Motorway overbridge design

johnbk

I have noticed two distinct types of over bridge designs. Roadbridge/Sisk use one with pillars in the centre of the road, while Ascon or Bam use bridges with no pillars in the centre but rather pillars to either side. Ason's solution is much neater I think as the road has to curve around the centre pillar that Roadbridge/Sisk use.

Has anyone any opinion on this or insight into why they can be different?

The Word Is Bor

The DMRB has recently changed to allow for a single concrete barrier/central pier giving a two span bridge. Before the central pier had to be protected and thus the barrier bowed out around it. I'm not too sure but I don't think that the combined barrier/pier would hold up to a full impact of a fully loaded artic, although I've yet to see any test data. I know that a normal N2 wouldn't hold up to an impact of an artic.

The other design restricts the width of the mainline for any possible future widening but allowing no structure in the median.

AFAIK it's up to the designer's to specify the type of overbridge and what can fit within the LMA, in the case of PPP/D&B.

johnbk

Interesting. I noticed the combined barrier-centre pillar on the Mitchelstown section of the M7 and the road still curves slightly around it :-(. It doesnt make a whole lot of differance but doesnt look great compared to the pillars either side. I think that the bridge with one pillar in the middle must be cheaper to build. I would imagine that the beams (I think they are all made by Banagher) are the most expensive single component of the bridge and that they must be cheaper on the bridge with centre pillars.

Tech3

johnbk wrote: »

Interesting. I noticed the combined barrier-centre pillar on the Mitchelstown section of the M7 and the road still curves slightly around it :-(.

Since when did the M7 go though Mitchelstown?

jack_reacher

I think its all down to cost & bridges using the central pillar are cheaper to build. Sisk did the overbridges on the Fermoy Toll road & used the design with pillars either side of the road. AFAIK since then they're using the central pillar.

Delta Kilo

Thats funny I was driving down from Dublin on the M6 today and I just noticed this very thing, Ascon use the columns either side and a single span in the middle, whereas Sisk use the column in the centre. I cant see how the latter is any cheaper. Surely the extra cost incurred by longer beams would be cancelled out by having less columns??

The N6 overbridge on the soon to be opened section from Athlone to Ballinasloe has one octagonal column in the centre of what is a pretty wide bridge. It seems to defy gravity. I cant see how it is strong enough to support the weight of a few HGVs on it!

D'Peoples Voice

tech2 wrote: »

Since when did the M7 go though Mitchelstown?

thinking the same thing, it must be some central pillar causing the road to swerve that far south:rolleyes:

mysterious

Central pier is much neater and sensible.


The Two pier span bridges are not able to facilitate widening unless been knocked.


This is why I cant understand why they are building them. The Biggest joke is the M7/M8 towards Portloaise none of the bridges are future proofed for widening. The M7/M8 multiplex is the most like section of route which may need widening 20 years down the road.

But ah well.

We live in Ireland.

Road-Hog

mysterious wrote: »

Central pier is much neater and sensible.


The Two pier span bridges are not able to facilitate widening unless been knocked.


This is why I cant understand why they are building them. The Biggest joke is the M7/M8 towards Portloaise none of the bridges are future proofed for widening. The M7/M8 multiplex is the most like section of route which may need widening 20 years down the road.

But ah well.

We live in Ireland.

Can you explain further why a 'two pier span bridge' would have to be demolished to facilitate widening?

Also I would think that the majority of single span overbridges are carrying very little traffic and will never need upgrading within their design life of 120 years (I think could be wrong)

From consultation with an engineer recently, the number of pillars/columns supporting the bridges is really down to the length of the span, the longer the span the deeper the supporting beam/thicker the deck has to be. The deeper the beam/deck the more expensive to construct etc, adding in an additional central support reduces the depths. Theoretically it would be possible to build every bridge as a single span type however the thickness of the deck increases dramatically as the span length increases and therefore economics dictates and means additional cols/piers are needed.

Also I'm sure it is possible to add lanes to a single span/non centrally supported bridge without demolition of the orignal structure, must run it by my structral engineer mate.

Carawaystick

Road-Hog wrote: »

Can you explain further why a 'two pier span bridge' would have to be demolished to facilitate widening?

If you have 2 columns just by the hard shoulders you can't widen the mainline and leave a hard shoulder/safety margin. If you have a single column in the centre of the Motorway, you can widen the hard shoulders to be an extra lane and reinforce the bank up to the bridge deck.

Road-Hog

Carawaystick wrote: »

If you have 2 columns just by the hard shoulders you can't widen the mainline and leave a hard shoulder/safety margin. If you have a single column in the centre of the Motorway, you can widen the hard shoulders to be an extra lane and reinforce the bank up to the bridge deck.

Maybe its just I don't know enough about bridge construction/widening but I can't fully understand what you are saying about, maybe like the interchange design thread you could do up a quick sketch in ms paint or whatever kind of package that may be suitable?

Amtmann

Road-Hog wrote: »

Maybe its just I don't know enough about bridge construction/widening but I can't fully understand what you are saying about, maybe like the interchange design thread you could do up a quick sketch in ms paint or whatever kind of package that may be suitable?

He's not talking about widening the bridge, but rather the motorway beneath the bridge. If the columns are at the side of the motorway (i.e, along its edge), then how can you add a third lane and keep the hard shoulder with the supports in the way.

Carawaystick

yeah, the bridges bring a minor road over the Motorway. It's the motorway that might need widening.

mysterious

They have the two pier span bridge on the Limerick SRR ph.2...

Arrrrrrrggh.