Simply supported structures (isostatic system) are easier to construct and require relatively simple construction and installation technique. They are mostly suitable for culverts and minor bridges in short and medium span range. They are also suitable in situations where the sub-soil condition is poor and not conducive for continuous structures where differential settlement may be large.
There are however some disadvantages of using simply supported bridges in case of long multiple span bridges as they require considerable number of expansion joints, which constitutes a serious inconvenience and entail continuous costs during operation.
It is thus recommended that simply supported structures be used for the project under following situations:
- For Culverts
- For minor bridges with span bridge length between 6m to 530m
- For bridges resting on open foundations where SBC is less than 25 t/m2.
- Bridges where the height of piers above the surrounding ground level is short.
In general the Design of Wells for Foundations shall conform to the provisions of IRC: 78-2000. The following points should be taken care off:
- All the Loads and Forces shall be evaluated as per provisions of IRC: 6 and their combination for the purpose of designing of Wells shall be taken as per provision of Clause 706.1.1 0f IRC: 78-2000.
- The permissible increase in stresses in various combinations will be 33 1/3 percent with Wind and 50 percent for combination with Seismic as per provision of Clause 706.1.2 of IRC: 78-2000.
- The permissible increase in allowable base pressure will be 25 percent for all combinations except for combinations without Wind & Seismic as per provision of Clause 706.1.2 of IRC: 78-2000.
- The external diameter of the wells shall be selected such that the allowable safe bearing capacity is utilised to the maximum.
- The thickness of the steining of the well shall be kept ‘ h ‘ = K x d x l 0.5 or 500 mm. whichever is more.
Where ‘ h ‘ = the minimum thickness of the steining
‘ d ‘ = external diameter of the well
‘ l ‘ = depth of well below top of well cap or L.W.L. which ever is more
‘ K’ = a constant , its value to be taken as per clause 708.2.3.1 of IRC: 78.
- The stability & design of well foundation shall be checked under most critical combination of Loads & Forces as per clause 706 of IRC : 78.
- The side earth resistance shall be calculated as per provisions of Appendix-3 of IRC: 78. The use of provisions of IRC: 45 shall be done for Pier well foundation that to in cohesionless soil only.
- The minimum diameter of dredge hole shall be kept as 2.00 meter.
- For Plain Concrete Wells, vertical reinforcements in the steining shall not be less than 0.12 percent of gross sectional area of actual thickness provided. This shall be equally distributed on both faces of steining.
- In case, where the well steining is designed as a reinforced concrete member, the amount of vertical reinforcement in the steining shall not be less than 0.2 percent of actual gross sectional area of steining. On the inner face, a minimum of 0.06 percent (of gross area) steel shall be provided.
- The vertical reinforcement shall be tied with up with hoop steel not less than 0.04 percent of volume per unit length of steining.
- Mild steel Cutting Edge to be provided shall be strong enough and not less than 40 kg/m to facilitate sinking of well through all type of strata expected to be encounteredwithout causing any damage.
- The internal angle of the Well Curb as ‘ ? ‘ as shown in the Fig.-2 of appendix-3 of IRC: 78-2000 shall be kept between 30 to 37 degrees.
- The Well Curb shall be in reinforced concrete of mix not leaner than M 25 with minimum reinforcement of 72 kg./cum. Excluding bond rods.
- The Bottom plug shall be provided in all wells and top shall be kept not lower than 300 mm. in the centre above the top of the curb.
- The filling of well if considered necessary, above the bottom plug shall be done with sand or excavated material free from organic matter.
- A 300 mm. thick plug of cement concrete shall be provided over the filling.
- The bottom of the Well Cap shall preferably be laid as low as possible taking into account the lowest water level.
- The well shall be designed taking into consideration a tilt of 1 in 80 and a shift of 150 mm. in the direction which causes most severe effect