"Christian Menn: Bridges"

I recently reviewed Philip Jodidio's Calatrava: The Complete Works, a shallow but brain-poundingly flashy review of the Spanish engineer / architect's grandiloquent oeuvre. Now here's a similarly weighty coffee-table recap of Swiss engineer Christian Menn's career. How do they compare?

Christian Menn: Bridges (Scheidegger and Spiess, 2015, 350pp) [amazon.co.uk] is in matching German and English throughout, so a good chance to find out more about the man who is generally recognised as one of Switzerland's finest bridge designers. The previous monograph, Christian Menn - Brückenbauer (2007) was in German only.

The book covers pretty much all of Menn's significant designs, interspersed with various essays, an interview, and tail-ended with a short biography, list of works and the like.

Menn explains his own philosophy of bridge design, which is also a philosophy of bridge procurement, with some thoughts on design competitions and design standards. For Menn, bridge design must be led by engineers, as structures must satisfy all applicable standards and be economical as well as beautiful. His view is that design is the art of achieving an appropriate balance, with economy and aesthetics apparently incompatible, but to some extent reconciled in the choice if appropriate structural form.

I've discussed Menn's philosophy in detail on a previous occasion, so won't repeat that here.

Menn's signature bridge design was the concrete arch, especially in its deck-stiffened guise as pioneered by Robert Maillart. The first project in this book is the magnificent 72.5m span Crestawald Bridge, and there are many more similar examples. They have been beautifully photographed by Ralph Feiner (all the photos I've used here are by Feiner), and generous space is given over to these images, as well as technical diagrams for each bridge in plan, elevation and cross-section.

With the exception of the Crestawald arch, most of Menn's bridges of this type use a polygonal arch, the engineer's rational alternative to a pure curve. The bridges therefore have a Germanic austerity to them which generally sits well within the mountainous Swiss landscape. I particularly admire the way in which they rely on the strength and purity of their structural form: the concrete finishes are sometimes poor and the detailing negligible, but that's simply not what they are about.

Menn has also been responsible for a number of admirable post-tensioned concrete box girder designs, again most notable for their simplicity and clarity of intent - no fussy cross-sections or pier sculpting, they rely on their visual directness for effect.

After a while, it's hard not to feel bludgeoned by repetition - most of Menn's career has been distinguished by monotony, and a number of basically very similar bridges are presented in perhaps more depth than is merited.

I also occasionally find myself doubting his aesthetic judgements. He is highly self-critical, an admirable trait, but is he always right in his view of his own work? He complains that the twin-legged piers of the Felsenau Bridge would have been better as single legs, but I find this hard to credit, as this is a very high quality bridge as built.

In the later years of his career, Menn's work was sometimes uneven. I greatly admire the elegant cable-stayed Sunniberg Bridge but find the Ganter Bridge to be over-rated in the extreme, with its stiff rectangular piers and their clumsy connection to concrete fins in which its support stays are embedded.

The Leonard P. Zakim Bunker Hill Memorial Bridge in Boston, USA, is for the most part a fine design, but it is cursed with a gargantuan 56m wide deck and I can't help wondering whether it would have been better as two smaller, parallel bridges.

Similarly, some of Menn's unbuilt designs in the middle-East show significant departures from the clarity of his earlier work. The pylon for the cable-stayed Al Showah Island Bridge consists of a spindle braced by concrete fins, and seems over-wrought, as does an arch design for the same scheme, where the rise of the arch is much greater than structurally necessary.

Setting all that aside, this is an excellent and well-produced book, and overall Menn's oeuvre certainly deserves this degree of attention. The details of his individual designs are not necessarily something to emulate, but this is largely because few readers will be designing works for the Swiss landscape, and most will be working in forms other than solely reinforced and prestressed concrete. What stays with me the most is his disciplined approach in finding the most appropriate structural form, and in then keeping it simple and unembellished. From experience, that's nowhere near as easy as it sounds, and something that few have achieved as successfully and consistently as Christian Menn.

Bridge criticism 10: Of Mice and Menn

In recent posts, I've discussed the views of Santiago Calatrava and Fritz Leonhardt on what makes a good bridge design. This time, it's the turn of the great Swiss designer Christian Menn, who was responsible for the excellent Sunniberg Bridge, amongst others.

Some engineers are highly prescriptive in their views on bridge aesthetics, and Menn is undoubtedly one. In his paper "Functional Shaping of Piers and Pylons" (in Structural Engineering International, 1998), Menn wrote:

"A truly well designed bridge balances economy and aesthetics while responding to the functional requirements and technical and environmental boundary conditions."

Menn considers the "functional requirements" to comprise the traffic, alignment and state-of-the art construction technologies. The "technical and environmental boundary conditions" include topography, geology, clearances, available programme, emission limits, impacts on adjacent buildings etc.

Menn continues:

"On the basis of the above considerations, the real art of bridge design is to elaborate a suitable technically appropriate structural system that aims at achieving an optimal balance of economy and appearance …This pragmatic, simple and purely functional approach not only leads to technically proper structures but also to aesthetically convincing ones."

According to Menn, any significant increase in cost above the "least expensive functional solution" (a 5% premium for larger bridges, or 20% for medium bridges) is unacceptable and "should be abandoned". (Image, right, of Menn's Boston bridge courtest of Ken Douglas at flickr, showing non-functional pylon caps).

Menn’s opinion is common amongst bridge engineers, with one typical example being the Billington & Woodruff paper discussed in a previous post. This moralistic position is also shared by Leonhardt.

Menn’s is the language of moral puritanism – bridges must be "proper"; ornamentation is improper; cost must be minimised; the most appropriate structural system will inevitably produce the best bridge. This back-to-basics approach continues to offer much of value in an age where architect-led bridge design has produced schemes which are unaffordable or unmaintainable, but it should not be the only game in town.

It's doubtful that in the modern era the assumption that a "simple and purely functional approach" automatically leads to "aesthetically convincing" structures. Writers such as Billington offer the greatest praise for the structures of designers such as Maillart, Candela, Nervi or Isler, but the conditions of production for such structures have changed irrevocably. These structures, for which the identification of an optimal form allowed forces and materials to be minimised, come from an age where least cost arose from least materials, and hence a technically efficient design would often coincide with one which is slender and elegant. Even in this pre-modern period, however, there were structures which are highly inefficient structurally yet which have become much loved icons, such as the Forth Railway Bridge (pictured above right, courtesy of Simon Bradshaw at flickr).

Modern technology has changed the conditions for least cost. In particular, mass-production, pre-fabrication, and automation mean that in most cases the least-cost solution is one that minimises site labour and maximises the use of off-site fabrication and assembly. In this situation, a parallel-flanged beam may cost less than a beam shaped to fit its bending moments, even though more material is used. Many structures which were efficient to construct have also been found to be expensive to maintain, and a structure with a lower whole life cost may well have higher initial cost e.g. the use of concrete and hence heavier foundations to avoid the cost of repainting structural steelwork.

The further difficulty with Menn’s prescription is the issue of public opinion. While there may be some common ground amongst structural engineers as to what constitutes a good design, it is far from clear whether our idea of good design is shared by the public who benefit from a bridge and who fund its construction. For example, the public may be thought to delight in Calatrava’s white skeletal frameworks, even though the costs of fabricating these complex geometries must frequently result in a project that greatly exceeds Menn’s "least expensive functional solution".

Ironically, of course, Calatrava was Menn's student, and Menn was rich in praise for him initially.

Who are engineers to judge the success of such a bridge if their opinion departs from the end-user? While clearly there are areas where engineers have expertise the public do not (on both capital cost and particularly on the likely maintenance liabilities), we should not deceive ourselves into thinking that a concentration on functionality automatically produces the "best" bridge.

Next: Michel Virlogeux

Swiss Bridges: 5. Sunniberg Bridge

From Salginatobel, we drove south towards Klosters and by far the biggest bridge on the study tour. Christian Menn's Sunniberg Bridge is, to my mind, his finest work, and since its completion in 1998, it has been a major landmark in the Landquart River valley.

The bridge is part of the Klosters bypass scheme, but at CHF 17m, its cost is dwarfed by the CHF 345m cost of the 4km long Gotschna Tunnel, which the highway enters at the south end of the bridge. Nonetheless, the bridge design was (rightly) chosen in place of a cheaper box-girder option because of the environmental sensitivity of the landscape.

Bizarrely, the bridge was opened seven years after completion, in 2005, by Prince Charles, that notorious foe of modern architecture. Prior to opening, the bridge had provided the construction access for the tunnel works. I find it hard to imagine the opening ceremony: "Yes, one is proud to open this monstrous carbuncle if it makes it ten minutes quicker to get to one's skiing holiday."

Approaching the bridge from the south, we drove down towards it from higher in the valley. From here, the bridge looked small, minimal, barely present in the landscape. It was only once we approached it from ground level that its scale became apparent. 526m long, 12m wide, with a longest span of 140m, and pylons up to 77m above ground level, it's a bridge on the heroic scale.

Menn's adoption of the extradosed bridge form allows the deck to be more slender than in a conventional box girder bridge (because it's supported by cables from above), while the overall bridge doesn't compete unduly with the mountain landscape (because the cables are at a much shallower angle than a conventional cable-stayed bridge). The extradosed bridge is really a special case of a post-tensioned bridge, where the cables are lifted out of the deck to allow the overall bridge deck to carry higher bending moments and shear forces at pier positions. Essentially, it's more complex and expensive than a post-tensioned bridge, but less efficient than a normal cable-stayed bridge. But when done well, it can certainly look very impressive indeed.

The bridge was designed without any expansion joints, which means that under temperature variations, the deck "breathes" in plan i.e. sways sideways. The piers are designed with relatively slender bases to be flexible enough to withstand this movement, and along with the need to maintain highway headrooms, this leads to their distinctive and elegant Y-shape. Everything about the piers and pylons is well thought out - their gentle curves, echoing the tall trees nearby; the way the bridge deck nestles between their arms; their continuity (many cable-stayed bridge have very different pylon forms above and below deck); the clever way the massive steel cable anchorages are hidden within them; and their delicate but robust proportioning.

Unfortunately, we spent so much time tramping around admiring the bridge from below, that we had no time to stop at deck level. However, a pretty good idea of the appearance can be found from this advertising poster, which I spotted when arriving at Zurich airport. Good to see the Swiss pride in this monument to engineering heroism.

Later in the day, we'd see one of Menn's concrete arch bridges at Viamala gorge, but it wasn't really flattered by the viewpoint, and certainly didn't compare well to Sunniberg. Sunniberg Bridge is a proper engineer's bridge - all the key elements of the design have a sound technical rationale, but the combination of choices made and the way they are each worked out is absolutely exquisite.



Further information:

• entry at structurae
• entry at wikipedia
• location on Google Maps (satellite photo predates bridge construction!)
• information posters for Klosters bypass (in German)
• project newsletters (see number 17 for the alternative design options; see number 33 for bridge design & construction details)
• Sunniberg Bridge, Klosters, Switzerland (paper in SEI by Menn et al)
• analysis of the Sunniberg bridge (student paper, PDF)
• Matthew Wells's book 30 Bridges
• David Billington's book The Art of Structural Design - A Swiss Legacy