Assessment and reinforcement of the 20 existing bridges included in the rehabilitation of 137 km of rural road between Bozoum and Bossangoa in Central African Republic. Presentation to the Ministry of Works.

1. THE ROAD (137 km) AND THE BRIDGES
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
BOSSANGOA
BOZOUM
OUHAM BAC

2. EXISTING BRIDGES - DESCRIPTION
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
STEEL GIRDERS
TIMBER
DECKING
STONE MASONRY
ABUTMENTS
• THERE ARE 20 BRIDGES ON THE BOSSANGOA-BOZOUM ROAD
• THE CONSTRUCTION METHOD IS THE SAME FOR ALL THE
BRIDGES: TIMBER DECKING ON 4 STEEL GIRDERS WITH STONE
MASONRY ABUTMENTS

3. EXISTING BRIDGES - CONSERVATION STATE
• TIMBER DECKING
– ON THE BOSSANGOA SIDE, THE TIMBER DECKING IS STILL
IN POSITION, BUT GENERALLY IN POOR CONDITION.
– ON THE BOZOUM SIDE THE ORIGINAL DECKING HAS
COLLAPSED AND HAS BEEN REPLACED BY WOODEN
BRANCHES.
• STEEL GIRDERS
– THE CONDITION IS GOOD.
• MASONRY ABUTMENTS
– THE CONSERVATION IS GENERALLY GOOD, EXCEPT FOR 7
BRIDGES WITH CRACKS THAT REQUIRE REINFORCING.
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

4. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 16 (BOSSANGOA SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

5. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 17 (BOSSANGOA SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

6. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 19 (BOSSANGOA SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

7. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 8 (BOZOUM SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

8. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 10 (BOZOUM SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

9. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 12 (BOZOUM SIDE) – THE TIMBER DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

10. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 20 (BOZOUM SIDE) – THE STEEL GIRDERS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

11. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 4 (BOZOUM SIDE) – THE STEEL GIRDERS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

12. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 7 (BOZOUM SIDE) – THE STEEL GIRDERS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

13. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 4 (BOZOUM SIDE) – THE ABUTMENTS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

14. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 16 (BOSSANGOA SIDE) – THE ABUTMENTS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

15. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 20 (BOZOUM SIDE) – THE ABUTMENTS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

16. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 3 (BOZOUM SIDE) – THE ABUTMENTS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

17. EXISTING BRIDGES - CONSERVATION STATE
PONT N° 18 (BOSSANGOA SIDE) – THE ABUTMENTS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

18. THE DESIGN
• GOAL OF THE DESIGN:
– REOPEN THE ROAD AND THE BRIDGES TO TRAFFIC IN A
SAFE, FAST AND COST-EFFECTIVE WAY. WE HAVE TRIED
TO CONSERVATE AND REUSE TO THE MAXIMUM THE
EXISTING BRIDGES, IDENTIFYING THE ALLOWABLE LOAD
OF THE STRUCTURES AND REINFORCEING THEM WHERE
NEEDED.
• METHOD OF WORK:
– FOR ALL THE BRIDGES WE HAVE CONDUCTED A
STRUCTURAL ANALYSIS WITH COMPUTER FINITE ELEMENT
MODELS TO DEFINE THE MAXIMUM LOAD THAT THE
BRIDGES CAN SUPPORT AND THEREFORE THE ALLOWABLE
LOAD OF THE ROAD. THE CALCULATIONS HAVE BEEN DONE
ACCORDING TO EUROCODES AND AASHTO “STANDARD
SPECIFICATIONS FOR HIGHWAY BRIDGES”.
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

19. THE DESIGN – THE STRUCTURAL ANALYSIS
• DESIGN LOAD:
(FROM AMERICAN AASHTO CODES)
• THE MATERIALS:
– TIMBER: MUKULUNGU
– STEEL: ? FOR THE EXISTING GIRDERS WE HAVE NO
INFORMATION ON THE GRADE OF THE STEEL AND ITS
RESISTANCE. WE HAVE CONSERVATIVELY ASSUMED THAT
THE GIRDERS ARE MADE OF S235 STEEL (yielding strength
= 235 MPa) WHICH IS THE LOWEST OF THE 3 COMMERCIAL
STEEL GRADES (235, 275 O 355 MPa)
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

20. THE DESIGN – THE STRUCTURAL ANALYSIS
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
FINITE ELEMENT MODEL (2D) FINITE ELEMENT MODEL (3D)
BENDING MOMENT CALCULATED DEFLECTION
MAXIMUM
CALCULATED
DEFLECTION

21. THE DESIGN – CONCLUSIONS
• REPLACE THE TIMBER DECKING FOR ALL BRIDGES
• REINFORCE THE ABUTMENTS OF 7 BRIDGES
• MAINTAIN THE STEEL GIRDERS FOR ALL BRIDGES
EXCEPT BRIDGE N° 18 WHICH HAS A LOWER LOAD
CAPACITY: 2 NEW GIRDERS WILL BE ADDED FOR
THIS BRIDGE.
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM

22. THE DESIGN – REPLACEMENT OF THE DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
MUKULUNGU TIMBER
FOR TRANSVERSE
MADRIERS AND
LONGITUDINAL
ELEMENTS: RESISTANT
AND DURABLE

23. THE DESIGN – REPLACEMENT OF THE DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
CLOSER MADRIERS (25 cm
INSTEAD OF 30 cm) TO
INCREASE RESISTANCE
AND SAFETY OF
PEDESTRIANS
RECONSTRUCTION OF
THE SAFETY POSTS ON
EACH SIDE OF THE
ABUTMENTS TO ENSURE
THAT VEHICLES TRAVEL
ON THE CENTER OF THE
BRIDGE

24. THE DESIGN – REPLACEMENT OF THE DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
RUNNING SURFACE MADE WITH
8 cm MADRIERS INSTEAD OF 3
cm PLANKS TO INCREASE
DURABILITY

25. THE DESIGN – REPLACEMENT OF THE DECKING
REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
LAG SCREWS INSTEAD OF
NAILS TO INCREASE
DURABILITY
RUNNING SURFACE MADE WITH
8 cm MADRIERS INSTEAD OF 3
cm PLANKS TO INCREASE
DURABILITY

26. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
THE DESIGN – REINFORCEMENT OF THE ABUTMENTS
• SIMPLE AND EFFECTIVE REINFORCEMENT
TECHNIQUES (FOR EXAMPLE CONCRETE
COATING) HAVE BEEN CHOSEN FOR THE
ABUTMENTS
EXAMPLE OF THE REINFORCEMENT FOR
BRIDGES 1, 2, 14 AND 20:

27. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
THE DESIGN – ALLOWABLE LOAD
• THE CALCULATIONS HAVE SHOWN THAT, ASSUMING A STEEL
RESISTANCE OF 235 MPa, ALL OF THE BRIDGES (EXCEPT ONE) CAN
SUPPORT AN AXLE LOAD OF AT LEAST 100 kN (10 tons)
• THE EXCEPTION IS BRIDGE n° 18 (pk 32.5) WHICH CAN SUPPORT AN
AXLE LOAD OF JUST 70 kN (7 tons)
• THEREFORE, BRIDGE N° 18 WILL BE REINFORCED BY ADDING 2 NEW
GIRDERS BESIDES THE EXISTING ONES, INCREASING ITS LOAD
CAPACITY TO 100 kN (10 tons).
• TO ACCOUNT FOR POSSIBLE OVERLOADED TRUCKS, WE WILL
CONSIDER AN ADDITIONAL 15% SAFETY FACTOR, THEREFORE THE
ALLOWABLE AXLE LOAD ON THE ROAD WILL BE:
85 kN (8.5 tons)

28. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
BRIDGE n° 18 – REINFORCEMENT
• TWO NEW GIRDERS WILL BE ADDED BESIDES THE
EXISTING ONES, INCREASING THE LOAD CAPACITY

29. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
TEST LOADS – DURING CONSTRUCTION
• DURING CONSTRUCTION IT WILL BE IMPORTANT TO
CHECK THAT THE BENDING RESISTANCE OF THE
MUKULUNGU TIMBER ISN’T LOWER THAN THE VALUE
CONSIDERED IN THE DESIGN (30 MPa)
• SIMPLE RESISTANCE TESTS WILL HAVE TO BE MADE ON
THE TIMBER MADRIERS

30. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
TEST LOADS – DURING CONSTRUCTION
• TEST LOADS ON THE TIMBER MADRIERS

31. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
TEST LOADS – AFTER CONSTRUCTION
• AFTER CONSTRUCTION IT WILL BE IMPORTANT TO CHECK THAT THE
RESPONSE OF THE BRIDGE CORRESPONDS TO THE EXPECTED
PERFORMANCE.
• LOAD TESTS WILL BE MADE ON THE BRIDGES TO CHECK THAT THE REAL
DEFLECTION CORRESPONDS TO THE CALCULATED DEFLECTION. A TRUCK
WITH THE SAME WEIGHT AS THE DESIGN LOAD WILL STAY ON THE BRIDGE
WITH THE AXLE PLACED IN MIDSPAN, AND THE MAXIMUM DEFLECTION WILL
BE MEASURED WITH A DIAL GAUGE.

32. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
MAINTENANCE AND MONITORING
IN ORDER TO ENSURE THE DURABILITY OF THE BRIDGES IT IS NECESSARY TO
REGULARLY MAINTAIN AND MONITOR THEM.
• TIMBER RUNNING SURFACE: IT IS ONE OF THE MOST EXPOSED PARTS TO WEAR
AND DEGRADATION, AND SHOULD BE CHECKED EVERY 4 MONTHS TO IDENTIFY
DAMAGED PARTS THAT SHOULD BE REPLACED.
• TRANSVERSE MADRIERS: THE TRANSVERSE MADRIERS SHOULD BE INSPECTED
EVERY 4 MONTHS TO FIND DAMAGED OR BROKEN ELEMENTS THAT SHOULD BE
IMMEDIATELY REPLACED.
• TIMBER CONNECTIONS: THE CONNECTIONS BETWEEN THE TIMBER ELEMENTS
ARE PARTICULARLY SENSITIVE TO VIBRATIONS AND TRAFFIC. THEY SHOULD BE
CHECKED EVERY 4 MONTHS; THEY SHOULD BE TIGHTENED WHEN NECESSARY,
AND REPLACED IMMEDIATELY IF FOUND TO BE MISSING.
• SAFETY POSTS: THE SAFETY POSTS ON THE ABUTMENTS ARE ESSENTIAL FOR
THE SAFETY OF THE BRIDGES: THEY SHOULD BE CHECKED EVERY 2 MONTHS
AND IMMEDIATELY REPLACED IF BROKEN.
• ABUTMENTS: ABUTMENTS SHOULD BE CHECKED REGULARLY FOR CRACKS,
DISPLACED STONES, SCOUR ON FOUNDATIONS, ETC. AND APPROPRIATE
RECONSTRUCTION/REINFORCEMENT SHOULD BE DONE. A COMPLETE
INSPECTION SHOULD BE DONE EVERY SIX MONTHS.
• VEGETATION: VEGETATION SHOULD BE CLEARED FROM THE BRIDGE
ANNUALLY. VEGETATION SHOULD ALSO CAREFULLY BE REMOVED FROM THE
ABUTMENTS WITHOUT DAMAGING THE MASONRY.

33. REHABILITATION DE LA R.R. N°8
BOSSANGOA – BOZOUM
THANKYOU
FOR YOUR ATTENTION