Assessment of strengths of sections of precast pre-tensioned units designed as continuous members

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On many occasions the support section will be designed as reinforced to carry hogging moments. In this situation. prestress may be ignored in the compression zone at the ultimate limit state. If no redistribution of moment is carried out, it is likely that at the serviceability limit state the compressive stresses are low. spailing will not occur andr creep will be low. Such a section will notneed a concrete stress check at the serviceability

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limit state. If compressive stresses are high. i.e. the transmission length may be short

compared with the span. some debonding of prestressing tendons may be necessary. In r any eventitis importantto recognise the problems discussed in the commentary to 5.2.6.1.

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5.4.6 Serviceability limit states 54.6.1 Serviceability

5.4.6J.1 General

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5.4.6.1.2 Prestressed precast units. The starting point in designing a prestressed concrete

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member will generally be the serviceability limit state involving the use of permissible compressive stresses as given in 4.3.4.2. Provided it can be shown that failure would be of the under-reinforced type, 5.4.6.1.2 allows an increase in these stresses of up to 50%;

this applies particularly to the situation where the prestressed unit is unpropped. This

means that the maximum compressive stress under all loads can be permitted to reach ^Lii a value of l).5f~~. As a substantial proportion of this stress will be due to permanent

loads, it is suggested that this enhanced stress should be used with caution and for

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particular situations where the detrimental effects due to possible excessive creep ~ilI

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be less severe. It has been common practice in bridge design in recent years to apply this enhancement allowance as shown in Figure H5 12. For those situations shown in Figure HS.12 (a). the full stress value of 0.5f~~ could be used; in Figure HS.12 (b) a25%

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in situ

in situ concrete concrete

-K -

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A A

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8 B precast

precast beam

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beam

ai maximum compressive siresa IbI maximum compressive Stress

in precast beam, at A-A in precast beam at A-A

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is 0.5 f.,~ is 042 f~

Figure 1-15. 12: Suggested enhanced compressive stress values in composite beams for

12 different forms of construction.

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Part I: SectionS increase of up to 0.42f~ is suggested. For other intermediate cases the level of stress allowed should depend mainly on the amount of lateral restraint provided to the compression flange of the precast beam by the in situ concrete.

5.4.6.2 Tension in in situ concrete

5.4.6.2.1 Prestressed precast units in direct contact with in situ concrete. The presence of a prestressed flange on the tension side of added concrete (as at B-B in Figure H5.12 (a)) considerably retards the formation of cracking of that concrete. The stress levels given in Table 5.4 should ensure that no significant flexural cracking due to the imposed loads will have occurred in the in situ concrete at level B-B in Figure HS.12 (a) and therefore the t~vo concretes should continue to act as one composite section. The tensile stresses given in Table 5.4 may be increased by up to 50% (provided that the permissible tensile stress in the prestressed concrete beam is reduced by the same numerical amount) and therefore more prestress is required; this recognises the fact that the greater the level of prestress at the contact surface. the greater is the apparent enhancement to the tensile strain capacity of the added concrete.

5.4.6.2.2 Prestressed precast units not in direct contact with in situ concrete.For the situation illustrated in Figure H5. 12 (b) the in situ concrete top flange should be treated strictly as a reinforced concrete section subjected to any local transverse bending and designed

in accordance with 3.4.7.

5.4.6.3 Tension in prestressed precast units

This is of relevance if the prestressed unit has thin exposed webs where cracks could be a visual or durability problem. So long as reinforcement is provided in the units or in the composite infill. tension in prestressed units is less of a problem in construction of

the type shown in Figure HS.12 (a).

Where continuity for live loading is achieved by placing reinforcement in the in situ concrete top flange and designing the support section to be reinforced, this will certainly induce tensile stresses in the top of the prestressed beams (A-A in Figure ff5.13) under the service loads and 5.4.6.3 suggests that these stresses be limited in accordance with 4.3.4. Even if a full allowance is made for transmission length and loss of prestress. this may be rather restrictive in design if straight tendons are used. It would not seem unreasonable. for the type of section in Figure H5.12 (a) only, to treat the ends of these prestressed units as being Class 3 under full service loading, as any cracking will be limited by the requirements of 3.4.7 for the support section and will be remote from the prestressing tendons; if this suggestion is adopted. it should apply only to a length on either side of the support centre-line approximately equal to twice the overall section depth at the supports. Another solution to this problem would be to taper the top surface of the prestressed unit towards the support.

5.4.6.4 Differential shrinkage

5.4.6.4.1 General. If an in situ concrete floor slab is cast on an older precast unit. the two concretes tend to shrink at different rates, because much of the creep and shrinkage

strain in the precast member will have taken place before the connection is made. The

~Kff~6~.Z~f Uuislsto_induce secotidary stresses in the composite section as a whole, the

~ K

rn~~nt,~~i4iicb is~1i~Wi~iih~iensde stress inducedii~ the bottom of the precast

centre-line

in situ~ concrete of continuity steel

support

---

- ~

precast beams

\4. positive connection required in this zone to guard against long-term effects

Figure H5. 13: continuity in composite construction.

i—-K,

Handbook to BS8I 101985

unit, which could be important if the section is designed right up to the limiting stresses

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in 4.3.4.3.

As suggested in 5.4.6.4.1 these effects are not generally of great significance in most

practical cases for simply-supported members. Theyare likely to be worse if the precast

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unit is prestressed. where the stress in the top fibre (i.e. at the interface) due to prestress

is near zero. becausethe greatest differential strain movement between the two concretes will occur in this circumstance. It is suggested that. even then, these effects require

investigation only if there is a difference of more than one strength grade between the

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two concretes andif the time interval between casting the precast unit and placing the in situ concrete is more than about S weeks. The t~-pe of composite cross-section most

susceptible to these effects is the composite T-beam illustrated in Figure 1-15.12 (bj.

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5.4.6.4.1 Calculation of tensile stress. A method for evaluating differential shrinkage effects is given in reference 5.26 which also gives some indication of how the differential

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shrinkage coefficient, referred to in5.4.6.4.2. can be evaluated for design purposes for

vanous types ot sectton. Further Intormation is given in reterence ~.27.

5.4.64.3 Approximate value of differential shrinkage coefficient for building in a normal

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environment. See above.

5.4.7 Ultimate limit state

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This section is a major departure in BS 8110 ~vhen compared with the previous method of shear connection design. The method of CPIIO was based on elastic section analysis and was thouizht appropriate therefore for the serviceabilirv loadings.

This new method involves the forces acting at the ultimate limit state as this is the appropriate limit state for the mechanism. The intention was not to make any change with respect to safety factors and this and the old CF 110 method were intended to produce similar results.

Reference 5.28 provides full details of the source of the new design method.

5.4.7.1 Horizontal shear force due to design ultimate loads

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The analytical method requires a different approach depending on the position of the compression zone relative to the plane under consideration. There may be occasions

C-where the plane under consideration is so low in the tension zone that 5.4.7.1 (a) is very conservative. In those cases, only the tension carried across the plane in shear needs to

be considered.

c

5.4.7.2 Average horizontal design shear stress

The definitions in Table 5.5are intended to represent real practical surfaces rather than idealised and unrepresentative surfaces mentioned in previous codes. The as cast or as extruded finishes are deliberately introduced to cover the finish produced by slip form or extrusion machines now commonly used to produce prestressed slab units.

In bridge construction the term “rough as cast~ is often used to describe surfaces at

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the top of the conventional cast where the vibrator is removed leaving a very rough surface with large aggregate particles on the surface. Where bridge beams with this finish are incorporated into buildings. the horizontal stresses from the roughest category in

Table 5.5 are appropriate.

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5.4.7.3 Nominal links

5.4.7.4 Links in excess of minimum

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5.4.7.5 Vertical shear

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5.4.7.5.1 General. Reinforced concrete composite members may be designed by using 3.4.5 and. as long as there is adequate longitudinal shear connection between the concretes. the gross section may be used in the design.

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The design of prestressed concrete compDsite members is a little more complicated

124

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Part]:Section .5

(ci (c) C S C

Figure HS. 14: Composite sections considered in designing for shear (5.4.7)^— (a) original member, (b) with composite infill. (c) with composite topping.

than the design of non-composite prestressed concrete members and the simplified methods in 4.3.8 do not necessarily apply.

The assessment of V~0, the shear capacity of a member uncracked in flexure. in 4.3.8 assumes that the member carries all the shear in its web and that the critical point of maximum principal tensile stress is at the centroid. In composite construction, it is ideally necessary to check all possible critical sections and to ensure that the principal tensile stress of all the structural concrete in the member is less than the permitted value of 0.24

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When in situ concrete is placed between precast prestressed concrete members, the precast concrete provides restraint to the infill and increases its capacity to carry tension. In these cases, therefore, it is generally considered satisfactory to check only the principal tensile stress in the precast concrete. For most practical cases, it will be found that the precast part of the composite section is capable of carrying all the ultimate shear load, and this is all that the Code requires. Further complete checks, on the composite section as a whole, will be required only if the ratio of imposed loading to dead loading is exceptionally high.

-The shear force at which flexure—shear cracks form. V~. may also be calculated by using 4.3.8. It is necessary to consider each composite section on its merits when deciding how much of it is resisting shear. Figure H5. 14 (a) shows an original precast prestressed member that is incorporated into a composite member in two ways: in Figure H5.14 (b) it has composite infill and in Figure H5.14 (c) it has composite topping. In (b), the infill concrete may crack before the original member and the post-cracking shear strength of the infill may not necessarily add to the gross shear strength of the member.

It is therefore wise to make some reduction in the infill concrete shear in calculating flexure—shear strength, the amount of the reduction depending on whether the infill is restrained between precast members or restrained by reinforcement cast into the precast member.

When the composite member has a structural topping. as in Figure 1-15.14 (c), the flexure-’shearcapacity, V~, may be calculated by using the gross section depth and the web width of the original member because. in this case. the additional concrete has been placed in an area where it can add to the shear strength of the member.

5.4.7.5.1 In situ concrete between precast prestressed units. See above.

5.4.8 Differential shrinkage between added concrete and precast members

In document BS8110 structure use of concrete (Page 118-121)