DESIGN OF STEP FOOTING: Problem: Design a step footing for the column of size 𝟐𝟑𝟎 𝐦𝐦 ∗ 𝟓𝟐𝟓 𝐦𝐦, reinforced
with 6-20mm ∅ bars and carrying an axial working load 𝟏𝟐𝟎𝟎𝐊𝐍 and a moment 𝑴𝒙 =
𝟕𝟓𝐊 𝐍 − 𝐦, 𝑴𝒚 = 𝟐𝟎𝐊𝐍 − 𝐦. The soil bearing capacity is 𝟐𝟓𝟎𝐊𝐍/𝐦𝟐. Use 𝐌𝟐𝟓 grade
concrete and Fe 415 bars. Effective cover at the bottom steel is 𝟕𝟓 𝐦𝐦.
Solution: 𝑭𝑐𝑘 = 25 N/mm2
𝐹𝑦 = 415 N/mm2
Service load = 1200KN
Assumed weight of footing and backfill (△ 𝑃)(10%) = 120KN
Safe bearing capacity of soil (S.B.C) = 250KN/m2
Eccentricity 𝑒𝑥 = 𝑀𝑥 /(P + ΔP) = (75000∗ 1.5 load factor )/(1200∗ 1.5 load factor ∗ 1.1(10%
load of footing and backfill )) = 56.81 mm
Eccentricity 𝑒𝑦 = 𝑀𝑦 /(𝑃 + Δ𝑃) = (20000∗ 1.5 load factor )/(1200 ∗ 1.5 load factor ∗ 1.1(10%
load of footing and backfill )) = 15.15 mm
The length and breadth of the footing are to be so selected that the eccentricity e is less than 𝐿/6
and 𝐵/6 to get the entire area of the footing in contact with soil.
Area minimum =
𝑃𝑢 +Δ𝑃
𝑞𝑎
=
1200+120
250
= 5.28 m2. As moments are also acting along with
axial load,so we assume L = 2.75 m