Feature Article : "The views expressed here are those of the authors and do not necessarily represent or reflect the views of Modernghana.com."

In the last two weeks or so, much has been made of the apparent imminent collapse of the Adomi Bridge at Atimpoku. I find the press reports very alarming and the explanations by quoted experts very disturbing indeed. The word 'collapse' from a structural engineering viewpoint envisages a catastrophic and violent occurrence, the consequences of which are irrecoverable.

I have watched pictures of the bridge and the location of the cracks like everybody else on the Television. I have attempted in this piece to present to the general public some information that may help them to understand what might have happened and the consequences therefrom.

I hope that by writing this piece, no one would feel that I am undermining them. Adomi Bridge is not only a vital link between the Volta/Eastern regions and the Greater/Ga-Adangbe regions; it is also part of our national heritage that must be preserved. I am therefore calling on other Ghanaians with the relevant experience (structural and bridge engineers, in particular) to contribute and pool knowledge and experience to save and preserve this national treasure.

Without the benefit of close inspections and with the guidance of TV footage (GTV 7 O'clock News, 13th July) and a photograph on Graphic Online (reproduced in Figure 2 below), I have tried to guess what the causes of the cracks in the transverse beams might be. I have then proceeded to discuss short-term repair measures. Of course, no repairs would be durable without effective maintenance so I have also discussed what could be the focus of any future inspection and maintenance regime.
Construction

The Adomi Bridge is an arch suspension type whereby the roadway is suspended off two giant arches via cables. According to a 1958 article in the Structural Engineer, the bridge has a span of 805 feet and the rise to the crown of the arches is 219 feet.

The bridge deck is probably of concrete with a wearing surface of tarmac. This concrete slab spans onto longitudinal stringer beams. The stringer beams also span onto transverse beams, which are the focus of the current problem. The transverse beams are simply supported between the cables, which are then hung off the latticed arches.


Figure 1: Adomi Bridge (Ghana) (Copyright acknowledged)

The transverse beams are probably fabricated plate girders, which normally are made of thin plates whose proportions make it susceptible to buckling. Hence the beam webs are stiffened at the locations where the longitudinal beams are attached. The transverse beams are tapered with the maximum depth at mid-span (where bending stresses are highest) and reduces towards the ends where the bending moment is theoretically zero.

The transverse beams are braced in plan (below the bridge deck) by X-bracing to maintain the 'squareness' of the steel grillage formed by the transverse and longitudinal beams. This also makes the deck into an effective girder to transfer horizontal forces due to wind and wheel tractive forces to the abutments.

It can be seen from Figure 2 that welded to the bottom of the flanges are some narrow cover plates. There could be two possible reasons for this. Either:
1. The plates are welded on to increase the bending capacity of the beam at the location where bending moment is highest and probably greater than the capacity of the I-section alone; or
2. It was considered that the rivet holes made for connecting the X-bracing had reduced the bending capacity of the beam at those locations and that the plates were needed to compensate for this reduction.

Finally all the structural members are connected by rivets, which is an old method of forming connections that preceded the wide use of bolting.
Gravity Load paths

To understand the relative importance of the structural elements making up the bridge, I will describe how the loads from the roadway gets transferred to the arches and finally to the ground.

Gravity loads generated on the roadway (which includes the weight of the roadway itself and that of vehicles) are spread out by the bridge decking onto longitudinal beams placed at relatively close centres over the width of the bridge. These longitudinal beams in turn discharge their loads onto the transverse beams.

As mentioned above, the ends of the transverse beams are supported by the cables (ropes) hanging off the two arches. Thus the loads on the roadway that are ultimately carried by the transverse beams are transferred to the arches, via the ropes acting in tension (i.e. stretched). The arches take all the loads and transmit them in compression to the arch foundations, which must be able to mobilise the soil to resist the thrust (i.e. kick-out) from the arches.

The latticed arches therefore form the primary load-bearing structural system and support the entire weight of the bridge. Each transverse beam only supports the load from the bridge deck acting over its tributary area in addition to maximum wheel loads from vehicles as they pass directly over.

From the above description, failure of a single transverse beam would not put the bridge in imminent danger of collapse. However, failure of several beams will lead to neighbouring beams becoming overloaded, which in turn could endanger the stability of the roadway. In other words, failure of a single transverse beam will lead to a local collapse (depression in the roadway) limited to the immediate area of the subject beam. This is what is currently evident, which has caused the bridge to lose its functionality.

The extent of any damage, and the risk to global stability of the bridge, however, increases as you go through the loadpaths in reverse. Thus the structural elements can be ranked in importance (from the most important downwards) as follows:

1. The arches and their foundations;
2. The cables (ropes) and all their attachments to both the transverse beams and the arches;
3. The transverse beams; and lastly,
4. The longitudinal stringer beams.

This hierarchy should also inform any inspection and maintenance regime that is set up. From Figure 1, part of the arch foundations is exposed to the elements and appeared weathered. This is not good from a durability point of view as water can collect in any crevices and cracks creating the conditions for the parts of the arches at the interface with the foundation blocks to corrode. Any loss of section in the arch members due to corrosion could lead to the arches losing their springing putting the bridge in danger. Consideration should therefore be given to protecting the foundation blocks by cladding them with some impermeable material shaped to drain water away from the steel interface.

Location and possible causes of the crack Continued    Source: Frank Ohemeng, Dr.

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