Archie_M_notes

Lane width and bridge width

Let's get one thing straight first. Archie-M works out the distribution for each load using the appropriate distribution rule. The lane width you should put in (Bridge Width from version 2.4) should be the maximum width available for a load to distribute to. This might be bounded by the edge of the bridge, an adjacent lane or a crack in the arch ring.

The sketch below might help.

New version screenshot

This is here mainly as a trial for the new OBVIS web. Seems useful, though, to let you see where Archie is heading.

 

Some History

2010 is a significant year for ARCHIE. in 1984, Arnold Hendry and his Edinburgh team tested the arch at Bridgemill for TRL. On site, Bill was able to run continuous checks on analysis using Jacques Heyman's model and an HP41CV pocket calculator. No graphics, but the essence of ARCHIE was there. The program output was a series of 12 offsets of the thrust line from the intrados exactly like the thrust in, thrust out columns in the Archie-M output.

In 1985 (I can't remember when but probably about November) there was a meeting at New St Andrew's House in Edinburgh to show the results. Two weeks before that. Bill took delivery of an NCR Decision Mate V (approximately) PC compatible computer. It had a colour screen (7 colours and black) and came with GW Basic which had graphics functions to draw on the screen.

In the two weeks available, Bill managed to produce CIRAR, a program to analyse circular segmental arches using the full zone of thrust model that became ARCHIE.

SO ARCHIE IS 25 YEARS OLD THIS YEAR

We will have to think of a way to celebrate.

Of course. Archie-M 2.4 is a long way from CIRAR, but it also reflects just under thirty years of active research and consultancy on arch bridges. Bill's first arch assessment was of Dairsie Bridge in Fife and it was done using the HP41CV and some plotter routines programmed into a Commodor Pet.

More on thrust issues

I have been looking further the complexity of thrusts in high rise bridges. If the thrust runs out of the arch in such a direction that it gets to be a modest distance above the segment concerned, and if that segment receives a large proportion of horizontal soil pressure, there will be problems with the thrust.

Clic on the images to magnify.

 

If this arch is given backing to the level where the vertical through the springing cuts the extrados, the thrust looks much more normal.

Here there is a very slight down-turn instead of a severe upturn.

this is all part of the issue of the thrust making no sense once it is outside the domain of the forces concerned. The forces are actually all associated with the arch segment and not with the soil segment (the arch feels soil pressure but does not know where it comes from). Once the thrust escapes the arch, we are dealing with a large block of material in which the divisins have a large effect on the flow of force. It only makles sense to look at the forces as a whole. Indeed, the interesting question is where does the force leave the arch/fill block. That raises questions of what is force and what is reaction. Where there is backing, as above, we can legitimately remove the horizontal soil pressure from the arch and instead apply it to the backing. We should then treat the whole mass from the point where the thrust leaves the arch as an single entity and work out whether the resultant force is guided into the abutment.

There must be more to come from this.

Bill

Thrust line issues

I have been exercised for some time about the issue of limitations on the powerful technique of thrust lines (or, in our case, zones of thrust) as tools for visualising force flow. One major issue is the question of what bit of force gets included at which stage of the thrust line. If the structure is relatively narrow (like an arch rib) and the thrust is contained within it, there is no particular problem. In a large blocky structure or where the thrust wanders into the backing on an arch, things get more interesting as this picture (and some explanation) shows.

First a picture of the traditional thrust line for Bargower with an HB load. This has been editted down to the bare essentials.

Now look what happens if we work out where the thrust leaves the fill part associated with the segment.

The upturned end looks very strange but is actually the same thrust shown above.

The picture below shows a bit more detail of what is happening at the end there.

The grey portion of the arch shows the last segment and the fill it is assumed to support. The yellow bit of thrust line is the related section of thrust calculated on the assumption that the action of the forces can be assumed to take place between the boundaries of the arch segment projected outwards. It seems that this is unsatisfactory because the effect of the forces on the thrust line is entirely separated from the area within which the forces are generated.

An alternative approach would be to work out where the thrust leaves the domain of the loads concerned. ie where it leaves the grey area. This occurs at the point where the last section of thrust projected backwards cuts the vertical division in the fill. That is an easy calculation but produces an unexpected result which can be seen in the red piece of thrust zone. The thrust moves rapidly upwards as it approaches the end of the arch.

This effect is created by the hockey stick shape of the grey zone. the blob at the bottom creates an anticlockwise moment that throws the thrust upwards. The calculation is correct but the division of the structure turns out to be unreasonable.

Actually, the point at which the thrust leaves the structure is sensible. It is the route it takes to get there that doesn't make sense.


We are working on this. Those few of you who have version 2.4 will note these strange upturns on arches which approach the vertical at the end. In previous versions the thrust has been allowed to continue out of the arch but this also has draw backs for the overall image produced

A new decade. Where are we?

I seem to have been very quiet over recent months. That is not because I have been ignoring Archie but rather because we have been tied up with endless niggles.

Most of you will know that we have been trying to get version 2.3 out for nearly 2 years. The biggest hangup has been trying to get it stable under Vista. To do that we had to buy a new version of microsoft C++ which was so different from the previous version it would not initially load the program. Eventually we got all that sorted out and by early November we were ready to wrap it in the security blanket and send it out.

Then it transpired that Aladdin had abandoned the old HASP4s that we were using and wanted us to buy new hasps for all our customers. that provided enough of a drive for us to change our protection system to a software only one.

Of course, that didn't go entirely smoothly, but just before Christmas we sent out one new version to a customer and the installation worked as planned.

Of course, throughout all this we have been making updates to the program. Some of the changes are quite small but should make a biggish difference to operation. For one thing we have changed the load files to include a vehicle width as well as a track width. This means that Archie can check that the vehicle can fit on the prescribed "Lane width" which we have now renamed Bridge Width. There should be no more cases of people applying a 3.5m wide HB load to a 2.5m wide default lane.

We have also changed the way the thrust position is calculated so that the thrust never extends beyond the sphere of the loads it is supporting. This will prevent the apparent error of thrust crossing and coming out of the next arch in a viaduct.

There are a number of other changes in hand and we hope also tohave a brand new web site up and running in the very near future.

Happy new year to you all.

Oh, and by the way, I think that 2010 marks 25 years since I first made a little program called CIRAR (in those days a file name was limited to 8 characters and it only dealt with CIRcular ARches). It was written in GW basic and was very slow but today's users of Archie-M would still recognise the thrust line output even if it is in coloured lines against a black screen.

Anyway, 25 years is a long time. Even the 10 years of Archie-M has seen some changes in my life. When we began writing it in 1999 we were told it was a wasted investment as "the arch work is al but finished".

This past couple of weeks I have been working on the design of a Flexi-Arch bridge for Macrete. So not just not finished but actually a broadening application.

Looking forward to another decade working with the friends I have made in the business.

Bill