FIG 2.7 AREA OF CROSS SECTION AT DIFFERENT LEVELS
3. COMPONENTS OF BRIDGE STRUCTURE
Let us now study the bridge components and its adaptability and suitability in particular site conditions.
3.1 FOUNDATIONS
3.1.1 Depth of foundations : The foundation shall be taken to such depth that they are safe against
scour, or protected from it. Apart from this, the depth should also be sufficient from consideration of bearing capacity, settlement, stability and suitability of strata at the founding level and at sufficient depth below it.
(A) Depth of foundations in soil (Erodible strata)
(a) Depth of shallow foundations : Foundations may be taken down to a comparatively shallow depth below the bed surface provided a good bearing stratum is available and the foundation is protected against scour.
R.L. of foundation = Designed H.F.L.(Tallied H.F.L.) - Maximum scour depth
- Depth of Embedment (D.E.) Where Depth of Embedment = Minimum 2.0 m for piers and
abutments with arches.
= Minimum 1.2 m for pier and abutments, supporting other type of superstructure. (b) Depth of deep foundations (in erodible strata)
R.L. of foundation = Designed H.F.L. (Tallied H.F.L.)- 1.33 * Maximum scour depth.
(B) Depth of foundations in rock
foundation R.L. in case of Hard Rock = R.L. of strata of Hard Rock - 0.60 m
foundation R.L. in soft rock / = foundation R.L. in soft rock / Exposed rock - 1.50 m. exposed rock (Scourable rock strata is not considered while taking
R.L. of rock top)
Selection of a particular type of foundation is a very important job as it affects the entire proposal for the bridge. e.g. if the rock is not available at shallow depth, the tendency may be to adopt well foundation and as well foundations are costly the situation may lead to adoption of bigger spans, with P.S.C. structures. On the other hand if scour depth is less and flood depth is also reasonably small the raft foundation could be the choice. This will result in smaller spans, less height of bridge, may be a submersible bridge with permissible interruptions is felt sufficient.
Presence of soft/hard rock within 5m would attract open foundation depending upon the scour depth, the type of bridge and height of the bridge above and below the bed level. Situation with 5m depth of foundation below bed and 2m to 3m height of pier above bed may not sound good. Alternative should be thought of in such cases. So look for the strata where foundation can rest. Start with open foundation. If the depth of strata is deeper than 6m to 7m think of wells or piles. Simultaneously study scour depth and height of the bridge above bed level. If the scour depth is within 3.0 m and there is no problem of standing water, consider the possibility of raft foundations.
3.1.2 Important Points
The following points are to be noted while preparing bridge proposal.
(a) Span to height ratio for Raft foundation be kept as 1.00 to 1.25 Open foundation be kept as 1.25 to 1.50 Pile foundation be kept as 1.25 to 1.75 well foundations it should be 1.50 to 2.00
The height of pier is measured from foundation to top of pier i.e. up to pier cap top.
(b) The dimensions of pier, abutment and well foundation to be taken from type designs or from the latest I.R.C. Codes.
(c) Proper uniform sitting of well foundation could be ensured by taking the foundation into rock by about 15 cm.
(d) The raft foundation details be taken from the type designs as applicable.
(e) Other similar designs prepared and approved by the Designs Circle should also be studied and referred to.
(f) Open foundations are comparatively easy to decide about.
(g) Anchorage of open foundation into the rock shall be as per IRC-78 i.e. minimum 0.60m into hard rock and 1.50 m into soft rock excluding scourable layers.
(h) Levelling course and annular filling should be proposed for open foundation. Annular filling should be done with M 15 concrete upto rock level.
(i) Stability of foundation should be worked out. The beginner should obtain the standard calculation sheets from office, and do the calculations manually to gain confidence. Further trials could be on computer. Software is available for checking the stability of the foundation. (j) Area under tension as per IRC:78-2000 clause 706.3.3.2 is allowed up to 33 % for load
3.1.3 FOUNDATION TYPES
Generally two types of foundations are adopted for bridge structures. (i) Shallow foundations - Open foundations
- Raft foundations (ii) Deep foundations - Pile foundations
- Well foundations
3.1.3.1 Open
Open foundations are preferred over any other type. These are to be provided when good-founding strata is available at shallow depth and there is not much problem of dewatering. R.C.C. footings are preferred over P.C.C. footing in case of RCC piers.
3.1.3.2 Well
The shape of well can be, Single Circular, Double D-Type,Dumbell Type, Twin Circular as shown in fig.3.1.The typical components of well and various methods of starting well foundations are shown in fig.3.2 and fig. 3.3
Some important points to be noted regarding well foundations are as follows –
a. If the external diameter of single circular wells exceeds 12 m relevant provisions of clause 708.1.2 of IRC: 78-2000 shall apply.
b. The steining thickness of well shall not be less than 500 mm and shall satisfy the following relationship
h =
where h = minimum thickness of steining in m d = external diameter of circular well in m
L = depth of wells in m below top of well cap or LWL whichever is more
K = constant(for wells in cement concrete 0.03,brick masonry 0.05 and twin D wells 0.39(For details refer to clause 708.2.3 of IRC: 78-2000).
c. In case of PCC wells the concrete shall not be learner than M-15. In case of conditions of severe exposure, steining shall not be leaner than M-20. The horizontal annular section of well steining shall be checked for ovalisation moments taking account of side earth pressure.
d. M.S. cutting edge shall not be less than 40 kg/m to facilitate sinking through all types of strata. In case of well curb the internal angle should be kept at about 300 to 370.Well curb shall not be leaner than RCC M-25.
e. The bottom plug provided should be such that the top is kept not lower than 300 mm in the centre above the top of the curb sump to be provided below the level of cutting edge. Well filling above the bottom plug shall be done generally with sand. Top plug of 300 mm in M-15 shall be provided over filling.
L kd