(Clause 10.2.2)
Material
Type of stress under design
Triangular or near triangular compressive stress distribution (e.g. due
to bending)
0.50fck 0.40fck
Uniform or near uniform compressive
Tension Not appli-cable
Deemed to be satisfied by 16.8.1
NOTE 1– The above stress limitations have been derived from 12.2 after making allowance according to Table 13 (see 12.4.2).
NOTE 2 – See 17.3.3 for limiting flexural stresses in joints for post-tensioned segmental construction.
10.3 Ultimate Limit States
10.3.1 Rupture or Instability – The assessment of the structure under design loads appropriate to this limit shall ensure that prior collapse of the structure does not take place as a result of rupture of one or more critical sections, buckling caused by elastic or plastic instability or overturning.
The effects of creep and shrinkage of concrete, temperature difference and differential settlement need not be considered at the ultimate limit state provided that these effects have been included in the appropriate load combinations to check the stress limitations given in 10.2.2.1 for the serviceability limit state.
10.4 Other Considerations
10.4.1 Deflections- The deflection of the structure or any part of it, shall not such as to affect adversely the appearance or efficiency of the structure.
10.4.1.1 The appearance and function of concrete superstructures are normally unaffected although calculations may be required in the following circumstances :
(a) where minimum specified clearances may be violated;
(b) where drainage difficulties might ensue;
(c) where method of construction may require careful control of profile, e.g. at discontinuities in serial construction, and where decks comprise abutting prestressed concrete beams.
10.4.2 Fatigue – The fatigue life shall comply with the requirements of 13.4.
10.4.3 Durability – The specifications in this code regarding drainage for the deck (see 15.2.2.1), concrete cover to the reinforcement (see 15.9.2) and acceptable crack widths (see 10.2.1) in association with the limits given in 5.4 are intended to meet the durability requirements of almost all bridge structures. Where more severe environments are encountered, however, additional precautions may be necessary, and specialist literature shall be referred to.
11. LOADS, LOAD COMBINATIONS AND PARTIAL LOAD FACTORS
11.1 Loads – The values of loads as given in IRS Bridge rules shall be taken as characteristic loads for the purpose of this code.
11.1.1 For design of concrete bridges of span 30m and larger, an appropriate temperature gradient shall be considered. In the absence of any data in this regard, depending on the environmental conditions, a linear gradient of temperature of 50C to 100C between the top and bottom fibres may be considered for design.
The effect of difference in temperature between outside and inside of box girders shall also be considered in design.
The values of the function YfL for the various loads are given in Table 12.
11.3.2 Serviceability Limit State – For the limitations given in 10.2.1, load combination 1 only shall be considered. For the stress limitations given in 10.2.2, load combinations 1 to 5 shall be considered.
The value of YfL for creep and shrinkage of concrete and prestress (including secondary effects in statically indeterminate structures) shall be taken as 1.0.
11.3.3 Ultimate Limit State – To check the provisions of 10.3 load combinations 1 to 4 shall be considered.
The value of YfL for the effects of shrinkage and, where relevant, of creep shall be taken as 1.2.
In calculating the resistance of members to vertical shear and torsion YfL for the prestressing force shall be taken as 1.15 where it adversely affects the resistance and 0.87 in other cases. In calculating secondary effects in statically indeterminate structures YfL for prestressing force may be taken as 1.0.
11.3.4 Deflection – Minimum specified clearances shall be maintained under the action of load combination 1.
The appearance and drainage characteristics of the structure shall be considered under the action of permanent loads only.
11.3.4.1 The values of YfL for the individual loads shall be those appropriate to the serviceability limit state.
11.1.2 Creep and shrinkage of concrete and prestress (including secondary effects in statically in determinate structures) are load effects associated with the nature of structural material being used; where they occur, they shall be regarded as permanent loads.
11.2 Combinations of Loads
11.2.1 Combinations of loads – Following five combinations of loads are considered.
11.2.1.1 Combinations 1 – The permanent loads i.e. dead load, superimposed loads etc. together with the appropriate live loads.
11.2.1.2 Combinations 2 – The load to be considered are the loads in combination 1, together with those due to wind/
earthquake, and where erection is being considered temporary erection loads.
11.2.1.3 Combinations 3 –The load to be considered are the loads in combination 1, together with those arising from restraint due to the effect of temperature range and difference and where erection is being considered temporary erection loads.
11.2.1.4 Combinations 4 – The load to be considered are the permanent loads, together with the loads due to friction at bearings.
11.2.1.5 Combinations 5- Dead load, superimposed dead load, together with derailment loads.
11.3 Partial Load Factors – The factors by which the design loads are obtained from the characteristic loads are specified in 11.3.1.
11.3.1 Design loads, Q* are the loads obtained by multiplying the characteristics load, Qk by YfL the partial safety factor for loads which takes into account the following:
-1. Possible unfavourable deviations of the loads from their characteristic values.
2. Inaccurate assessment of the loading, unforeseen stress distribution in the structure and variation in dimensional accuracy achieved in construction.
3. Reduced probability that various loads acting together will all attain their characteristic values simultaneously.
TABLE 12
LOADS TO BE TAKEN IN EACH COMBINATION WITH APPROPRIATE YfL (Clauses 11.2 and 11.3)
NOTE 1-ULS : Ultimate limit state SLS : serviceability limit state
NOTE 2- Superimposed dead load shall include dead load of ballast, track, ballast retainer, precast footpath, wearing course, hand rails, utility services,kerbs etc
NOTE 3- Wind and earth quake loads shall not be assumed to be acting simultaneously.
NOTE 4- Live load shall also include dynamic effect, forces due to curvature exerted on track, longitudinal forces, braking forces and forces on parapets.
LOAD LIMIT
STATE
YfL TO BE CONSIDERED IN COMBINATION
1 2 3 4 5
Dead weight of concrete ULS
SLS
Superimposed dead load ULS
SLS 2.00
Wind During erection ULS
SLS with dead and superimposed dead
loads only and for members primarily resisting wind loads.
ULS With dead plus superimposed dead
plus other appropriate combination 2 loads.
Relieving effect of wind ULS
SLS With dead and superimposed dead
loads only With dead plus superimposed dead
plus other appropriate combination 2 loads.
Restraint against movement except frictional Differential temperature effect ULS
SLS
As specified by engineer
Earth Pressure
Fill retained and or live load surcharge
relieving effect ULS 1.00 1.00 1.00 1.00
-Erection temporary loads (when being considered) ULS - 1.30 1.30 -
-Live load on foot path ULS
SLS Derailment loads (As specified by bridge rules for combination 5 only)
12 CHARACTERISTIC STRENGTHS AND PARTIAL