[Back]

D. Adam, R. Markiewicz:
"Design of Foundation for Ada Bridge";
Talk: Belgrade Bridge Convention, Belgrad (invited); 2010-10-01.

The bridge over the river Sava in Belgrade is the main structure on the inner semi-ring road from Omladinskih brigada Street to Pastrovićeva Street, and it crosses the river Sava in the zone of the downstream tip of Ada Ciganlija. The Sava Bridge is a 7 span, continuous superstructure with an overall length of 964 m between deck expansion joints. The main support system is a 200 m high single pylon (pillar No. 6) asymmetric cable stayed structure with a main span of 376 m and a back span of 200 m.
Due to investigated soil conditions and loads from bridge superstructure all piers were founded with piles. At piers 1 to 5 as well as 7 and 8 pile groups were designed. At pier 6 the designed piles and diaphragm walls form a box-shaped foundation, the soil inside the box takes part for load transfer.
The foundation depth of piers 1 to 6 is within marl and the foundation depth of piers 7 and 8 within sandstone. At each pier the pile heads are connected by a pile cap for load transfer from the superstructure to the single piles.
At pier 6 the box-shaped foundation is composed of a compound body consisting of diaphragm walls and piles and the enclosed soil. This quasi-monolith transfers high vertical and horizontal forces. Piles and capping raft form a box, which acts physically like a "pot" turned upside down. Consequently, the settlements are expected to be smaller than for conventional pile groups, and the earthquake resistance is significantly higher. Box-shaped foundations represent a special form of piled raft foundations utilising the enclosed soil core as an integrated load transfer member.
The following two calculation models were considered for the box-shaped foundation at pier 6:
. Calculation model A: Verification of vertical bearing capacity of single piles and diaphragm wall according to ÖNORM EN 1997-1 (EC 7-1).
. Calculation model B: Quasi-monolith. A full bond effect between deep foundation elements and the closed soil was assumed. This compound body comprises the outer circumference of the foundation if secant piles or diaphragm walls are installed.
For seismic design of foundations two approaches were investigated: On the on hand dynamic soil parameters were determined from geophysical field tests (e.g. cross-hole and down-hole tests) and/or dynamic laboratory tests (e.g. resonant columns test). Field tests delivered elastic data (dynamic shear modulus and dynamic elastic modulus) at small shear strains. On the other hand seismic calculations were performed by variation of the static stiffness since radiation damping could be neglected. In general, the quasi-static stiffness increases when dynamic loads are applied. In the scope of the seismic design of the foundations of the Sava Bridge parametric studies were performed taking into account both approaches. Dynamic soil parameters were derived from geophysical tests and data given by the Serbian Seismic Institute in Belgrade.
In order to verify soil parameters used for calculation and design of pile foundations of Sava-Bridge four trial piles at piers 5, 6 and 7 have been installed and tested.
In the static model of the superstructure a stiffness matrix was integrated at each pier in order to take into account the forces and deformations of the pile foundation. Furthermore, additional differential settlements were considered at each support using the elastic stiffness of the structure.
In order to check the assumptions in the static model of the superstructure settlement calculations were performed. The foundation of the piers is within rock-like marl at piers 1 to 6 and within sandstone at piers 7 and 8. Therefore, the piles predominantly work as end bearing piles. Total settlements of the foundation consist of settlements of single piles and settlements of the pile group.
The settlements of a single pile were derived from pile load tests (trial piles) taking into account the maximum working load of a single pile. For settlement calculations of the pile group the envelope area of all piles (resp. diaphragm walls at pier 6) in the foundation depth was taken as a basis. For the settlement calculations of this quasi-monolith a shallow foundation with a deep foundation level was assumed. Settlement calculations were performed with working loads without safety factors. Therefore, separate static calculations without any safety factor have been carried out in order to calculate working loads of each pile.

Ada Bridge Belgrade; Sava; Design of Foundation;