|Three cell box culvert which I designed for NRCP|
As local practice is based on British codes, the most relevant document is identified as part 2 & part 4 of Bs 5400. The additional guidelines are provided by BD 31/01.
Appendix A of BD 31/01 shows many diagrams on load cases which are to be considered. It includes following combinations.
1.) Max. Horizontal loads with Max. vertical loads ( No Traction)
2.) Minimum vertical load with max. horizontal Loads (No Traction)
3.)Maximum vertical load with minimum horizontal loads ( No traction)
4.)Traction with max. vertical Loads
5.) Traction with min. vertical loads
The checks on stability is conducted as follows;
6.) Sliding : min. vertical loads with buoyancy on foundation slab. ( primary live loads are included with min. factors as it is necessary include traction forces)
7.) Bearing : max vertical loads with buoyancy on foundation. All loads are nominal.
In all above conditions , it is necessary to concern whether to include live load surcharge on to both abutment walls or not.
In the first five of loading cases, BD 31 is silent on applicability of Hydro static Pressure on walls.
However elsewhere , in cl 3.1.4 it says, /// when appropriate , the effects of hydro-static pressure and buoyancy shall be taken in to account. The increase in pressure on the back of the walls due to hydro static pressure at a depth Z meters below water level shall be taken as 10Z(1-K) kN/m2///
Please see the following figure which i took form my note book and captured from my mobile camera. it explains how this 10Z (1-k) is derived.
However BD 31 does not well explain that how hydrostatic forces do applicable in the first five load cases.
Contrary , the american LRFD documents which are basically based on Marston's studies on the subject , provide more details on this.
The following picture that referred from a design guidelines of Kansas Department of Highways is self explanatory.
1.) Maximum vertical load in deck slab and Max. outward load on the wall ( DL max+ Earth vertical max+earth horizontal minimum+hydrostatic max)
2.) Minimum vertical load and maximum load on inward wall of the abutment ( DL Min + Earth verticle Min+ earth horizontal max)
3.) Max vertical load and max load in inward wall of the abutment ( Dl max+Earth verticel max+Earth horizontal max+ live load max)
It is clear that there is not general agreement between distinguish design guidelines on the load cases and more specifically on the hydrostatice loads and the uplift /buoyancy.
I am of the view that hydrostatic pressures become critical when a sudden discharge flows through the barrel without no time allowing Ground Water level to rise. Such a phenomena could happen in the cases of upstream flood or owing to spill of upstream reservoir.
In the same line of argument , water level can be differ in two sides owing to rise of ground water level but stream flow is at regular levels. I am in doubt whether is this could happen in real?
In other cases hydrostaic forces are in balance or have little impact on the total effects.
|construction of the 3 cell|