Expert: Arshad Khan - 7/1/2009

Question
QUESTION: I am designing a pipe bridge across a river. I need to calculate the area of the footing for the Pad foundation. I will have one on either side of the river. The pipe is a 600mm diameter DI and there is soil overburn on each pad foundation. do i need to consider ecentricity and moments. and how far away from the footing do i calculate the weight of the pipe and overburn. the foundations are 12m apart. should i assume that the loads are through the center of the footing and therefore no eccentrcity and moment problem.

ANSWER: Hi Francis,

Assuming that the pipe is straight, without any bends near the pad footings, then you do not need to take eccentricity into account along the pipe axis (apart from nominal as per code recommendations). However, at right angles to the pipe, you need to take into account wind loading and (depending on relative level to flood water level), you may need to take account of water/debris pressure/loading horizontally, which will exert moments on the pad footings.

Regarding the second question, this depends on the type of joints in the pipe, and their locations, as well as the degree of compaction of the sub-base- flexible joints over the footings mean you may not need to take loading for much more than say 3 to 4 times diameter away from the footing.

---------- FOLLOW-UP ----------

QUESTION: the pipe is at a gradient of 1 in 600. the pipe will sit and will be supported on a conrete column (about 1.5m high which will transfer the loading onto the footing. Am i correct that for the area of the footing i need to consider vertical loading to the footing agianst the bearing capacity of underlying soil. I am confused as to how i will include eccentricity and moments in calculating the required area of the footing.


The joins of the pipes are rigid, how far from the center of the footing should i take the loading.

Thanks

Francis


Answer
Hi Francis,

The 1:600 slope is insignificant. Moments will be incurred orthogonal to the axis of the pipe, as I explained erlier- horizontal loading due to wind/water, applied for exposed length, multiplied by (1.5m plus 0.3m to centre of pipe plus thickness of pad).

If joints are rigid and soil is weak, then assume 1/4 span of pipe's weight outside of span as an additional vertical load on to each pier.