Why design a cable stayed bridge with pylons inclined towards the obstacle being spanned?

Question

Here's a YouTube video with a concept of a bridge which is currently under construction in Saint Petersburg. The bridge largest span is cable stayed and designed to drive traffic over the Neva river fairway.

The largest span rests on two pylons placed symmetrically such that they are inclined towards the fairway axis. Something like this:

This differs much from "usual" design where pylons are built upright. They may have "A" shape but still don't have inclination along the road axis.

The usual strategy of building cable stayed bridges is that you first build the pylons and temporary supports, then assemble the deck on those supports, then mount the cables, then remove the temporary supports. Maybe the pylons become evenly loaded once construction is complete but clearly building those inclined pylons of dozens tons of reinforced concrete poses a challenge - both the pylons and their foundations need to support the extra loads that appear simply because the pylons are inclined. Building an upright pylon looks much easier.

It looks like this design just asks for extra problems and doesn't provide any benefit compared to pylons built upright.

Why design a bridge with pylons inclined towards the river fairway instead of upright pylons?

Answer 1

Why build pylons which are inclined away from the obstacle being spanned?

As well as being aesthetically interesting, this can also be structurally efficient. Personally I love the many bridges by Calatrava using this concept, in particular the Puente del Alamillo. The pylon is actually fully in compression under dead load: the cable stress and the self-weight of the pylon resolve into a force directly down the axis of the pylon.

Why build pylons which are inclined towards the obstacle being spanned?

Unfortunately the main answer is aesthetics. "Art" taking pride of place, increasing costs. Because here we have the self-weight of the pylon and the cable stress acting together, both bending the pylon in the same direction. To counteract this you will typically use a lot of prestress.

Ok, so I can see that it could make engineering sense if, due to your site constraints, you had a short main span and long backspans; because then the backspan cable force (acting against pylon self-weight) could be greater than the main span cable force. But that would be very unusual and isn't the case in the picture you've provided.

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A note on construction of inclined pylons

Your "usual strategy" is, in fact, not common for larger cable-stayed bridges. Far more common is building a section of the pylon and a section of the deck, joining them with a cable, and then repeating. Following this method, the out of balance load of the self-weight on an inclined pylon is much reduced.

Answer 2

As AndyT says - it seems it's aesthetics in most cases.

Given the above as the answer, the following is essentially comment, but seemed worth posting as it gives a lot of detail of what goes into a design and how there are issues that are not apparent to "outsiders" but which can be important. eg in this example the span is very slightly curved resulting in a 66 mm deflection in the roadway (under 3 inches) but it caused extra difficulty. And the effort needed to construct this bridge and the tight tolerances are not at all obvious from inspection.

This seemed worthwhile as while the original question asks about just the angled towers this shows how many other unseen factors can affect the design.

ORMISTON ROAD CABLE STAYED BRIDGE
This paper provides an overview to selected construction challenges encountered during the building of the Ormiston Road cable stayed bridge, an iconic cable stayed composite steel and concrete bridge constructed in the Sir Barry Curtis Park, Manukau City, Auckland.

If I understand the description correctly, one tower is in considerable compression and the other in tension. This is a small road bridge but various constraints make technical difficulties greater than in some much larger bridges.

Some key comments - the whole paper is worth reading.

• The construction of the cable stay bridge was technically very complex due to the asymmetric geometry and very tight tolerances specified. The bridge deck is on a radius of approx 37kms, which sounds very flat but results in variations in levels due to curvature of 66mm along the length of the bridge. The 45.5m pylons are made up of a 28m section of reinforced concrete tapered from 1.8m diameter at the base to 1.3m diameter at the top, with a 5.5m high structural steel box to provide anchorage for the stay cables and topped with a 12m lattice spire made of stainless steel and glass. To further complicate matters both pylons are inclined back longitudinally at 15 degrees and angled together at 5 degrees and were not self supporting.

• There was very little tolerance in ensuring the stay cables were correctly aligned between the pylon and the deck anchorages. The angular rotational tolerance of 0.25 degrees commonly specified for cable stayed bridges required the positional tolerance of the stay anchorages to be within 3mm. With this level of accuracy much of the construction effort and risk mitigation was focussed on survey integrity and conservation of construction tolerances.

• The concrete pylons are angled in two directions providing a dynamic element to the bridge. They are also positioned closer to the western abutment than the eastern meaning the back span is considerably shorter than the fore span. This asymmetry generates considerable uplift on the western abutment which is resisted with deep tension piles.

• Normal Drossbach ducting could not be used as the tendon sheathing after research showed that Drossbach could collapse at about 12m head of concrete. 100NB steel pressure pipe was used as an alternative, which could cope with the high hy

• Tendons were assembled on the ground prior to lifting and placing inside the piles which already had the reinforcing cage installed. It took a synchronised effort of 3 cranes using 6 snatch blocks and an excavator to successfully lift the 45 m long flexible tendons from horizontal to vertical, without kinking the tendon, so that they could be lowered into the pile casing.

• The pile tendons pass through the western abutment and terminate in the deck. This meant that the tendons could not be stressed and grouted until the deck had been poured, some 9 months later. As a temporary measure to prevent corrosion of the strand, **a sodium hydroxide solution was introduced to the pile tendons to create a protective alkali environment. Regular pH testing was used to monitor and maintain alkalinity.

• While the bridge span is short at 70 m, the effective tributary load area for the cables was of similar magnitude to a much larger cable stayed bridge because of the large deck width and resulted in similarly sized cable stays.

Footbridge with "leaning tower" at Brown Owl (hoo?) in New Zealand.

Location on Google maps

Answer 3

I believe there is a sensible engineering reason no one has pointed out yet. In the picture in the original question the central span appears to be slightly longer than twice length of each outer cable-supported span. This implies a greater load from each half of the central span than from each outer cable-supported span. Furthermore, the cables from strictly vertical towers would have to become more shallow to reach the greater distance to the center of the central span which would further increase the tension necessary to support the same partial vertical load.

That would result in unbalanced tension on vertical towers and tend to pull them inward and distort the bridge. Having the towers lean outward--and/or be pulled outward by additional tension into the ground support--may be one way of accommodating the imbalance (such as in the asymmetrical example in the answer by @RussellMcMahon), but it may be that the level of tension required becomes impractical for the load and span distance required and given the supporting structure on the riverbed for the bridge in the question. In any case, it would certainly seem to require more structural support--and thus expense--to have the towers lean outward against even greater tension to support an even further reach to the center of the long central span. (This may be why conventional wisdom was having trouble coming up with a workable and affordable design, if that was true of this case.)

Instead, it appears that by having the towers tilt inward the cables are able to maintain a more balanced profile with less stress being added to the design just to balance it out. The tops of the towers are near each midpoint between the center of the central span and the outer reach of each outer cable-supported span, so the cables under the greatest tension (and having the greatest horizontal component) are the most symmetrical... to then keep the lateral forces on each tower balanced out. It's more like the base of vertical towers were simply slid farther apart while keeping the tops fixed, which means the structure and expense are more like that for a shorter central span using symmetrical cables from vertical towers rather than the accelerating expense for the longer spanned distance with conventional designs.

The exact distance between cable mounts on the span may not be exactly the same for the central span and outer spans, and may vary slightly across each span to slightly vary the load each supports as the mounting point on the tower becomes farther from the center between the pair of partial loads. Each incrementally closer pair of cables can then be placed so as to balance out the lateral tension at the tower and keep the load on the tower directed along its axis of compression strength. The engineering math to work out the optimal placements is beyond me. It's possible that the cable load spacing is uniform after all; it just doesn't necessarily have to be with this approach.