A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
In BS8110 for reinforced concrete design, it states that longer tension lap
lengths have to be provided at the top of concrete members. The reason
behind this is that the amount of compaction of the top of concrete
members during concrete placing is more likely to be less than the
remaining concrete sections [49]. Moreover, owing to the possible effect of
segregation and bleeding, the upper layer of concrete section tends to be
of lower strength when compared with other locations.
When the lap lengths are located at the corners of concrete members, the
degree of confinement to the bars is considered to be less than that in
other locations of concrete members. As such, by taking into account the
smaller confinement which lead to lower bond strength, a factor of 1.4 (i.e.
40% longer) is applied to the calculated lap length.
8. What is the purpose of setting minimum amount of longitudinal
steel areas for columns? (SR2)
In some design codes it specifies that the area of longitudinal steel
reinforcement should be not less than a certain percentage of the sectional
area of column. Firstly, the limitation of steel ratio for columns helps to
guard against potential failure in tension. Tension may be induced in
columns during the design life of the concrete structures. For instance,
tension is induced in columns in case there is uneven settlement of the
building foundation, or upper floors above the column are totally unloaded
while the floors below the column are severely loaded. Secondly, owing to
the effect of creep and shrinkage, there will be a redistribution of loads
between concrete and steel reinforcement. Consequently, the steel
reinforcement may yield easily in case a lower limit of steel area is not
established.
In addition, test results showed that columns with too low a steel ratio
would render the equation below inapplicable which is used for the design
of columns:
N=0.67fcuAc+fyAs
9. Why does the presence of tension reinforcement lead to increasing
deflection in concrete structures?
In BS8110 a modification factor is applied to span/depth ratio to take into
account the effect of tension reinforcement. In fact, deflection of concrete
structure is affected by the stress and the amount of tension reinforcement.
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