The KNUST walkway roof collapse: what are the causes?

I have read with great shock the news of the collapse of the roof to a walkway at Indece, KNUST that killed a Level 100 student. I extend my deepest condolences to the family of the young lady and to the Royals of Queens Hall where I was President during the 1985/86 academic year. Those there at the time would have known me as ‘ABUSCO’.

I have been troubled by this tragedy not only because I have an association with Queens Hall where this young lady resided, but I also studied Civil Engineering at KNUST with the likes of Kwabena Agyepong (of NPP fame) and Albert Abongo, the immediate former Minister of Works and Housing. There have been numerous criticisms directed at the university authorities but I have particularly felt those directed at civil engineers trained at this university; hence my decision to write this article.



When photographs of the collapse appeared on JOY FM’s website I rang a few of my course mates from our days at KNUST to discuss what the causes might have been. I have decided to share some of the outcome of our deliberations. I apologise in advance if the language is too technical for the layman to understand.



The following observations can be made from the photos on the JOY FM website:



1. There are no reinforcing bars sticking out of the ends of the roof slab, suggesting that the roof was of precast construction with no structural topping tying the units together;



2. The entire roof fell off as one piece with rows of tubular columns bent sideways suggesting part of the collapse mode was sway; and



3. There is a near clean break in a longitudinal beam indicating shear failure.



The sudden nature of the failure, as reported, suggests that the longitudinal beams supporting the slab units sheared through followed immediately by the sway mode failure of the tubular columns.



Possible causes of the collapse



I will try below to discuss the possible causes of the collapse. This is derived from the observations listed above (some of which may not be accurate without much closer inspection) and from my recollection of similar structures in all halls of residence from some 25 years ago.



In my opinion the failure could have been caused by the accumulation of several factors that can be attributed to the following:



1. The design concept - the roof essentially was a short-span shed, and at that, quite a simple structure. It appears to consist of discrete precast slab units spanning onto longitudinal steel beams supported by very slender tubular columns. The decision to use heavy precast concrete slabs (reported to be 9-in thick) for a roof carrying next to nothing by way of imposed loading was flawed but this could have been the norm at the time of construction. From a durability point of view, the use of precast units has enabled the structure to remain in service for more than 50 years, which is nominally taken as the design life for normal building structures. The flaw, however, lies in the decision not to tie the discrete precast units together with a structural topping.



2. Design provisions - A careful study of the photographs shows that the provisions for horizontal stability of the shed were grossly inadequate. Transversely and longitudinally, stability was provided via the fixings connecting the top of the slender columns to the slabs. In effect the discrete slab units were meant to act as the top beams of transverse and longitudinal ‘steel-slab’ portal frames. The problem with this structural arrangement is two-fold:



a. First the slab is far too stiffer than the slender columns and if there were to be any significant movement in either horizontal direction, the elements most likely to fail were either the fixings connecting the top of the columns to the roof slab or the columns themselves; and



b. Secondly there is no redundancy in the system by way of alternative horizontal loadpaths were these critical elements to fail.



Thus at the first sign of distress, there was most likely to be loss of vertical support, which would almost invariably be catastrophic.

3. Lack of maintenance – with the inherent flaws in the design (as described above) any lack of adequate and/or appropriate maintenance would contribute in no small way to making a potentially unsafe structure even more dangerous. It is evident from one of the photographs that at least one of the longitudinal beams supporting the precast units failed in shear (as evident by the clean break). This might have been caused by loss of cross sectional area due to corrosion.



The roof appears to be flat; hence it was essential that rainwater do not pond on the slab in order not to add weight. This required that the roof drains were clear of any blockages. I rather suspect that there might have been growth of moss and algae on the roof and within drainage outlets such that anytime it rained the roof loading was increased only to be relieved solely through evaporation rather than being drained away. Some of the press reports have it that the collapse happened after a heavy down pour and probably this was the proverbial last straw that led to the corroded longitudinal beam failing in shear.

How did all these factors come together to cause the collapse?

The steel elements might have corroded over the years the structure had been in service. This would have led to loss of section in some elements. The topping and the joints between the precast units (which might have been plain in-situ concrete without rebars) might also have deteriorated.

So following the heavy downpour, and with the drains probably blocked, there was standing water on the roof. Consequently one or more of the longitudinal steel beams failed in shear caused by the combined effect of increased loading and reduced shear area. This led to loss of vertical support to a slab panel, which also broke the span of the concrete roof diaphragm. The entire structure then behaved like a heavy slab supported on ‘jelly-like’ legs with little or no horizontal stiffness. Under the twisting action of the falling slab and probably wind blowing at the time, the columns failed in a sway mode in the weaker transverse direction, as shown in the photographs.

The Way Forward

The university authorities have done the right thing by calling for immediate structural inspections (not geotechnical inspections as reported by GNA) of all buildings on the campus. I have no doubt that structures of similar construction would form the major focus of these inspections. I recommend that other institutions across the country, including ministries and hospitals, where similar walkways abound, should also carry out these structural inspections. As a matter of urgency though, walkways of similar construction should be ‘barriered’ off to prevent usage as the inspections proceed to establish the safety or otherwise of using them.

In the short-term, structural modifications should be carried out on similar structures to prevent this from happening again. This would require the introduction of the following stiffening measures into such structures:

? Cross beams should be welded to opposite columns (below the roof) to convert the separate columns into transverse portal frames; and

? Diagonal braces should be welded between some of the columns on each longitudinal side of the walkway to provide shorter and alternative horizontal loadpaths in the longitudinal direction.



In the long-term, though, the roof concrete units should be removed to be replaced with lightweight metal cladding (i.e. aluminium or steel roofing sheets). There is however no substitute for regular inspections and responsive maintenance and this culture ought to be adopted to prevent any future occurrences.



I once again extend my condolences to the family and friends of this young lady. I hope that the causes of the collapse are established quickly and that the findings would be shared with the wider community and country. More importantly, I hope that the relevant lessons would be learnt such that this would never occur again in Ghana, let alone at KNUST.



Dr Frank Yaw Ohemeng

(Manchester, UK)