Infrastructure management – Structural engineering – Case study Morogoro, region of Tanzania

January 1999 – January 2002

Assuring mobility through sustainable transportation systems –
A supply and demand system approach to the development of bridge management strategies


Organism: Government of Tanzania
Institute: Regional Engineer’s Office of the Morogoro Region of Tanzania
Address: Eng. Johnson B. Kiiza, Regional Engineer
P.O. Box 91, Morogoro
Phone: +255 – 56 48 90
Fax: +255 – 56 47 80
Direction: Mr. J. Kiiza


Institute: Institut des structures (IS)
Laboratoire de maintenance, construction et sécurité des ouvrages (MCS)
Faculty: Environnement Naturel, Architectural et Construit (ENAC)
Address: Ecublens
1015 Lausanne, Suisse
Phone: +41 21 – 693 28 82
Fax: +41 21 – 693 58 85
Direction: Prof. Eugen Brühwiler
Coordination: Dr. Bryan Adey


In the last two decades, under the pressure of deteriorating infrastructure and constrained budgets in North America and Europe, computer aided bridge management systems (BMSs) have been developed. These systems allow decision makers to analyse numerous management strategies in the search for the optimal allocation of resource budgets that ensures adequate bridge performance, i.e. the optimal management strategy.

The current approach to bridge management incorporated in these BMSs considers bridges independently of their role in the network. This individual bridge approach is focused on keeping existing bridges between their original as-built condition states and certain minimum acceptable condition states. It is assumed that an adequate level of service is provided, i.e. user demands are met, as long as the bridge condition is equal to or better than the minimum acceptable condition state. The condition states are determined based on visual inspections.

The individual bridge approach limits the ability of existing BMSs to determine management strategies that ensure the maximum benefit for the users of the network for the allocated resources. Optimal management strategies (OMSs) cannot be found if an adequate level of service is not provided, if multiple bridges are affected simultaneously and if bridge condition cannot be determined by visual inspection. In this thesis an approach is presented that can deal with these limitations: the supply and demand system approach. The supply and demand system (SDS) approach to the determination of OMSs alleviates these limitations by taking into consideration:

  • the ability of the transportation network as a whole (system) to provide an adequate level of service,
  • the role and performance of the bridges in the network (supply), and
  • the consequences if an adequate level of service is not provided (demand).

The ability of the transportation network as a whole to provide an adequate level of service is evaluated by determining how the network is affected by hazards to which it is subjected. This involves consideration of the performance of the individual bridges in the network, the network connectivity, i.e. the number and location of alternate routes on which detours can be made if bridges are non-operational, and the network functionality, i.e. the capacities of, and traffic on, the network links. The role of the bridges in the network is taken into consideration when the ability of the network to perform adequately is determined. By modelling the entire network the location of each of the bridges and the consequences of inadequate bridge performance can be included. Bridge performance in the network is evaluated by determining their ability to perform adequately during the investigated time period with respect to the hazards to which they are subjected. Bridge performance is quantified using probabilistic methods.

The consequences if an adequate level of service is not provided are evaluated by assuming that the network has different network condition states, NCSs, and estimating the user costs in each of these NCSs. This involves the simulation of the traffic on the network, the modelling of the road deterioration on the network links and determination of the relationships between vehicle operating costs, travel time costs and accident costs, and vehicle speed, road deterioration and traffic flow in the possible NCSs.

The SDS approach is illustrated by determining the OMSs for five bridges in a real transportation network, in the Morogoro region of Tanzania. The performance of the bridges and the network over a 15-year time period is investigated using probabilistic methods with respect to excessive loads due to traffic and excessive scouring due to floods. The consequences to the users if adequate service is not provided and detours are possible are evaluated using the Highway Design and Maintenance model developed by the World Bank, and when detours are no longer possible, using benefit/cost ratios.

The sensitivity of the OMSs determined using the SDS approach to changes in traffic volume is investigated by re-determining the OMSs for the five bridges in the real transportation network with the traffic volume on a number of the links in the network near capacity. By determining OMSs for the same bridges with small and large traffic volumes the effect of congestion on OMSs is evaluated.

OMSs determined using the SDS approach are compared with those determined using two variations representative of the individual bridge approach used in existing BMSs: the supply bridge and the supply and demand bridge approach. Single intervention and multiple intervention management strategies are determined for four groups of bridges in the real transportation network for a range of bridge vulnerabilities.

The main conclusion of this research is that the SDS approach can be used to determine OMSs when the bridge performance cannot be determined solely by visual inspection, without the assumption of an adequate level of service, and when multiple bridges may be adversely affected simultaneously.

The OMSs determined using the SDS approach are sensitive to the traffic volume on the network links. This sensitivity is because the costs incurred on the network are not linearly proportional to the number of vehicles. As the traffic volume on a link nears capacity costs increase dramatically. The use of the SDS approach to determine OMSs instead of the common approaches used in BMSs can result in increased savings.


Roelfstra, G., Adey, B., Hajdin, R., and Brühwiler, E. The condition evolution of concrete bridges based on a segmental approach, non-destructive testing and deterioration models International Bridge Management Conference, Denver Colorado, United States, April 26-28, 1999

Adey, B., Hajdin R., Kiiza J., Brühwiler E., 2000, Societal benefits of maintenance strategies for civil infrastructure, Transdisciplinarity Conference: Joint Problem-Solving Among Science, Technology and Society, Zurich, Switzerland, February 27 – March 1.

Adey, B., Hajdin R., Brühwiler E., 2001, Comparison of hazard scenarios using probabilistic methods, Safety, Risk and Reliability Trends in Engineering, IABSE, CIB, ECCS, fib, RILEM International Conference, Malta, March 21-23.

Adey, B., 2002, A supply and demand system approach to the development of bridge management strategies, thesis number 2519, The Swiss Federal Institute of Technology, Lausanne, Switzerland, January.

Adey, B., Bailey S., Hajdin R., Brühwiler E., 2002, Updating estimates of bridge reliability, First International Conference on Bridge Maintenance, Safety and Management, IABMAS, Barcelona, Spain, July 14-16.

Adey, B., Hajdin, R., and Bruhwiler, E., 2002, « A supply and demand system approach to bridge management » in the Probabilistic Safety Assessment and Management 6 Conference, San Juan, Puerto Rico, June 23-28.

Adey, B., Hajdin, R., and Bruhwiler, E., 2002, A system approach to the reduction of damage costs due to natural hazards, IABSE Symposium, Towards a Better Built Environment – Innovation, Sustainability, Information Technology, Melbourne, Australia, September 11-13.

Adey, B., Hajdin, R., and Brühwiler E., 2002, Effect of common cause failures on indirect costs, Journal of Bridge Engineering (accepted for publication in Bridge Engineering ASCE) also presented at the 2001 Structures Congress, Washington D.C., USA, May 21-23, 2001