Composite construction

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AL-NAHRAIN UNIVERSITY COLLEGE OF ENGINEERING CIVIL ENGINEERING DEPARTMENT ADVANCE STRUCTURAL DESIGN Ι Composite Construction Prepared By: Hashim Dheyaa

Transcript of Composite construction

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AL-NAHRAIN UNIVERSITYCOLLEGE OF ENGINEERINGCIVIL ENGINEERING DEPARTMENTADVANCE STRUCTURAL DESIGN Ι

Composite Construction

Prepared By:Hashim Dheyaa

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Composite Construction1. INTRODUCTIONComposite construction, as defined herein, is the use of a cast-in-place concrete slab placed upon and interconnected to a prefabricated beam (Fig. 1) so that the combined beam and slab will act together as a unit. The prefabricated beam may be a rolled or built-up steel shape, a precast reinforced concrete beam, a prestressed concrete beam, a timber beam, or even light-gauge steel decking. The interconnection to obtain the single unit action is by combinations of mechanical shear connectors, friction, and shear keys.In the early 1900s, a type of composite construction was used where a steel I-shaped section was fully encased in concrete placed integrally with the slab. The use of encased beams is still permitted but such use is rare. The composite beam and slab con struction presently used began to appear in the 1930s. Since about 1940, nearly all usage has been with a slab attached to one flange of a prefabricated beam by means of me chanical connectors. This type of composite construction has been widespread in bridge design since the early 1950s and in buildings since about 1960. Present design methods are the result of extensive research into composite section behavior.

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Figure 1 (Concrete slab and prefabricated beam.)

Figure2 (Comparison of deflected beams with and without composite action.)

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2. COMPOSITE ACTION Consider a concrete slab atop the flange of a steel or precast concrete beam as shown in Fig.1. First, if the system of slab and beam is not acting compositely, only friction, will provide interaction; thus little of the longitudinal action is carried by the slab. The static system with friction neglected is shown in Fig. 2. (a), wherein the slab and the beam each carry separately a portion of the load. When the noncomposite system deforms under vertical load, the lower surface of the slab is in tension and elongates while the upper surface of the beam is in compression and shortens. Thus a discontinuity will occur at the plane of contact. Since friction is neglected, only vertical internal forces act between the slab and beam.

When a system acts compositely [Fig. 2, (b)], no relative slip occurs between the slab and the beam. Horizontal forces (shears) are developed which would shorten the lower surface of the slab and elongate the upper surface of the beam. Thus the discontinuity at the contact surface may be eliminated when sufficiently large horizontal shear resistance can develop. It is noted that the deflection of the composite system will be significantly less than that of the noncomposite system.

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3.ADVANTAGES AND DISADVANTAGES OF COMPOSITE CONSTRUCTION. The significant feature of a composite system is a stiffer and stronger structure than can be obtained from the same beam and slab acting noncompositely. In general, the advantages over noncomposite construction are (1) smaller and shallower beams may be used, (2) longer spans are possible without deflection problems, (3) the toughness (impact capacity or energy absorption) is greatly increased, and (4) the overload capacity is substantially greater. Some of the factors that tend to weigh against this construction are (1) the cost of the connectors which offsets some of the saving in beam material, (2) the cost of placing the mechanical shear connectors, and (3) the erection and construction difficulties when the projecting connectors impede or prevent workmen from walking on the beams. Most indications are, however, in favor of designing for composite interaction wher ever a cast-in-place slab is used.

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4. EFFECTIVE SLAB WIDTH

A slab acting compositely with a beam behaves the same as in an ordinary reinforced concrete T-section. Referring to Fig.3 the variables that control the effective slab width are (1) the ratio of slab thickness to total beam depth, t/h, (2) the ratio of beam span to beam width, L/bw, (3) the ratio of beam span to beam spacing, L/bo, (4) the type of loading, and (5) Poissons ratio. Just as for the T-section the effective width bΕ is to be taken in accordance with ACI-8.10 as the smallest of the following for interior beams: (1) one-fourth of the beam span length, L/4, (2) center-to-center spacing bo of beams, and (3) beam web width bw plus 16 times the slab thickness t, i.e., (bw + 16 t) .

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5. COMPUTATION OF SECTION PROPERTIES The elastic section properties (area, moment of inertia, section modulus) of the composite section are needed for computation of actual working stresses and deflections under service loads. For such section properties, the transformed section concept is used to convert all areas of the composite section into an equivalent homogeneous member. When a steel beam is used, the concrete slab is converted into equivalent steel by using a slab width equal to bE /n, where n = Es/Ec, the ratio of the modulus of elasticity of the steel beam to that of the concrete slab. When the prefabricated beam is either reinforced or prestressed concrete, the 28-day compressive strength f' is frequently different for the beam and slab; thus Ec is different. In that case the slab may be converted into equivalent beam material by using a slab width of  

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6.SHEAR CONNECTION Strength Concept The shear connectors share equally in carrying the total compressive force developed in the concrete slab as the nominal moment strength Mn is approached. This means, referring to Fig. 4, that shear connection is required to transfer the compressive force developed at midspan to the prefabricated beam in the distance L/2, since no compressive force can exist in the concrete slab at the end of the span where zero moment exists. The compressive force at strength Mn to be accommodated could not exceed that which the concrete can carry,

or, if the tensile force below the bottom of the slab at strength Mn is less than Cmax , Thus, for individual connectors each having a strength qult when failure is imminent, the total number of connectors N required between the points of maximum and zero bending moment is Whichever is smaller.

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Figure 4 (shear force and bending moment of concrete composite section.)

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Connection of Slab to Beam

It can also be noted that the connection and the beam must provide the same nominal moment strength Mn. Under working loads, however, the beam resists dead load and live load, but the connecting may need to resist only the load coming on after the slab has acquired its strength (primarily the live load). If the connection is designed to carry only the live load, a higher factor of safety should be used. Approximately the same result is achieved if the connection is designed to carry dead load as well as live load with the usual safety provisions.

Several types of connectors are as follows:1.Reinforcing bar stirrups from the precast beam, fully anchored into the slab.2.Friction, or bond, in combination with vertical ties, for slab on precast reinforced or prestressed concrete beam. This type of shear connection is adequate for most of these cases. While friction, or bond, alone maybe sufficient, at least a minimum amount of vertical ties must be used (ACI-17.6) unless the contact surfaces are subject to a nominal stress Vu/(Фbvd) not greater than 80 psi, and “clean, free of laitance, and intentionally roughened.” The width bv is at the contact surface being investigated for horizontal shear (ACI-2.1). Minimum ties are to provide a sort of clamping action to prevent buckling of the concrete slab, which would suddenly break the bond.3. Shear keys, for all cases of concrete-to-concrete composite action where friction, or bond, is inadequate. Since these keys are acting very nearly in pure shear rather than in diagonal tension.

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7.SLAB ON PRECAST REINFORCED CONCRETE BEAM -STRENGTH DESIGN

To assure the composite action, the horizontal shear must be transferred across the contact surface. a horizontal shear nominal strength Vnh computed as:

Where ….eq.(1) vnh = nominal unit stress capable of being transmitted on contact surface. bv = width of cross-section at contact surface being investigated for horizontal shear. dc = distance from extreme compression fiber to centroid of tension reinforce ment, for the entire composite section; when determining nominal horizontal shear strength over prestressed concrete elements, dc is not to be taken less than 0.8h.

  The maximum values of vnh are as follows: 1. When the contact surface is roughened, clean and free of laitance, with no vertical ties used, max vnh = 80 psi

2. When the contact surface is clean and free of laitance, but not rough ened, and when vertical ties having a minimum area Avmin

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and are spaced S at not more than four times the slab thickness (i.e., the least dimension of the supported element), nor 24 in.

3. When the contact surface is roughened, clean, and free of laitance, and minimum ties as in (2.) are used, the maximum nominal stress vnh permitted is

where = 1.0 for normal-weight concrete, 0.85 for “sand-lightweight” concrete, and 0.75 for “all-lightweight” concrete. The quantity pv is the ratio of tie reinforcement area Av to area bvs of contact surface.

4.When the nominal stress vnh exceeding 500 psi is desired design for horizontal shear. Thus, the design requirement may be stated as

where V = total shear force at section due to factored loads = 0.75, strength reduction factor for shear Vnh = nominal strength computed according to Eq. 1  

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EXAMPLE: Design a composite slab on a simply supported precast reinforced concrete beam span of 24 ft. The spacing of beams is 8 ft center-to-center. The cast-in-place slab is 4 in. thick, and the live load to be carried is 200 psf. Use f'c (slab) = 3000 psi, f'c (precast beam) = 4000 psi, fy = 40,000 psi, and the strength method of the ACI Code.

 SOLUTION (a) Loads and solution procedure. As a preliminary to the actual solution, it is to be noted that the use of precast members speeds construction and the use of composite action reduces the required depth of the beam. To design a composite beam without using temporary shoring, the precast beam is first designed to carry its own weight plus the weight of freshly placed concrete. Of course, the noncomposite system must also carry temporary load due to workers, equipment, runways, and impact, plus the dead weight of forms.

The loads are:

Loads on the noncomposite precast beam,

 4-in. slab, (4/12) (0.15) (8) = 0.4 kip/ft

estimated beam weight = 0.2 kip/ft

dead load = 0.6 kip/ft

temporary load, 0.050(8) = 0.4 kip/ft

Load on the composite section,

live load, 0.200(8) = 1.6 laps/ft

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Temporary live and dead construction loads frequently are not included in the design of the precast noncomposite section, but rather the overload that may occur is accepted as a short-duration reduction in the factor of safety. When deflection is to be investigated, service load moments are needed; so they could be computed first, and then overload factors are applied. For permanent loads on the noncomposite section, MD = (0.6)(24)2 = 43.2 ft-kips For the live load on the composite section,  ML = (1.6)(24)2 = 115 ft-kips

(b) Moment on precast noncomposite section. Assume a desirable reinforcement ratio p about one-half the maximum permitted by (see Table 3.6.1 for ACI-10.3.5 maximum). Then the desired Rn is:

 

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The precast beam must be at least 14.4 in. deep (ACI-Table 9.5a) unless deflection is computed even if the member is not supporting or attached to construction likely to be damaged by excessive deflection. If h = 15 in., d ~ 12.5 in.; then

(c) Determine reinforcement for composite section.

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Assuming the neutral axis to be in the slab when nominal strength Mn is reached,

If a is typically somewhat less than t/2, say 1 to 2 in., two layers of steel will be required even if the beam width is increased. Try b = 12 in. and h = 15 in. for precast beam

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Try 6-#9 bars in two layers (As = 6.00 sq in.). A check by basic statics may be made as follows:

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(d) Investigate construction loads.

For the precast section,

Though no special safety requirements are given for temporary loads, it may be reasonable to use the dead load factor 1.2; thus

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(e) Investigate shear transfer. Compute the nominal shear stress vnh over the contact area,

The maximum vnh permitted by ACI-17.5.2.3, with ʎ = l.0 for normal-weight concrete, is

……eq.3

…okUse 6-#9 in two layers.

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Without the reinforcement term in the above equation, the computed nominal stress is only slightly over the 260 psi limit. Use minimum #3 ties and then check eq.3

Or but, in any case, not greater than 24 in. Check Eq. (3) using s = 12 in.

The computed maximum vnh of 297 psi does not exceed the upper bound of 500 psi (ACI-17.5.2.3) and the computed nominal stress (282 psi) does not exceed the limit (297 psi). Use of 12-in. tie spacing is acceptable. Use #3 ties @ 12 in.