] 3.1 General
This Section contains a substantial amount of explanatory material—far. more than is commonly provided in Part 1 of the Code. Additional information is therefore only
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required for relatively few clauses.There is. however, one general point about serviceability calculations which, even though it is touched on in the Code, needs reiterating. By their nature, serviceability
.4~] calculations cannot be accurate. This arises from our inability to predict the properties of concrete which influence the deformations of the structure. Some of the problems
.7 with prediction of these properties will be considered below.
(1) Tensile strength
The deformation under load, particularly of lightly reinforced members, is critically affected by the tensile strength of the concrete. This is illustrated schematically in Figure H(2)3.1. Normally, the information about the concrete available to the person doing the calculation is just its compressive strength. There is no reliable, unique relationship between the compressive and tensile strength of concrete. Unless a great deal more is known about the concrete than just its cube strength. it is doubtful if the tensile strength can be estimated to a better accuracy than ±30%
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LOAOService load Range of 1-~
uncertainty in cracking load Li
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(2) Behaviour of concrete in the tension zone after cracking
LI After cracking, the concrete between the cracks continues to carry some tension
and therefore this concrete adds significantly to the overall stiffness. The amount of this contribution is very variable and cannot be predicted with precision.
] (3) Creep and shrinkage
These characteristics are not known with any precision in normal circumstances and yet contribute 50% or more to the total deformation. They will depend on the exact
j details of the mix used, the loading history and the environmental history. There
can be significant differences in behaviour between members cast in early summer compared with those cast in late autumn. Differences in formwork striking times and propping procedures could also have a substantial effect. The person attempting calculations is unlikely to be able to define any of these factors, but needs to put
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bounds on their effects.
The calculation methods used in this Section are based on simplified assumptions about section behaviour; however, in view of the major uncertainties discussed above,
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Figure H(2)3. 1.- Influence of uncertainty about tensile strength of concrete on deformation.
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Another feature of serviceability calculations is that they can be checked against the actual behaviour of the structure. Since calculation and reality are most unlikely to a~rcc.
this tends to undermine the confidence of the designer. Calculations cannot be expected to predict what the deflection or crack width will actually be: they can be used to set bounds on the likely values, and it is important that they are used in this way. so that positive practical action is taken by the designer to ensure serviceable and durable structures (see Section 7, regarding the required accuracy of calculations).
3.1.1 Introduction
3.1.2 Assumptions
3.2 Serviceability limit states
32.1 Excessive deflections due to vertical loads
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3.2.1.1 Appearance
The limit on sagging of span/250 follows the recommendations of a committee of the Institution of Structural Engineers on the testing of structures’3-’1. A survey of beams and slabs in Germany conducted by Mayer, where sag had given rise to complaints’3’
produced about 50 examples. The measured sag was less than span/250in only two of these examples. and span/300 was the smallest sag which gave offence-. The selected limit thus has some practical justification. When the designer can show that greater sag is unlikely to give rise to trouble, this limit might be increased to span/200. A limit to precamber is not given; ho~vever. a reasonable limit would seem to be about L/250. If greater precambers are needed, then the structure must be a verv flexible one and could give rise to problems due to general liveliness’.
3.2.1.2 Damage to non-structural elements
The basic problem is that partitions, since they tend to be vertical, are very stiff and generally cannot follow the deflection of the floor or beam which supports them. If the partition possesses a reasonable degree of tensile strength. the floor can deflect away from the partition, leaving a gap between the partition and the floor of much the same magnitude as the deflection. This, of course, can be hidden by skirtings or similar details.
Unfortunately, with permanent partitions (e.g. blockwork), the bottoms of the partitions
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Figure H(2)3. 2.- Partition wall damage— cracks between to a self-supporting wall.
wall and floor due
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Figure H(2)3.3: Partition it-all damage— cracks at joints between wall and ceiling and towards exterior wall due to rotation or movement of individual wall panels.
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Figure H(2)3.4: Partition it-all damage — inclined cracks due to shear.
Figure H(2)3.S: Partition ‘tall damage —t-ertical cracking due to f7exure.
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Figure H(2)3.6: Partition wall damage— n’pes of damage related to different structural configurations.
are frequently held down by the floor screeds. In this case. the bottom of the partition is pulled down and cracks appear in the body of the wall. These can be large and unsightly. The presence of openings (e.g. doors, windows) in partitions tend to produce weaknesses and will often act as crack initiators. A paper by Clarke, Neville and Houghton-Evans~33~ gives illustrations of the type of damage that can occur in particular cases and the relevant Figures are reproduced here (H(2)3.2—H(2)3.6).
Masonry is generally fairly brittle and it seems clear that, if a floor or beam below a masonry partition deflects. then that deflection will be accommodated very largely by cracks (i.e. 10mm of deflection over the length of a wall is going to produce a total crack width of the order of 10mm somewhere). While the accommodation of vertical deflection by horizontal cracking can produce large cracks, the cantilever example shown in Figure H(2)3.5 is capable of producing quite startlingly large cracks even where the deflection is relatively small.
Figure H(2)3.7, taken from reference 3.4, shows the damage produced in model walls where various forms of deformation are imposed on the lower edges of the walls. It can
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Figure H(2)3. 7: Cracking of model walls due to sagging or hogging (from Ref.
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Part 2: Section 3Ibe seen that hogging deformations cause much greater disruption than sagging deformations.
This concludes the discussion of the types of damage that can commonly occur. It is 1 far from complete but 2ives a general idea of the general forms of damage which canbe met. The next question to consider is the limitations to deflections required to keep
damage within acceptable limits.
Much of the work done on damage to partitions has been concerned with allowable 1 settlement of foundations rather than the deflection of members supporting walls:
ne~’ertheless. the data should have some relevance.
One of the earliest investigations of this problem was that by Skemptont3-5~. He related 1
damage to angular distortion (Figure H(2)3.S).
angular distortion = L
Figure H(2,l3.8 Skempton ‘s definition ofangular distortion
He concluded that cSIL should be limited to £/300. This would appear to be more equivalent to a limit to mid-span deflection of a beam of L/600.
Mayer. who collected information from buildings in which deflections had caused complaint, reported the results shown in Figure H(2)3.9. The deflections occurring after construction of the partitions could not. of course, be measured and are therefore estimated values. It will be seen that these results, obtained from studies of deflection problems in buildings. are not inconsistent with the results obtained from considerations of damage due to foundation settlement. Both sources suggest that deflections would have to be limited to around £11000 if damage is to be avoided with any certainty.
It must be concluded that it is impossible to give universally applicable limits for allowable deflections and the designer should really establish limits appropriate to the particular structure and type of partition. The values given in this clause are those given in the ISO standard ISO 4356-1977 which has been approved by the UK, but the point is made that the values are only indicative.
3.2.1.3 Construction lack offit 3.2.1.4 Loss of performance
A cracks between wall and support o cracks within wall
C bending and shear cracks
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Figure H(2 13.9: Damage to partitions as a function of calculated deflection of supporting srructure (Ref 3.2,.
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3.2.2 Excessive response to wind loads 3.2.2.1 Discomfort or alarm to occupants 3.22.2 Damage to non-structural elements 3.23 Excessive vibration
Research has been carried out on the response of humans to vibration. For further information on this, for example, see reference 3.6.
3.2.4 Excessive cracking 3.2.4.1 Appearance
Clearly the width of crack which will be acceptable is very dependen on the particular circumstances and a code could not possibly give more than a guide. Factors likely to influence acceptable crack widths are:
(a) surface texture of concrete (b) distance of observers from surface
(c) exposure conditions (in surfaces exposed to weather, cracks can become accentuated
by dirt and by exudations of calcium carbonate)
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(d) aesthetic importance of element (a crack in. say. the entrance lobby of a prestige office block is likely to lead to more complaints than the same crack in, say, a pigsty)
3.2.4.2 Corrosion
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In recent years a considerable effort has been put into establishing what relationship, if any, exists between crack width and corrosion~3-71. The general conclusion from cuch
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studies is that small cracks (say less than 0.5mm) very rarely pose any particular corrosionU~
risk. whatever the nature of the eiivironment. However, very few studies have been carried out in circumstances where the cracks follow the line of a reinforcing bar rather than crossing it. In the absence of reliable information on this question, it therefore seems prudent to limit widths to about 0.3mm.
3.2.4.3 Loss of performance
Leakage is probably the commonest manifestation of loss of performance caused by cracking. Investigations of leakage through cracks have been carried outt38~ but have not been entirely conclusive. It seems probable that cracks passing right through a section
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with widths less than 0.2mm will fairly rapidly seal themselves and would therefore not Eu
cause significant loss of water in a water-retaining structure.
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3.3 Loads
3.3.1 General
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3.3.2 Dead loads
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3.3.3 Live loads