Structural System

Method of sub-framing and sub frame analysis

Monolithic braced framea. Division into subframes- Dividing into subframes- beams, columns etc.- Moments and shears – elastic analysis

2. Critical load arrangement

i. Full span loads – 1.4Gk + 1.6Qk (max.)

ii. Alternate span loads - 1.4Gk + 1.6Qk (max.) and 1.0Gk (min)

c. Simplification for individual beams and columns

- Simplified subframes- Columns (ends of beams)- Fixed and half stiffness of beams

d. Continuous-beam simplification- Continuous over supports providing no

restraint to rotation

e. Asymmetrically loaded column

Rigid frames providing lateral stability

- Stability must be analyzed for horizontal and vertical loads

Alternatively;1. Max. design loads (1.4Gk +1.6Qk) for vertical loads

on elastic analysis2. Summation of moments from – elastic analysis of

sub frame -1.2Gk +1.2Qk and complete frame assuming points of contra-flexural at the centers of all beams and columns for winds loads 1.2 Wk.

Horizontal Loads

Redistribution of moments-Plastic analysis based on stress-strain curve.-Max. moments and moment distribution changes from elastic to plastic-Only allow 30% of reduction at peak moment.

Elastic and Plastic Moment Distributions

Requirement applying moment distribution

Concrete cover to reinforcement

Nominal cover-design depth of concrete to all reinforcement

including links.- Actual cover to all reinforcement should never

be less than the nominal cover 5 mm

• The nominal cover should : - be in accordance wit recommendation for bar

size and aggregates size of concrete cast against uneven surfaces ( Cl 3.3.1.2- Cl 3.3.1.4)

- Protect steel against corrosion ( Cl 3.3.3)- Protect steel against fire ( cl 3.3.6)- Allow for surface treatment such as bush

hammering

Bar size - nominal cover > main bar size - If the bar are in pairs or bundles, the nominal cover

should not be less than the equivalent cross sectional area of the bars.

Nominal maximum size of aggregates- Nominal cover > nominal max size of aggregates- Nominal max size of coarse aggregates should not

normally be greater than one quarter of minimum thickness of concrete section.

- For most work, 20mm aggregates is suitable.- For thin sections, consideration should given to use

14mm or 10 mm nominal maximum size.

Concrete cast against uneven surfaces- Specified nominal cover should be increased

beyond values of Table 3.3. - For concrete cast directly against earth, the

nominal cover should be not less than 75mm.- For concrete cast against an adequate

building, nominal cover less than 40mm (excluding blinding)

.

Ends of straight bars- Not required cover because the end of the

straight bar not exposed to the weather.Cover against corrosion- Cover used depend on exposure condition and

quality of the concrete as placed and cured immediately surrounding the reinforcement

- Table 3.4 show the limiting value for the nominal cover for concrete made with normal weight aggregates.

Method of specifying concrete for durabilityMix proportions- Use of these designation ensure that the respective

water/cement ratio and minimum cement content are included as part of the specification and have to be separated detailed

Permitted reduction in cement ratio- When concrete with free water/cement ratio significantly lower

than the maximum value in Table 3.3, the cement content may be reduced provided the following requirement :-

• reduction of cement content does not exceed 10%• Corresponding free water /cement ratio is reduced by not less

that percentage of the cement content• Resulting mix can be placed and compacted properly• Establishment of systematic control to ensure the reduced limit

are met in concrete as placed

Permitted reduction in concrete grades- A systematic checking regime is established to

ensure adherence with the free water/cement ratio and cement content.

Cover as fire protection- The values is given in table 3.5 and figure 3.2 will

ensure that fire resistance requirement are satisfied.

Control of cover-to endure that the reinforcement is properly placed

and that the specified cover is obtained.