Question

On the areas immediately behind external anchorage, the permissible unit bearing stress on the concrete, after accounting for losses due to relaxation of steel, elastic shortening, and seating of anchorages, shall not exceed ______.

1.

048  f_ci   \(\sqrt{\frac{A_{bearing}}{A_{punching}}}\)or 0.8 f_ck

 Whichever is smaller

2.

 0.48 f _ci  \(\sqrt {\frac{{{A_{bearing}}}}{{{A_{punching}}}}} \)  or  0.40 f_ck
Whichever is smaller

3.

 0.48 f _{ci  \(\sqrt {\frac{{{A_{bearing}}}}{{{A_{punching}}}}} \)}  or  0.76 f_ck

 Whichever is smaller 

4.

 0.48 f_{ci  \(\sqrt {\frac{{{A_{bearing}}}}{{{A_{punching}}}}} \)} or 0.78 f_ck

 Whichever is smaller 

Answer 1

Correct Answer - Option 1 :

048  f_ci   \(\sqrt{\frac{A_{bearing}}{A_{punching}}}\)or 0.8 f_ck

 Whichever is smaller

Concept:
Bearing Stress:

As per IS 1343:2012, Cl. 19.6.2.1.

The permissible unit bearing stress on the concrete after accounting for losses due to relaxation of steel, elastic shortening and seating of anchorages etc. on the areas immediately behind external anchorages is given by –

\({\sigma _{b,allowable}} \le \left\{ {\begin{array}{*{20}{c}} {0.48{f_{ci}}\sqrt {\frac{{{A_{br}}}}{{{A_{pun}}}}} }\\ {0.80{f_{ck}}} \end{array}} \right\}\)

Here,

f_ci – cube strength at transfer

f_ck – characteristic strength

A_br – Bearing area

A_pun – Punching area

Other codal provisions:

• During tensioning, the allowable bearing stress can be increased by 25% but it should not be greater than f_ci­. In this case this bearing stress is known as temporary bearing stress.
• The bearing area shall be the maximum area of that portion of the member which is geometrically similar and concentric to the effective punching area.
• The effective punching area shall generally be the contact area of the anchorage devices which, if circular in shape, shall be replaced by a square of equivalent area.
• When the anchorages are embedded in concrete, the bearing stress shall be investigated after accounting for the surface friction between the anchorage and the concrete.
• The bearing area shall not overlap where number of anchorages are used.