of stud connectors in one rib at a beam intersection, not to exceed three in computations regardless of the actual number of studs installed, wr is the average width of the concrete rib or haunch, hr is the nominal rib height, and Hs is the length of stud connector after welding, not to exceed the value hr + 3 in. (75 mm) in computations regardless of the actual length.

For full composite action, the number of connectors required between the maximum moment point and the zero moment point of the beam is given by

For partial composite action, the number of connectors required is governed by the condition f bMn > Mu, where f bMn is governed by the shear strength of the connectors.

The placement and spacing of the shear connectors should comply with the following guidelines:

• The shear connectors shall be uniformly spaced between the points of maximum moment and zero moment. However, the number of shear connectors placed between a concentrated load point and the nearest zero moment point must be sufficient to resist the factored moment Mu.

• Except for connectors installed in the ribs of formed steel decks, shear connectors shall have at least 1 in. (25.4 mm) of lateral concrete cover. The slab thickness above the formed steel deck shall not be less than 2 in. (50 mm).

• Unless located over the web, the diameter of shear studs must not exceed 2.5 times the thickness of the beam flange. For the formed steel deck, the diameter of stud shear connectors shall not exceed 4 in. (19 mm), and shall extend not less than 1j in. (38 mm) above the top of the steel deck.

• The longitudinal spacing of the studs should fall in the range six times the stud diameter to eight times the slab thickness if a solid slab is used or four times the stud diameter to eight times the slab thickness or 36 in. (915 mm), whichever is smaller, if a formed steel deck is used. Also, to resist uplift, the steel deck shall be anchored to all supporting members at a spacing not to exceed 18 in. (460 mm).

The design flexural strength fbMn of the composite beam with shear connectors is determined as follows:

In regions of positive moments: For hc/iw < 3.76/^/ (E/—f), fb = 0.85, Mn is the moment capacity determined using a plastic stress distribution assuming concrete crushes at a stress of 0.85f/ and steel yields at a stress of —y. If a portion of the concrete slab is in tension, the strength contribution of that portion of concrete is ignored. The determination of Mn using this method is very similar to the technique used for computing moment capacity of a reinforced concrete beam according to the ultimate strength method.

For hc/tw > 3.76/^/(E/Ff), f b = 0.90, Mn is the moment capacity determined using superposition of elastic stress, considering the effect of shoring. The determination of Mn using this method is quite similar to the technique used for computing the moment capacity of a reinforced concrete beam according to the working stress method.

In regions of negative moment: f bMn is to be determined for the steel section alone in accordance with the requirements discussed in Section 4.5.

To facilitate design, numerical values of fbMn for composite beams with shear studs in solid slabs are given in tabulated form in the AISC-LRFD Manual. Values of f bMn for composite beams with formed steel decks are given in a publication by the Steel Deck Institute (2001).

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