Design Of A Steel Footbridge For University Of Birmingham Students

Access and Safety Requirements

A footbridge is defined as a bridge which is solely for use by the pedestrians. They are usually constructed over roads with heavy traffic flows to enable people to cross from one side of the road to the other side. Footbridges are also constructed over physical obstacles such as rivers and valleys (Russell et al. 2017). These footbridges usually carry loads which are modest in nature thus the structure needed is to be relatively light. The footbridges are also expected to be of a relatively long span and offer good stiffness and at the same time be of clear view to the public thus attracting their attention. Steel therefore is the best material to be used for the construction of footbridges since it meets all the requirements named above. Steel also provides better economic advantage.

In this research study, a steel bridge is to be designed that will enable the University of Birmingham (UoB) students to safely cross the Lewes Road. It is therefore anticipated that the footbridge will be relatively light since there will not be too much load carried over the bridge as it is only the students who will be using the bridge. The principal span in the footbridge will be simply supported and the bridge will also be constructed in such a way that the cyclists and those who use wheelchairs will easily access it. The width of the bridge will be such that it can enable the passage of students in both directions while also taking into consideration the interests of the cyclists and those who will be using the wheelchairs. The bridge will have segregated provisions at some occasions to prevent the collision between the Cyclists and the Pedestrians. Parapets will also be constructed to guarantee the safety of the UoB students crossing the bridge and the traffic flow below the bridge.

For the analysis of the footbridge’ truss, the method of sections and the method of joints will be implemented. For the method of section, a theoretical section of the truss will be cut and named through the members forming it including the members whose forces are to be determined thus dividing the framework into various parts. The forces in the selected sections are treated as external forces and then the laws of static equilibrium are used to find the unknown forces. The second method to be implemented for the analysis of the footbridge is the method of joints where each joint is kept in a state of equilibrium and principal laws of static equilibrium used to find the unknown forces. 

The first step of designing a footbridge is the determination of the access and safety requirements such as the width of the bridge and its form access which all depend on the traffic flow of the pedestrians that is expected. For this footbridge, we will use the minimum requirements of the width and form of access. The clear width will be 3.5 m and the parapets at the footway sides will be 1.15 m high. This height is used since the footbridge will be over a road and the area is also not prone to vandalism. The width is designed to be 3.5 m because the pedestrians and the cyclists will be sharing the pathway. A 50 mm upstand will also be constructed on the footbridge using an edge beam. The upstand will be for drainage purposes and also for run-off prevention over the carriageway in the road below the bridge. 

Truss Construction Type

The span will be dependent of the width of the Lewes Road that the footbridge is designed to cross at the point of intersection. The span will be designed such that the stands of the bridge are 4.5 m away from the Lewes Road in order to avoid the risk of collision from a rambling vehicle. In the case of this footbridge, a span of 30 m will therefore be constructed. A clearance of 5.8 m will be used in the footbridge since it passes over a highway, Lewes Road. 

To provide access to the footbridge, stairs and ramps will be used. Ramps will be designed to suit the needs of the disabled students who will in most cases be using the wheelchairs and it is a general policy for it to be constructed in every footbridge. The ramps constructed will have a gradient of 0.05 and it will be done in a series of spiral sections due to availability of enough space in the university premises. The ramp will be arranged such that the change of direction at any intermediate landing will be 180⁰. This design is referred to as scissor fashion. The inside radius of the ramp will be 5.6 m. The stairs will be arranged in two flights with handrails on the parapets inside faces.   

The construction type to be used in this case will be truss construction type with a warren truss used. Since the bridge will be clad such that it provides complete enclosure to the students, through construction will be employed. A depth of 1.5 m will be used since the span of the footbridge is 30 m.

A footbridge which is any structure that enables pedestrians to cross over man-made or natural obstacles with less risk are always exposed to various types of loads. These load types include vertical loads, horizontal loads, self-weight loads, and loads as a result to earthquakes. Load evaluation is very crucial for any footbridge design (Fatemi, Mohamed Ali & Sheikh 2016). In this section, we will only be evaluating the vertical loads that the footbridge will encounter during its service life.

A vertical load is defined as any load acting perpendicularly onto the floor of the footbridge. The vertical loads likely to be encountered by the footbridge include the human traffic, wind load and self-weight of the bridge and its auxiliaries. The self-weight loads will however be ignored.

For footbridges, there are three vertical load models that are encountered. These load models include a uniformly distributed load that represents the static effects of a dense crowd, one concentrated load representing the effects of maintenance load and finally one or more mutually exclusive load representing standard vehicles passing the bridge for maintenance purposes of emergency vehicles that might cross the footbridge (Marecik & Pa?tak 2018). 

The dense crowd representing the students crossing the footbridge will result into a uniformly distributed load.  The National Annex will be used to define the longitudinal and the transverse loads i.e., qfk since the footbridge will not be densely crowded because it is not open to the general public, the longitudinal and the transverse loads (Shen et al. 2017). The recommended UDL value will therefore be; 

From the equation above, it is deduced that the minimum crowd load that the bridges encounter irrespective of their span is  and the maximum crowd load is . The equation 1 above can be used to check the span length effect on crowd load value. For our footbridge, the span was 30 m.

Hence in our case, we will be using a uniformly distributed load of  to be acting on the bridge. 

A 10 kN concentrated load representing maintenance load is usually considered during evaluation of the local effect (Almasri & Halahla 2019). The load is considered to be acting on a square surface of 10 cm sides.  is however usually not taken into account once the service vehicle is accounted and it is also not incorporated with the other variable non-traffic loads. 

The service vehicles will be assigned for this project. These vehicles are for maintenance of the bridge and emergencies in case of accident or fire. 

Conclusion

The maximum allowable deflection in a steel footbridge of span 30 m as that we designed should be 0.038m while in our case the maximum deflection is 0.16m hence it exceeds the required standards. The thickness of the longitudinal beams and cross beams used should therefore be increased to improve its load bearing capacity so that the footbridge does not fail. Further study should also be conducted to find out ways of mitigating steel corrosion to increase the durability of the bridge. 

Reference List

Almasri, A & Halahla, A 2019, ‘Effect of Tension Stiffening on the Deflection of a Tapered Reinforced Concrete Cantilever Under a Concentrated Load’, International Review of Civil Engineering (IRECE), vol. 10, no. 2, p. 56.

Do 2014, ‘Analysis of Natural Frequency According to Span of Foot-bridges’, Journal of Korean Society of Steel Construction, vol. 26, no. 5, p. 375.

Fatemi, SJ, Mohamed Ali, MS & Sheikh, AH 2016, ‘Load distribution for composite steel–concrete horizontally curved box girder bridge’, Journal of Constructional Steel Research, vol. 116, pp. 19–28.

Marecik, K & Pa?tak, M 2018, ‘A comparative analysis of selected models of pedestrian-generated dynamic loads on footbridges – vertical loads’, in J Pas?awski (ed.), MATEC Web of Conferences, vol. 222, p. 01009.

Russell, J, Wei, X, Živanovi?, S & Kruger, C 2017, ‘Dynamic Response of an FRP Footbridge Due to Pedestrians and Train Buffeting’, Procedia Engineering, vol. 199, pp. 3059–3064.

Shen, JP, Li, C, Fan, XL & Jung, CM 2017, ‘Dynamics of silicon nanobeams with axial motion subjected to transverse and longitudinal loads considering nonlocal and surface effects’, Smart Structures and Systems, vol. 19, no. 1, pp. 105–113.