In this article we will discuss about:- 1. Introduction to Composite Construction 2. Design Principles for Composite Construction 3. I.S. Recommendations.
Introduction to Composite Construction:
Composite construction consists of providing monolithic action between (i) Prefabricated units like steel beams, precast reinforced or prestressed concrete beams and (ii) Cast-in-situ concrete. This method is found to provide a greater structural efficiency compared with the conventional methods of construction.
We know, in the conventional method of the steel beam and slab construction, there is no composite action between the two and each component carries the entire load transmitted by the slab. It can be realized that if sufficient shear connection be provided between the prefabricated beam and the cast-in- situ slab, the two will act as one unit and the load will be resisted by composite action as in an R.C. tee beam.
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Only prefabricated or only cast-in-situ construction have their own advantages and disadvantages. But composite construction will have both the advantages of prefabricated and cast-in-situ construction. Composite construction has the added advantage that the prefabricated units can be used also to serve as form work for the cast-in-situ work.
There are two methods of assembly adopted in composite construction.
These are:
(i) The unpropped method, and
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(ii) The propped method.
(i) The Unpropped Method:
In the unpropped method, the prefabricated units are made strong enough to support the dead load of wet concrete and any constructional live load and load due to accidental form work.
(ii) The Propped Method:
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In the propped method, the prefabricated units remain supported on the props even during the laying as well as curing of cast-in-situ concrete. Hence, when the props are removed the whole unit i.e., prefabricated and the cast-in-situ unit, will act as a monolithic unit strong enough to carry the dead load and the live load.
Design Principles for Composite Construction:
Consider the case of a composite construction in which the prefabricated unit is a steel beam. The construction involves the three main elements; namely-
(i) The reinforced concrete slab.
(ii) The steel beam,
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(iii) The shear connectors connecting the steel beam and the R.C. slab.
Again consider the composite section and take moments about the neutral axis of the composite section.
I.S. Recommendations for Composite Construction:
The Indian standard code of practice for composite construction (IS: 3935) has made the following recommendations:
ADVERTISEMENTS:
(1) Composite Action:
For design purposes, in case the prefabricated unit is adequately supported before placing the in-situ concrete, it shall be designed to sustain self load only. If the load of the form work, constructional live load and the in-situ concrete, is carried directly by the prefabricated unit without adequate props, this additional load shall also be taken into account in addition to self-load.
The composite section shall be designed for all the loads imposed on the member taking note of the fact that the composite action of the member is effective only for the loads imposed after the composite action has started to function.
In prescribing the requirements of the I.S. code, full composite action has been assumed between the prefabricated member and the in-situ concrete. For such full composite action to be considered effective, the in-situ concrete shall have attained at least 75 percent of the designed 28 day strength of 150 mm cubes.
The composite action should preferably be proportioned in such a manner that the neutral axis of the composite section is generally located below the in-situ concrete slab.
If the neutral axis is located inside the in-situ concrete slab, the portion of the slab below the neutral axis shall not be considered effective for computing moments of inertia or resisting moments except for deflection calculations.
(2) Equivalent Section:
For prefabricated units in prestressed concrete or reinforced concrete, consideration shall be given to the different moduli of elasticity of the concrete of the precast and of the in-situ portions.
For prefabricated units in steel, the effective gross area of the concrete slab shall be converted into the corresponding equivalent area of steel. This shall be done by dividing the effective area of the concrete slab by the modular ratio.
(3) Differential Shrinkage and Creep of Concrete:
The effects of shrinkage and creep of the cast in-situ concrete on the prefabricated member shall be considered. It shall be ensured that the stresses in the prefabricated member do not exceed the safe stresses by more than 25 percent when these effects are superimposed on the stresses caused by the worst combination of other loads.
(4) Deflection:
Live load deflections. Live load deflections shall be calculated on the basis of the moment of inertia of the transformed composite section using the full value of the modulus of elasticity of the concrete.
Dead load deflections. For beams shored during construction the dead load deflections shall be calculated on the basis of the moment of inertia of the transformed composite section using one-half the value of the modulus of elasticity of concrete.
For beams not shored during construction the dead load deflection shall be calculated on the basis of the moment of inertia of the prefabricated beam alone except that deflections due to dead loads applied after the concrete slab has attained 75 per cent of the specified 28-day strength shall be calculated.
Steps such as giving a reverse camber to compensate for the full dead load plus half the live load deflections shall be taken in the design and construction in order to prevent excessive.
(a) Dishing of the slabs and beams built with shores,
(b) Thickening of slabs and beams built without shores, and
(c) Deflection of beams in services.
Limiting Deflections:
For simply supported beams the total deflection due to dead load, live load and impact should preferably not exceed 1/600 of the span, or the deflection due to live load and impact should preferably not exceed 1/800 of the span. The deflection of cantilever arms due to dead load, live load and impact shall not exceed 1/600 of the cantilever arms and due to live load and impact shall not exceed 1/400 of the cantilever arm.
(5) Design of Slabs:
In continuous spans, the effective span of the slab shall be taken as follows:
(a) Central distance between the outstand of supported flanges of the steel fabricated units. (Fig. 16.2).
(b) Clear distance between the webs of precast reinforced concrete or prestressed concrete units, reduced by two-thirds of the total thickness of the slab and flange of the precast at the face of the web. (Fig. 16.3).
(6) Flange width of Composite Beams:
Beams having Flanges on Both Sides:
The width of flange (slab) considered effective in the design of the composite beam except in the case of edge beam shall not exceed the least of the following:
(a) One-fourth the span of the beam,
(b) Distance, centre-to-centre of the beams, and
(c) The web (or rib) thickness plus twelve times the least thickness of the slab plus, in the case of slabs resting on wide flange (Fig. 16.3), two-thirds the total thickness of slab and the flange of the prefabricated unit at the face of the web (or rib).
For Edge Beams:
The effective flange width for inner and outer parts (measured from the centre line of the beam) to be taken in the case of an edge beam shall not exceed the following:-
(a) One-twelfth the span of the beam (for both the inside and outside parts);
(b) Half the distance to the adjoining beam (for the inner part) and the actual width (for the outer part); and
(c) Six times the least thickness of the slab plus half the web (or rib) thickness plus, in the case of slabs resting on wide flanges (Fig. 16.3), one-third the total thickness of the slab and the flange, at the face of the web (or rib) (for both the inside and the outside parts).
Allowance for Openings:
Any permanent openings which exist shall be deducted from the calculated width of the flange (slab) as the section under consideration. The loss of section due to the openings may, however, be compensated by other suitable provisions, such as trimmer beams, in which case the full flange width shall be taken into account.
(7) Prefabricated Steel and In-Situ Concrete Composite Members:
Steel structural members. The steel structural members may be of rolled steel joists or any other built-up sections. The structural members shall preferably by symmetrical about the vertical axis. The top flanges and web plates shall be able to absorb and transmit the force from the connectors. The minimum thickness of the free overhang shall not be less than one-tenth of the free overhang (Fig. 16.3) so that heavy distortion at the junction with the connectors does not occur.
With steel prefabricated units, the depth of steel section should preferably be not less than 1/90 of the span and the depth of the composite section should be not less than 1/95 of the span. If depths smaller than these are used, the section should be adequate to limit deflections to the values obtained with the limiting depth specified above.
Slab and Haunch:
The minimum thickness of the concrete above the steel structural member shall be not less than 100 mm and, therefore, haunches should necessarily be provided where thinner slabs are used. The slope of the haunches shall not be greater than one vertical to three horizontal for slabs thinner than 100 mm.
The depth of the haunch shall be chosen so that the composite structural member is not greater than one and half times the depth of the steel structural member and further the depth of the haunch shall not be greater than one and half times the thickness of the slab.
Where a supporting fillet is provided between the prefabricated steel section and the concrete slab, its section shall be ignored in computing the total composite section.
Shear Connectors:
In the case of connections between in-situ concrete and the prefabricated steel units, resistance to horizontal shear shall be provided by mechanical shear connectors at the junction of the concrete slab and the steel beam or girder.
The connectors shall be capable of resisting the shear force between the slab and the structural steel member and at the same time prevent the vertical separation of the slab from the structural steel member at the inner face. The shear connectors shall be of the type which permit a thorough compaction of concrete in order to ensure their entire surfaces are in contact with concrete.
The shear connectors shall be of weldable steel and shall be end-welded to the structural members. The capacity of the welds at permissible stress shall be not less than the shear resistance of the connectors.
Note:
In the case of studs, specialized fusion welding will be necessary and hence expert advice and necessary equipment should be available. To permit satisfactory welding of studs, the gap between the heads of two adjacent connectors should not be less than 15 mm.
Studs and channel shear connectors shall not be spaced further apart than 600 mm. The clear distance between the edge of a beam flange and edge of the connectors shall not be less than 25 mm. The concrete cover over the shear connectors in all the directions shall not be less than 25 mm.
In order to ensure that the concrete slab is sufficiently tied to the steel flange the overall height of the shear connector (that is, the length of stud, distance of the helix, height of the channel hoop etc.) should not be less than 50 mm nor project less than 25 mm into the compression zone of the concrete slab. The thickness of the compression zone shall be that at the section of maximum bending moment.
Types of Shear Connectors:
Shear connectors shall consist of any or a combination of the following types:
(a) Rigid Connectors:
These consist of short lengths of bars, angles or tees welded to the flange of the steel fabricated units (Fig. 16.5).
These connectors derive their resistance to horizontal shear from the bearing pressure of the concrete. Failure or slip is generally associated with the crushing of concrete. Some suitable means (anchors capable of preventing the separation on the in-situ concrete) from the prefabricated units in the direction perpendicular to the contact surface should be introduced with these connectors.
(b) Flexible Connectors:
Flexible connectors such as studs and channels welded to the contact surface of the prefabricated units are also used. These derive resistance entirely through the bending of the connectors.
(c) Bond or Anchorage Connectors Consisting of:
(i) Mild steel bars welded to the flange of the prefabricated unit in the form of vertical or inclined loop stirrups, or
(ii) Inclined bars with one end welded to the flange of the steel unit and the other end suitably bent, or
(iii) Bar stirrups welded to the flange of steel unit at each loop. These derive their resistance through bond and/or anchorage action.
(d) Any other mechanical device to resist horizontal shear and to prevent vertical separation of in-situ concrete from prefabricated unit.
Connectors, such as channels, tees and angles, more closely spaced with small faces are preferable to those with larger faces and widely placed, since the former arrangement induces a uniform distribution of shear stress in the concrete. The spacing of the connectors shall not exceed three times the thickness of the slab. Connectors should be as stiff as possible so that an even distribution of stress on the surface is achieved. Welding seams should be taken around the connectors as continuous.
Design Requirement of the Connectors:
The connection between the steel prefabricated unit and the in-situ concrete slab shall be checked for integral action of the composite structure at all loads such that-
(a) Shear along the contact surface is transferred without slip, and
(b) Separation of the prefabricated unit and the in-situ slab in a direction perpendicular to the slab is prevented.
Horizontal Shear Force:
The horizontal shear to be transferred by the shear connectors i.e., horizontal shear at the plane of contact of the prefabricated and in-situ unit shall be computed from the equation-
where, Sh = Horizontal shear per linear mm at the plane of contact of the in-situ concrete slab and the prefabricated beam at the cross-section of the composite beam under consideration.
V = The total external (vertical) shear due to the superimposed load acting on the composite section.
I = Moment of inertia of the transformed composite section.
ms = The static moment of the transformed area on the slab side of contact surface about the neutral axis of the composite section or the statical moment of area of reinforcement embedded in the concrete slab for negative moment.
Note 1:
For beams erected without temporary props the total external shear V is the total external shear from the live load and impact plus any shear from the dead load added after the concrete has attained a strength compatible to the composite action assumed above. For beams provided with properly designed props during construction, V is the external shear from dead load, live load and prop removal loads.
Note 2:
The compressive concrete area is transformed into an equivalent area of steel by dividing effective concrete area by the modular ratio m.
When negative (hogging) moments are to be resisted by the prefabricated section alone, shear connection between the prefabricated section and the slab need not be provided in the regions of negative bending moments.
When negative moments are to be resisted by the composite section, shear connection should be provided throughout the full length of the beam, but the concrete on the tension side of the neutral axis shall not be taken as effective except as a device to develop the full stress in the reinforcing steel embedded in.
(a) Rigid Connectors:
The safe shear resistance capacity of a rigid connector is given by the equation-
The spacing and size of the inside connectors shall satisfy the following requirements:
(i) The bearing pressure on the face of the connector should not exceed the permissible value
(ii) The longitudinal shear stress along the shearing surface between two successive connectors should not be greater than two and a half times the permissible shear stress for concrete. This condition shall be deemed to be satisfied, if-
2.5 b q = Sh
where, b = width of the steel flange of the rigid connector at surface of contact.
q = permissible shear stress measured as indirect tension in concrete.
Sh = the horizontal shear per linear mm at the plans of contact slab and the prefabricated beam at the cross-section of the composite beam under consideration.
(iii) The projected area along a slope of 1 in 5 from one rigid connector on to another should be at least three times the area of the face of the connector (Fig. 16.10).
The following precautions are necessary with rigid connectors:
(i) Angular or wedge-shaped placing of the connectors will tend to split the concrete slab and shall, therefore, be prohibited.
(ii) The area of the bearing face of the connectors shall not be less than one-fifth of the area to which the bearing force is transmitted.
(iii) As far as possible, rigid connectors shall be associated with anchors so that shear is resisted partly by bond of the concrete and partly by the bearing pressure of the concrete against the face of the inside connectors.
(b) Anchor Connector:
Anchor connectors are either used alone or are used in conjunction with rigid connectors.
The safe shear resistance of an anchor connector is given by the following equation:
The above expression for the shear resistance of the anchor bar is independent of the angle of inclination of the bar. But anchors may generally be either vertical or inclined at about 45°.
Spacing of anchors when used alone shall not be less than 0.7 times the depth of the slab and shall not be greater than two times the depth of the slab.
Spacing of anchors when associated with rigid connectors shall not be greater than two and a half times the depth of the slab. The minimum spacing of anchors associated with rigid connectors shall be governed by the design conditions of rigid connectors.
The anchors may be welded to the beam or the ends bent over and well placed completely around. The anchors shall be brought up to the top surface of the flange, then bent over and ends hooked. The bond length should be adequate to develop the full bond strength, as in accordance with the provisions of I.S. 456 with further provisions that the portion from the upper bend to the hook is atleast ten times the diameter of the bar.
(c) Flexible Connectors:
(i) Welded Steel Connector:
The safe shear resistance of welded connectors (Fig. 16.5) with minimum stud head diameter of d + 12 mm and stud head height of 12 mm and of steel of ultimate strength of 460 N/mm2 yield point of 350 N/mm2 and an elongation of 20 percent is given by the following equation:
(ii) Channel Flexible Connector:
In the case of channel connectors made of steel with minimum ultimate strength of 420 N/mm2 and an elongation of 21 percent, the safe shear resistance is given by the equation (see Fig. 16.5).
(iii) Spiral Connectors:
For spiral connectors the shear resistance shall be given by the equation-
The diameter of the spiral bars shall preferably be between 12 and 20 mm; smaller diameter bars may be used but care shall be exercised in welding as the welding heat is likely to impair the ductility of small bars. In all composite beams the spirals shall extend at least half way in the slab.
The ratio of the pitch of spiral to the diameter shall be between 0.5 and 4.0. The developed length of one spiral per pitch shall not be less than 20 times the diameter of the bar. From fabrication consideration, the spiral pitch shall be within the limits of 100 mm and 400 mm.
Spacing of Connectors:
The aggregate capacity of all connectors located at a transverse section of a beam shall be equal to the horizontal shear provided by the pitch. Thus, the required pitch or spacing parallel to the beam axis of the connectors may be determined by the equation.
End Shear Connectors:
At each end of a simply supported girder in composite construction in steel and concrete, end connectors shall be provided to counteract the effects of temperature, shrinkage and creep in addition to the external shear forces. The shear resistance of such an end connector shall have the following values unless otherwise determined by rational analysis.
Shear resistance of end connector-
where, M = maximum bending moment on the composite beam due to additional loads operating after the composite action has been effected.
ms = the static moment of the transformed area on the slab side of the contact surface about the neutral axis of the composite section or the statical moment of area of reinforcement embedded in concrete slab for negative moment.
The end shear connector shall consist of mechanical device having adequate shear resistance as calculated above. Such mechanical device may consist of cut piece of rolled steel with the bearing face directed towards the centre of the beam or it may consist of anchor bars spreading out into the slab away from the centre of the beam.
Prefabricated Prestressed or Reinforced Concrete and in Situ Concrete Composite Members:
Composite structures in which the in-situ concrete is assumed to act integrally with the precast beam shall be inter-connected to transfer the horizontal shear along the constant surfaces and to prevent the vertical separation of these units. Shear transfer shall be by shear bars, castellations and by bond. The units shall further be tied together by the extension of web reinforcement.
Ties:
Separation of the component elements in the direction perpendicular to the contact surface shall be prevented by ties adequately embedded on each side of the contact surface. The spacing of such ties shall not exceed four times the thickness of the slab or 600 mm whichever is less.
The minimum cross-sectional area of the ties, in each metre of the span shall not be less than 0.15 percent of the contact area or 130 sq. mm. All web reinforcements of the prefabricated unit shall be extended into the cast in-situ concrete.
In the case of either dynamically or statically loaded structures where the horizontal shear at the interface at ultimate load is less than ƒck/40 it is not necessary to provide vertical ties.
Bond Strength at the Interface:
The bond strength at the interface shall be checked for ultimate load. The interface shall always be made rough for effective bonding.
The ultimate values of the horizontal shear stress at the interface shall be calculated by using the formula-
If the calculated shear stresses are more than the values given under no slip condition in table below, it shall be taken that the slip has occurred. The design shear then be made taking a frictional shear resistance of 1 N/mm2 and the balance stress to be resisted by steel shear connectors stressed to a maximum of 130 N/mm2.
The interface shall not, however, exceed the value given under the maximum permissible shear stress in the table show:
Shear Bars:
The shear bars at the ends of the girders to a length of one-half to three-fourth depth of the girder shall be spaced closer and designed to take full shear force under ultimate conditions.
