DCOM Volume I Appendix D

Appendix D: Structural Design of Concrete

Concrete members used in water structures range from Walls, Slabs, Roofs and Floors. Design of Reinforced Concrete should ensure these structures have sufficient resistance to cracking, adequate strength and does not allow leakage. Three methods or approaches are available for structural design of concrete, these are described below:

Working Stress Method:

• Produces uneconomical section,
• Produces stable section.

Ultimate Load Method

• Produces cheaper section,
• Produces unstable section.

Limit State Method

• Produces economical section,
• Produces stable section.

For design of water structure it is recommended to use Limit State Design Method, design of structural members should consider two design limits;

Limit of Collapse Design:

• Take care of safety of structure
• Deals with all types of forces Shear Force, Bending Moment, Torsion Moment ,
• Design criteria is the resistance offered by structure should not be less than limit value set in design code
• The appropriate loading value in the structure is based on loading combination of Dead Loads, Live Loads, Wind Load and Earth Quake Load as provided in BS 8110 code.
  

Limit of serviceability Design:

• Take care of control, deflection, cracking, abrasion and corrosion,
• The calculated values of deflection shall be less than the permissible values of deflection,

Design Requirements and Safety Factors
Design requirements for water structures should be according to BS 8110 but modified for the limits state of cracking to take care of crack width under the effect of applied loads, temperature and moisture content. Details of design requirements and partial safety factors as per BS 8007 is shown below.

Criteria for Sizing of Concrete Slabs and Walls Slabs Resting on Firm Ground

• Concrete slabs casted to rest directly over firm ground should be designed with nominal percentage of reinforcement provided that it is certain that the ground will carry the load without appreciable subsidence in any part.
• Concrete slabs should be cast in panels with sides not more than 4.5 m with contraction or expansion joints between.
• A screed or concrete layer less than 75mm thick should first be placed on the ground and covered with a sliding layer of bitumen paper or other suitable material to destroy the bond between the screed and floor slab.
• In normal circumstances the screed layer should be of grade not weaker than grade 10, where injurious soils or aggressive water are expected, the screed layer should be of grade not weaker than grade 15 and if necessary a sulphate resisting or other special cement should be used.

Slabs Resting on Support

• When structures are supported on walls or other similar supports the slabs should be designed as floor in buildings for bending moments due to water load and self-weight.
• When the slab is rigidly connected to the walls (as is generally the case) the bending moments at the junction between the walls and slab should be taken into account in the design of slab together with any direct forces transferred to the slab from the walls due to suspension of the slab
• If the walls are non-monolithic with the slab, such as in cases, where movement joints have been provided between the slabs and walls, the slab should be designed only for the vertical loads.
• In continuous T-beams and L-beams with ribs on the side remote from the liquid, the tension in concrete on the liquid side at the face of the supports should not exceed the permissible stresses for controlling cracks in concrete. The width of the slab is determined in usual manner for calculation of the resistance to cracking of T-beam, L-beam sections at supports as given in BS8110 design code.