28 February 2012

Scottish Bridges: 20. Cambus O'May Footbridge

This will be the third and final of these Deeside suspension footbridges that I'll cover for now: there are more, but not in my old photo albums.

Like the nearby Polhollick Bridge, the bridge at Cambus O'May was built by James Abernethy & Co., a little later, in 1905. Both bridges were gifted to the public by a Mr Alexander Gordon. As at Polhollick, Louis Harper may have been involved in the design, as cited at the Harper Bridges website.

Two later Abernethy bridges in the same style were built at Charlestown of Aberlour, undated, and Invercauld, the latter in 1924.

The Cambus O'May span is a much more elegant structure than its cousin at Polhollick. It lacks the peculiar "ladder rungs" between the main cables, and the towers have a more pleasing appearance, largely down to the detailing of the crossframes and finials.

Although the bridge is Listed, it was largely reconstructed in 1988, replacing the riveted deck trusses with welded construction. A steeply stepped staircase on the north approach was removed at the same time (photos at RCAHMS and Bridgemeister show the former arrangement).

Further information:

22 February 2012

Scottish Bridges: 19. Polhollick Suspension Bridge

Head east from the Crathie Suspension Bridge, along the River Dee towards Aberdeen, and you will come to the Polhollick Footbridge, near Ballater.

At first sight, this is a much less charming bridge than the one at Crathie. It was built later, in 1892, and is usually credited to James Abernethy and Co, although the Harper Bridges website suggests it may have been designed by Louis Harper, who worked for Abernethy but also built bridges under his own name.

It's neither pretty nor was it particularly well cared for, at the time I took these photos a few years ago. The tower legs and parapets are very basic, and the sawtooth profile at the top of the towers looks awkward. The oddest feature is the set of "ladder rungs" between the two main cables. I'm not entirely sure what benefit they provide.

As of 2012, I read that the bridge is restricted to 4 pedestrians at a time, but that money has now been allocated towards its refurbishment.

Further information:

20 February 2012

Scottish Bridges: 18. Crathie Suspension Bridge

As with the Craigmin Bridge, I don't have any really good photos of the suspension footbridge at Crathie.

This bridge spans the River Dee in Aberdeenshire, just outside the grounds of Balmoral Castle. It was originally built in 1834 to carry carriages, to a design by John Justice Jr, although today it is open only to pedestrians. A wrought iron girder road bridge to Balmoral was built nearby in 1858 to a design by I.K. Brunel.

Justice was a highly innovative engineer, experimenting with variations on rod-stayed and suspension bridge form. His bridge at Haughs of Drimmie, Perthshire, combined a rod-stayed system with an under-deck suspension truss, while the earlier (and charmingly primitive) 1824 span at Kirkton of Glenisla, Angus, combined rod stays with shallow suspension bars incorporated into the parapets. Both are probably unique in their form, and it's amazing that they have survived to the present day.

As originally built, the Crathie bridge was very similar to the Haughs of Drimmie design, consisting of rod-stays supplemented by lightweight under-deck trusses. It seems I didn't take any photos which actually show the bridge in elevation, so you'll have to visit one of the links at the end of the post to understand what happens below the deck.

Although originally built as a "cable-stayed" design, the bridge was "renewed" in 1885 by Blaikie Bros., which is presumably when the iron flat link suspension chains were added.

Crathie Suspension Bridge is featured in several books, but astonishingly, Justice's other bridges are less well recognised. The Haughs of Drimmie bridge isn't even in Civil Engineering Heritage: Scotland, and none of these bridges are recognised in Walther's or Troitsky's books on cable-stayed bridges. It makes you wonder how many other remarkable bridges have escaped the attention of the historians.

With the other Justice Jr bridges, this is one of the earliest "cable-stayed" bridges built in the UK which still survives, although not the earliest unaltered example (that will be a story for another day).

Further information:

16 February 2012

Scottish Bridges: 17. Craigmin Bridge

I've been having a look through some of my older photos and thought I'd post a few on here, bridges I've not visited recently but which are interesting enough to feature. The first two or three are all from Scotland.

Craigmin Bridge is one of Scotland's great hidden secrets. I always think of it as Scotland's Pont du Gard, although it's clearly not as spectacular. I discovered it thanks to Gillian Nelson's brilliant book Highland Bridges, and without prior knowledge, it's not something you'd stumble upon, located as it is on a private estate at the end of a farm track near Buckie in north-east Scotland.

My photos only really hint at what is there: to properly understand it, you could try the photo linked from the Google Map below, or Nelson's book has an excellent picture.

The bridge is in multiple tiers. The lowest is a single segmental arch spanning the Burn of Buckie. Above this sit two semi-circular arches, each resting on the centre of the lower arch. The central pier between these arches is pierced at a higher level by a further arch, and above that, still another tiny arch forms a niche within the parapet.

The centre pier on the middle level is hollow, containing a small room which can be reached by doorways in the mid-level arch barrels. The purpose of the room is unknown, as this is not a bridge which has any form of recorded history.

Nor is it clear whether the lowest arch was built first, with the others added later, or whether they were all built together, perhaps at the same time as the nearby Robert Adam-designed Letterfourie House (1773).

It's an excellent bridge, and well worth a visit.

Further information:

13 February 2012

Bridge design competitions in the USA

I've been contacted by a civil engineering student at MIT who is writing their Masters thesis on the subject of bridge design competitions in the US - why are there so few? Should there be more? Can they learn lessons from elsewhere?

Rather than simply offer my own (largely ignorant) views, I've agreed it may be better to open the topic up to you, the readers of this blog. You can respond either through the blog comments, or if you prefer, by email directly to cee.student.2012@gmail.com.

Some of the questions raised include:

  • What are some known and/or interesting U.S. bridge design competitions (ideally ones that have been built or are being built) - are there any examples?
  • Is information on the design and the competition process available?
  • An understanding/synopsis of the design competition process (both in the U.S. and abroad)
  • How do the competitions work? Time frame, participants, judges/jury, budget, financing, awards, etc. Are these competitions widely publicized?
  • A feeling for the the design competition culture (both in the U.S. and abroad) as well as opinions of the process - are competitions helpful for the field? Do they incite conversations and innovation?
I am sure any responses would be very greatly appreciated.

For my part, one thing I note about US bridge design culture relative to that in Europe is the much greater obsession with community participation, often to the detriment of what many engineers would consider good design (e.g. the adding of irrelevant ornamentation, even to the extent of adding fake arches, so as pacify the voters). I've never understood this inability of designers and bridge promoters to make their own decisions, but it does seem to be at odds with the culture of design contests, which is generally heavily authorial - in search of a designer with a strong vision.

12 February 2012

Bridges news roundup

Unique rolling bridge gets Listed status
Very rare retractable bridge in Cumbria gets heritage protection.

New North Sheen footbridge designs to go on display
I mentioned the winner of this mini-bridge competition before, it would be interesting to see how the design has developed but I can't find a current image on line. Details of one of the unsuccesful competition entries can be found online.

New city centre footbridge lands
The image of a new bridge in Salford looks like it could be quite an interesting design - is it that rarity of rarities, an underspanned suspension bridge?

A bridge built to sway when the earth shakes
It's nice to see the New York Times offer a readable but informative overview of some of the engineering behind the new San Francisco - Oakland Bay Bridge.

Calgary Peace Bridge delayed, again
A dog-bites-man sort of story. It's nice to see that after seemingly dozens of stories about problems with the steelwork, it's that weird sister concrete who is to blame this time.

Historic Bridge and Infrastructure Awards 2012
What the title says.

Mahakam II bridge plans needed as evidence in collapse disaster
The title may exaggerate the problem, which seems to relate to missing performance specifications for the failed Kutai Kartanegara suspension bridge. The police chief is reported as saying: "Those papers have been hidden away for years and it won’t be easy getting access to them". A further report in the same newspaper appears to be pointing the finger squarely at negligence by the maintenance contractor involved.

01 February 2012

Kutai Kertanegara bridge collapse

New Civil Engineering magazine has published a summary of the official investigation report into the 26 November 2011 collapse of the Kutai Kartanegara suspension bridge in Indonesia (image below courtesy of Ezagren at Wikipedia). You may need to be an NCE subscriber to see their report, unfortunately.

The 470m long bridge collapsed while maintenance work was ongoing, killing at least 20 people and injuring many more. Only the bridge towers and main cables were left in place after the collapse (image courtesy of Katakutu on Wikipedia).

The NCE report isn't completely clear, but it indicates that the bridge was originally built with a 3.7m rise in the deck at midspan, which by late 2011 had reduced to 3m. Maintenance work was instructed with the aim of restoring the original profile, apparently by jacking at each vertical hanger. NCE states that the sag was the result of a 200mm horizontal movement of one tower foundation, but this appears incorrect, and movement of one of the main cable anchorage blocks seems the more likely cause (see below).

The report notes that collapse was initiated by shear failure of one of the hanger-to-deck connections, presumably near the tower where the hanger loads will have been increased most severely by any sag. The loads were then exacerbated when the maintenance contractor jacked on one side of the bridge deck only - while traffic continued to run on the other side. Reportedly, no calculations were made as to what effect this jacking sequence would have, although it seems intuitively obvious that it would lead to greatly increased hanger loads.

The bolts connecting the hangers to the deck were also found to be very brittle, almost like cast-iron in quality. This is quite remarkable, as use of ductile material in bridge works is just a commonplace in modern construction - to use something different is almost perverse.

The entire bridge apparently collapsed in about 30 seconds - after the first hanger connection failed, its load was redistributed to adjacent hangers, which also failed, unzipping the entire bridge deck very quickly. Although the initial failure may have been at deck level, the photos clearly imply that further failures occurred at the connection of the hangers to the main cable.

A little more light is shed on matters by an unofficial report from Professor Sohei Matsuno of IBA University, available online as a PDF. This notes that the anchor blocks had moved 100mm horizontally, causing the towers to tilt and the deck to sag. Matsuno blames this on the absence of raking piles from the anchor block foundations, which strikes me as a reasonable complaint.

Matsuno's report notes that the main cable sag will have redistributed forces in the hangers, such that some will have lost most or all of their tension, and become prone to significant movement under live load. Alternate loading in the cables may have damaged the joints. In any event, failure at a joint is normally something which is prevented in design: joints are generally designed to be stronger than the parts that they connect, which does not appear to have been the case on this bridge.

If a hanger was sufficiently overloaded to fail in a well-designed bridge, you would expect the cable to fail (probably right next to the joint), not its connection. You would also expect the bridge deck and adjacent hangers to have been designed to cope with loss of one (and possibly more) hangers. This is normal design practice both to safeguard against disproportionate collapse due to material failure, impact damage, or sabotage, as well as to make hanger replacement more straightforward.

To summarise, the bridge deck failed because the hanger joints at top and bottom were too weak, possibly exacerbated by the use of brittle components; hanger loads were increased by the bridge deck sag and by the irrational jacking sequence; hanger movement may have caused damage; and there was insufficient ductility and redundancy in the bridge to cope with the loss of one or more local components.

Although the reports available don't entirely clarify what went wrong technically (whether it was poor design or poor construction, for example), I think the non-technical reasons for failure may be more interesting.

The UK's Standing Committee on Structural Safety (SCOSS) promotes the view that structural failures all result from the "3 Ps" - people, process and product. An obsession with the technical mode of failure (the product) may obscure wider issues. The questions for the Kutai Kartanegara collapse therefore include: Were the designers, original contractors, and maintenance contractors competent? Did they understand the structural behaviour of the bridge, and how the anchorage movement had altered this behaviour? How was competence ensured - was it a condition of the procurement documents, and how was competence tested, monitored or audited? What was the technical approvals process - was there an independent body scrutinising the original design and construction proposals, or scrutinising the maintenance methodology, as would be the case in the UK? Were the permanent or temporary proposals adequately checked?

The use of brittle components, the odd jacking sequence, the seeming failure to properly consider the altered structural behaviour, these are all symptoms of inadequate people and processes. It is possible that the design was entirely satisfactory, only compromised by deliberate or accidental "errors" in construction, but these still point to issues with supervision or approval, not simply with poor materials. There are few, if any, structural failures which can be attributed to a single cause - factors of safety in design, and safeguards of quality in construction, are normally such that a number of separate errors have to coincide to cause failure, particularly disproportionate collapse as happened in Indonesia.

I imagine some engineers have the "it couldn't happen here" mentality. Does an incident in the developing world really hold lessons for the developed world? I've read several comments from Indonesian sources suggesting their initial reaction is to attribute the blame to corruption or incompetence, at a level which you may not expect to find in countries like the UK. But there are plenty of first-world examples where an unanticipated combination of errors in design and construction lead have led to structural failure.

In my own experience, I can think of cases where temporary works and operations have not had the same level of care devoted to them that the design of permanent works receives. I have also seen cases where poor materials or construction errors have either been deliberately introduced on site or deliberately covered up.

I think a key issue is what the incentives are on a project which might encourage or discourage safe practice. These generally relate to procurement methods, and I have to say that my experience is that safety is not an issue procurement specialists think about, certainly not in the context of incentives, which are generally devoted to programme (liquidated damages for delay) or cost (pain/gain sharing). It strikes me as obvious to anyone other than a procurement specialist that such incentives must be a positive disincentive to enhance safety (or quality), as safety is positively related to taking time to be careful, and to spending money on training, checking and supervision. The increasingly common view seems to be that the only incentive to ensure satisfactory quality and safety is the fear of civil or criminal prosecution.

It's not that long ago that these were understood to not always be effective as incentives - the degree of fear present relates to the perception of risk, and structural collapse is generally seen as such a remote risk that the incentive to prioritise actions against it may be weak. As a result, clients were in the past prepared to pay to put additional safeguards in place: independent checking of permanent and temporary works designs, and independent inspection and auditing of construction quality. These safeguards have been steadily eroded, with increased reliance on a contractor's self-certification of their own compliance with quality control requirements.

The Construction Design and Management Regulations (CDM) were one attempt to make sure that all parties in construction recognised their role in ensuring safety. However, in practice, they seem to have deflected attention away from the parties responsible for most construction accidents (contractors and their employees), added to the burdens on designers (the case that CDM in design has saved many lives seems at best unconvincing), and been seen by clients often as an unwelcome and unnecessary cost. However, it is the clients, through their approach to procurement, who can have the greatest opportunity to incentivise safety. Imagine a world where there were contract penalties for hours lost due to accidents - would this be more or less effective than relying only on the threat of prosecution for serious safety lapses? Would an Indonesian CDM system have prevented the Kutai Kartanegara collapse? I doubt it.

Another missed opportunity to improve the role of people and processes in structural safety came with the introduction of the Eurocodes - not just the structural design standards, but also the accompanying European execution standards ("execution" here means "construction", for anyone not yet versed in Eurospeak). These standards say quite a bit about how requirements for competence, supervision, checking and quality control can all be varied to suit the probability and consequences of failure. In general, they imply the adoption of more onerous procedures for more complex or significant structures. In practice, the design and execution standards are not properly tied together, and implementation of both by national authorities has only muddied the waters. What could have been a great opportunity to focus attention on the role of design and construction management in enhancing safety, and to make that more prominent in the procurement process, is being missed.

The technical specifics of the Kutai Kartanegara failure are unlikely to be repeated in the UK or similar countries. We are, however, yet to learn what really went wrong, particularly in regard to the people and process issues. I suspect that when we do, we should not be in too great a hurry to assume that some of the same problems "could not happen here".