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24.07.2008 Feature Article

Adomi Bridge – Possible Causes and Potential Repairs

Adomi Bridge – Possible Causes and Potential Repairs
24.07.2008 LISTEN

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.

adomebridgefigure2


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

From Figure 2 and TV footage, the crack is located at the mid-span of one of the transverse beams. Recent press reports suggest that there are two additional beams with this sort of fracture. I do not have any information on the location of the crack in these additional beams. My discussions are thus limited to this one beam shown in Figure 2.

There are speculations that the cause of the crack may be due to overloading as the axle weights of vehicles using the bridge now exceed what was envisaged during design. Others also claim that this crack may be due to the long queue of vehicles on the bridge as they wait to pay tolls. The former reason may be a direct cause whilst the latter may indirectly be affecting the fatigue tolerance of the beams (I will explain later).

If the loads were static (i.e. still), as applied to buildings, overloading would only cause the steel to yield rather than crack.

adomebridgefigure1


Figure 2: Crack through bottom flange and web of one of the transverse beams (Courtesy of Graphic Online)

Beams are normally designed to be stronger in shear than in bending to ensure that they fail in a safe ductile manner when design loads are exceeded. All that this means is that the bottom flange should undergo plastic deformation (i.e. show some elongation accompanied by thinning) with excessive vertical deflection but no cracking.

The fact that the beam is showing cracking instead of yielding suggests that the steel has become brittle. There are two main causes of steel embrittlement: brittle fracture (which occurs under lower temperature conditions) and fatigue fracture. The cause of this crack is most likely fatigue fracture since low temperature conditions (below freezing) are not prevalent. To understand the term “fatigue” one can visualise the situation where the material becomes “tired” as the beam experiences pulsating loads, and hence fails at a stress level below its actual strength.

Three basic factors are necessary for fatigue cracks to initiate:
• The loading pattern should have minimum and maximum peak values and the difference between them must be sufficiently large;
• The peak stress levels (especially the tensile stress) must be of sufficiently high value for if they are too low, no crack initiation will occur; and
• The material must experience a sufficiently large number of cycles of the applied stress.

This means that when a structural member is subjected to loads that vary between a maximum and minimum value of large enough difference and if the loads are applied over a large number of cycles, the member can fail below its strength.

There is a trade-off between either peak stress or stress range and number of cycles of the applied loads. As a bridge ages, the number of cycles of applied stress accumulates. Thus any increase in the value of the peak stress (due to increase in applied loading) and/or the difference between maximum and minimum peak values could shorten its fatigue life.

In addition to these three basic factors above, there are variables that can affect the tendency for fatigue fracture to occur such as stress concentration, metallurgical structure and residual stresses. I will not go into the details of these except to say that it is interesting to note that the crack appears to have started at the ends of the cover plate (see Figure 2). Three things are happening, which helps to explain why the crack formed at this location:

• There is a change in stiffness of the beam as the plate terminates producing stress concentration at this location;
• The welding at the ends of the plate has to some extent changed the metallurgical structure of the steel at this location; and
• The weld between the bottom flange and the web of the fabricated plate girder contains some residual stresses, which is adding to the applied stress from loading of the bridge.

Repairs

So given the nature of the cracks and the possible causes, what are the options available for repairs? I think the Engineers are pursuing the right track in terms of what they are currently doing. Because this is a bridge in service, it cannot be out for too long. However, if the problem is to be permanently solved, some period of complete closure should be envisaged to enable long-term repairs.

Long-term repairs would require information to be gathered about the bridge in service. There would be the need to carry out structural analysis of the bridge to identify the driving force for the fatigue cracks. This would require instrumentation of the affected beams to identify the actual peak maximum and minimum stresses and the loading cycle due to everyday traffic. The stresses definitely would need to be reduced to increase the fatigue life of the structural members. This may require the introduction of additional beams on either side of the transverse beams to relieve some of the loads. The effectiveness of various options can be investigated with the help of finite element analysis packages.

In the meantime, a quick visual inspection of all the transverse beams should be undertaken to enable short-term repairs to be implemented. I am sure this has already been carried out. This will not be sufficient for long-term repairs, though, as some of the cracks would be microscopic and would require appropriate inspection methods to identify.

In the short-term, however, visual inspections should be used to identify all transverse beams with cracks. Holes should then be formed at the tips of the cracks to stop further propagation. Research evidence suggests that a 200mm diameter hole is sufficient for this purpose. It is visible from Figure 2 that some of the rivets connecting the ends of the cross braces under the bridge are missing. I have no idea whether these were removed recently or have dropped out as a result of corrosion. This could worsen the situation if there is any resulting movement which could cause out-of-plane distortion of the transverse beams. Any missing or loose rivets should be replaced with HSFG bolts as part of the short-term repairs.

The beam shown in Figure 2 and beams with similar cracks have almost lost the capacity to resist bending and the shear resistance of the section is considerably reduced. The repair should be viewed as splicing the two halves together to restore both bending and shear capacities. Ideally, it would be better if the connection can be formed with High Strength Friction Grip (HSFG) bolts rather than welding, as bolting offers a better fatigue detail than welding.

Before attaching the web and flange splice plates though, the affected beams should be jacked up to their original positions. It is important that the spliced connection in each case is designed to resist the full shear and bending capacities of the section at mid-span, to restore the continuity that was there before.

I hope I have provided some insight into what is happening and that my suggestions would help in the search for a more permanent solution to save an important national icon. I once again repeat my call for all Ghanaians with relevant knowledge and experience to contribute to this effort.

Dr. Frank Ohemeng, Senior Engineer, British Nuclear Fuels Plc, Manchester, UK

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