STRUCTURAL CHARACTERISATION AND PERFORMANCE EVALUATION OF THE FAILED MANAFWA HISTORIC BRIDGE IN UGANDA

Abstract

The Manafwa Bridge is located in Busiu Sub County located in Mbale District. A part of the bridge located on the Mbale – Tororo Highway caved in leaving a very large hole in the middle of the bridge resulting into paralysis of traffic along the road that links the Northern part of Uganda and Southern Sudan to the Central part of Uganda. Consequently, traffic along the road was disturbed hence impacting on the economy as the route is a key linkage to Southern Sudan and to Kenya. Uganda has many similar historic bridges that were constructed in the middle of the 20th Century and yet the demand for new bridges in many other areas is very high. In order to meet future transportation demands, the study was conducted to find means of improving the existing historic bridges. One way to achieve this is by predicting the failure mechanism of the Manafwa bridge and others that collapsed in the recent past and to investigate means of increasing the load carrying capacity of existing bridges to allow heavier traffic to pass. The study involved bridge inspections and condition assessment surveys of the bridge and carrying out of field testing (both destructive and non-destructive tests) on the two-span Manafwa River Bridge along Mbale - Tororo Road to assess the extent of damage caused by traffic to the bridge superstructure. The superstructure slab developed a hole between two longitudinal steel beams located adjacent to the carriageway centre line on the Mbale bound lane on the Tororo span. There was noticeable cracking of the deck surfacing at a corresponding location on the Mbale span. Traffic counts were also carried out to determine the traffic loading on the bridge and axle load information generated. This study included structural assessment with experimental model analysis of the punching shear capacity of the bridge deck slab model. A numerical simulation of the entire bridge superstructure was carried out and of the failed deck slab area using ANSYS 16.0. The main findings of the study show that the deck slab failed because of deterioration of the concrete with in-situ results showing compressive strength of 9MPa in contrast to the design strength of 30MPa and the lack of hogging reinforcement. Punching shear due to the wheel loads exerted on the road led to the eventual development of the hole in the deck slab. The study recommends that a 200mm structural topping slab can be applied to the existing slab of minimum class 30/19 structurally bonded to the existing slab instead of demolishing the bridge. The study also recommends that this approach should be taken up for all other Historic Bridges to improve their performance and service life.