arches

A note from Gez Pegram, Alan Wood & Partners

I am looking at a 1901 extension to the rear of Mount Grace house.

There is a lintel over a window which appears to be
constructed from tiles and cement. It spans around 2.1m and is only 200mm
deep. 

We have removed a brick next to the timber beam bearing over the centre of
the lintel, expecting to reveal an embedded RSJ - the former owners were
apparently closely connected with the steel industry. With very restricted
access, the lintel appears to be solid.

This would seem very unlikely from an engineering viewpoint! Has anyone
come across a tile lintel like this, and could we expect hidden ironwork
somewhere inside it? It is Grade II* listed and we cannot remove more fabric
without good reason.

The concern is the central longitudinal line of cracking to the soffit, revealed
when finishes were removed for conservation. I thought initially this was to do
with expansive rusting of ironwork within. The crack is wider under the beam,
so it could also be indicative of a fracture due to the applied point load.

Thank you for your help!

Gez Pegram, Alan Wood & Partners

Vaults at the National Museum of Scotland

It is far too long since I ave been here. Whether taking time is a right on a holiday, or whether this is just a displacement activity is something I am not sure about. Anyway:

A year or so ago I got involved with my friend David Narro and his colleague Ben Adam on some alterations to the main floor of the NMS. The floor is supported by a very elegant set of vaults which have been obscured in a store until now. A new entrance is being constructed by piercing the basement wall and removing the supporting walls from the vaults. The original plan was to core needles into the vaults and lift from there but we eventually determined that it was possible and desireable to avoid such damage to the remaining structure. Inclined props were used to lift the vaults near the springings and the walls could then be removed and replaced with a steel frame without damaging the vault. . The pictures are a bit blurry but show the scheme quite well. I will look out my pics of the original structure shortly.

Pyramid Building

Watched a stupid program about Pyramid construction on Saturday. What a frustrating waste of time. About 2 minutes worth of content spun out over an hour by flash graphics, none of which showed the critical details.

Internal ramps could certainly provide additional access to the lower levels. At those levels, the amount of material is huge so multiple ramps would be vital. But the Egyptians were perfectly capable of building vertical walls and a 2m wide ramp would only need a 2.6m high wall which is not a major thing. So the perimeter would allow the installation of 20 vertical sided ramps. Perhaps it would be more sensible to start with 16. They could be continued to about 48m by which level 2/3 of the volume would be complete.

At that point, the base of the walls reach the edge of the ramp below so some ramps have to be taken out. Reducing to 4 will take us to 90m. That leaves only 1/20th of the volume to finish. 2 ramps can continue to 139m leaving only 0.25% of the pyramid to finish.

Actually, the block sizes reduce as the ramps climb so it might be that they were reduced in width as well as number.

Whatever else happens, it would be necessary to have a crane for the very summit and that means some sort of platfrom wider than the pyramid.

Remote deflection measurement

Anyone who knows me knows how much time I spend playing with (I prefer to think of it as developing) ways of measuring things on big structures. I recently had occasion to measure sideways deflections at the top of tall bridge piers. There will be more details later but I have found it possible to measure deflections from up to say 200m using a video camera, a telescope and a target. the video Download moire_target_trials.wmv (349.5K) shows the results of an early trial on a small pier. The first 9 seconds is a plain video the repeat has an overlay to create a fringe pattern to amplify the movement. The video is an iPod grade one so you might need quicktime or an ipod to be able to play it. It is clearly possible to see the circular motion as the passing train pushes the pier to the left, down and round.

The Maidenhead Centre

There are a number of drawings from the Brunel era on display in the ICE library just now. David Greenfield raised a question about this on CEHX. The pictures can't be displayed there so here they are.

Looking at the full picture first, the centre is shown in two halves with a break line jst to the left of the mid span. The right hand section is supported on 3 pies such that 5 would be used for the full span. There is a branching tree support from each pile with folding wedges for adjustment and decentring. This section is drawn in beautiful detail and colour washed.

The left hand section is shown plain but also carries dimensions. I think the dimensions are in a different hand so they raise a number of questions, see below. Also, the left had section has 2 piles for support and is set out so that 4 are needed for the full span. ie there would be four trees rather than three and two bits. Also the base of this frame is much higher.

One question, then, is, is this a drawing showing two alternative centres, or were the two frames used side by side so that 9 plies appear to obstruct flow and traffic in the river?

The dimensions are also interesting. I used Excel to convert them to decimals and plot them. At about the quarter point, the resulting line is about 9in below the semi-ellipse that passes through the springings and the crown. So, is the arch not semi-elliptical at all? Did brunel set out a cusp at the crown? The dimension lines have no arrows to indicate where they are measured from and to. It may be that there is say 4in of lagging above this profile. that would mean that Brunel expected a 7in drop at the crown to bring the rise to 24ft (which sounds more reasonable than 24ft3in). That would also imply a slight rise at the quarter point. I haven't checked that out yet, but it seems to hang together.

The list below gives the setting out dimensions and the elliptical equivalents for the half span.

                                                                       

Chainage	Ellipse	Setout
0	0	0
4	8.438643	7.833333
8	11.73998	11
12	14.13681	13.35417
18	16.86026	16.05208
24	18.93015	18.15625
32	21.00112	20.29167
40	22.48035	21.91667
48	23.47996	23.05208
56	24.0598	23.82292
64	24.25	24.25

Cobwebs

Sometimes you see something you just want to share. Yesterday we went to Killerton House to the craft fair. On the way back to the car we saw the sung glinting off cobwebs and eventually realised thta the whole field, perhaps 500m square, was covered. You can see the webs clearly in the front of this photo, but I guess it is harder to believe that the more distant highlights are also cobwebs.

How many spiders is that?

The place of Computers

I have long thought that computers are wrongly seen as a replacement for the brain instead of a lever. Pleased, therefore, to see a similar view expressed in different, but very tangible form in the Guardian on Saturday.

Silverton Mortar

I will check out dates as soon as I can. In the mean time, here is a closer look at the two maortars used.

And, of course, dates are pretty easy. The Landmark Trust have a page about the stables.

Flat arches and corrosion

The stables at Silverton are magnificent. They were in full view of the house and were designed to be imposing. The whole was intended to be Stucco but it never was. The portico faced the main house and has no entrance behind it. The pillars are surmounted by "safe" lintels of flagstone. Brick arches carry the main load. There is a band of "cement" mortar through the arches and about 3 courses above them.

Wrought iron cramps tie the lintels end to end. These have corroded and lifted the whole pediment about 2mm so the whole structure is supported on about 10000mm2 of iron at each pier.

Can the corrosion be arrested by cathodic protection?

Truth

Truth has been much in mind recently. A night out at Pride and Prejudice was one thread "It is a truth universally acknowledged". But truth in engineering is something that has exercised my mind for someyears without really coming to a head.

Last night I watched Andrew Graham-Dixon on The Secret of Drawing. He spoke of truth in a way that jelled some thoughts.

In court we are asked to speak "The truth, the whole truth and nothing but the truth". In engineering, such a demand would create a real problem because we can so rarely know the whole truth. What is vital, though is that we use nothing but the truth, or at least, where we have doubts, we acknowledge and respect them.

This reflects on the current debate about changes from UK to Eurocodes. there can be little doubt that many code changes are driven by developing ideas. If the established "truth" becomes known to be false it must be rejected whatever the cost, unless it is also known to be safe.

For some years now, I have been concerned about the "truth" of the models used in arch assessment. Not in the underlying statics of the 2D models thenselves, but in the way 3D behaviour is re-structured into a digestible 2D problem. There is an argument that we cannot set aside the existing flawed rules until we have something to replace them. Arch builders, as late as the mid 19th century, relied on rules of thumb. There is some evidence that those rules of thumb were more reliable than the calculations we do now. Is it not better even to make a judgement, knowing that you are doing so, than to make a calculation that is spurious.

I suspect I wil come back to this (I have to prepare a paper for IStructE for 16th Feb) but in the mean time see:

A Three Dimensional Model for Masonry Arch Bridge Behaviour

B. Harvey, A. Tomor, F. Smith,  UK