Addendum to Technical Report 43

(ProQuest-CSA LLC: ... denotes formulae omitted.)

The ongoing use of Concrete Society Technical Report 43 Post-tenstoned concrete floors Design handbook (Second Edition) has highlighted certain areas where additional clarification is required. This note sets out to address this and to correct

JOHN CLARKE, THE CONCRETE SOCIETY

Clarifications

Allowable stresses in flat slabs without bonded reinforcement

Unlike Edition 1, Edition 2 gives only one value for the permissible stress, without bonded reinforcement, for the support and span locations of flat slabs (Tables 4 and 5). This is because the report requires bonded reinforcement in the support zone, thus the number given is only relevant to the span location.

Limitations to the use of Table 4

Table 4 has been developed assuming an equivalent frame analysis is appropriate. Therefore, it should only be used when:

* all loads are uniformly distributed

* there are no section changes that significantly affect the moment distribution, e.g. band beams.

In addition:

* the characteristic combination of loads should be used

* the stresses should not be exceeded without moving to the strip approach and therefore the limits in Table 5.

Allowable stresses in flat slabs using 'design strip'approach

Table 5 is intended to be used with finite element or grillage analysis design. It has been derived from limited current experience and is likely to give a more conservative design than using the crack width calculation method.

Using f^sub ctm,fl^

In accordance with the Eurocode, Edition 2 of TR 43 allows the direct mean tensile strength f^sub ctm^ to be replaced by the mean flexural tensile strength f^sub ctm,fl^ in some circumstances. This is only allowed when relaxation losses, shrinkage (including early-age thermal shrinkage) and creep effects are taken into account in the structural analysis. It is emphasised that it is not only the loss of prestress but also the tension forces due to restraint of these phenomena that need to be taken into account. A method for allowing for restraint forces, specifically for early-age thermal shrinkage, is given in Appendix H; a similar method may be used for other losses. Where significant restraint exists, this should always be taken into account but due to the difficulties in accurately predicting long-term movements in the structure, long-term restraint stiffnesses and the distribution of restraint forces within the structure, it is likely that the enhancement of f^sub ctm^ to f^sub ctm,fl^ will only be appropriate in relatively simple situations.

Designed flexural untensioned reinforcement

When using Clause 5.8.7 the following points should be noted:

* When checking stresses at the transfer condition, the value of f^sub ctm^ used should be based on f^sub ci^, the concrete strength at transfer.

* When checking stresses for the transfer condition, an enhancement in the allowable stress to 1.2f^sub ctm^ for bonded tendons is only applicable where the bonded tendons are in the tensile zone for the relevant transfer load case.

* In bullet point four, the area of any bonded tendons in the tensile zone may be deducted from the area of reinforcement calculated, providing that the tendons are placed at a spacing of three times the slab thickness or 500mm, whichever is the lesser.

Corrections

Page 25, lines 6 and 7; amend definition of 'Equivalent UDL' to read:

P ? total drape ? 8/L1^sup 2^

Page 35, section 5.8.7; amend third bullet point to read:

"support zones in all flat slabs (less areas of bonded tendons)"

Page 102, recommendation 2; amend text to read:

"The steady state acceleration response at a position i, for the n^sup th^ mode of frequency f^sub n^, at a given excitation frequency hf^sub p^, can be obtained from Equation G3 as follows:

... (G3)

Here, Hf^sub p^, is the harmonic excitation frequency (where f^sub p^ is the walking frequency and the harmonic number is h = 1, 2, 3 or 4) and M^sub n^ is modal mass of mode n. The harmonic excitation force of amplitude P^sub j,h^, is applied at location j (at which the mode shape amplitude is ?^sub j,n^). The mode shape amplitude ?^sub j,n^ is at location i at which point the response is to be calculated. DMF^sub n^ stands for dynamic magnification factor for harmonic response which, for the n^sup th^ mode, is given in Equation G4 as:

... (G4)

...

Page 102, recommendation 3; amend text of first two paragraphs to read:

"The steady state responses calculated using Equation G3 above will be relatively small for many walking rates, but when the frequency of a harmonic of the footfall rate is close to a natural frequency of the floor, then a larger resonant response will arise at that frequency. If there are several modes with closely spaced natural frequencies, a harmonic force in the region of these frequencies may induce nearresonance in each of these modes. In this case the combined response may be found using the complex number form of the standard steady state harmonic dynamic magnification factor DMF^sub n^.

... (G5)

This is required so that phase information between the contributions from various modes at each harmonic frequency hf^sub p^ is maintained as required to calculate the amplitude of the total harmonic response

...

at that excitation frequency."

Page 102; delete Equation G7 and replace with:

... (G7)

Page 103; delete Equation G14 and replace with:

... (G14)