Haugh of Drimmie Bridge

1. 2022

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

The remarkable Haugh (or Haughs) of Drimmie Bridge, or Glenericht Bridge, carries a private road connecting Glenericht Lodge with the A93 road, a few miles north of Blairgowrie.

It has much in common with the smaller and simpler Kirkton of Glenisla Footbridge constructed by John Justice of Dundee.

It was originally built in 1823, and rebuilt after severe floods in 1847.

The Happy Pontist blog provides an excellent insight into aspects of this, and many other bridges.

1827 Description

The most modern, cheapest, strongest, and best made iron ones I have seen, are those erected by Mr. John Justice of Dundee; a short description of them may not be uninteresting to many of my readers, and of some importance to such as may be intending to erect any similar. The suspension bridges, designed and executed by Mr. Justice, differ from others, owing to the ingenious way they are constructed ; in my opinion, more strength is also produced from less iron than in any that I previously knew of. I have recently seen one erected by him in 1823, for Colonel Chalmers, at Glenericht, the country seat of that gentleman. The span of which is 104 feet, the road 10 feet wide, with a carriage way in the middle, and a narrow footpath on each side. This bridge is capable of bearing any weight that can be put on it; indeed, the first trial was seven loaded carts, each drawn by one horse, so as they could be all on it together.

'This bridge is supported by two strong chains under the roadway, each of them proved, before putting up, to carry a strain of 25 tons ; but the principal strength lies in twenty-eight suspension rods, each of them was also proved, before being put up, to carry 20 tons ; so that the strength is superior in a ten fold degree to any weight that can go on it.

'Notwithstanding the great strength produced, owing to the way it is constructed, there are only 2 tons of fine iron in the suspension rods, bottom chains, back stays, and every thing else in any way suspending or bearing the strain of that ponderous weight; and about 3 tons more of common English Iron, applied, in main posts, at the ends, (to save mason work,) and the cross beams or joists for supporting the road-way, and other parts where tenacity is not required.

'All parts of this bridge contribute jointly to the general strength, as well as severally to that of their respective places; and any part, except the main posts, may be taken away at pleasure without weakening the general structure, more than the mere use of that part taken away ; which must be of great use in case of repairs being required in the lapse of time, and I believe the expense was not one third of what a stone bridge would have cost, although plenty of stone is to be found on the spot.

'Besides bridges for carriages, Mr. Justice has erected a number of foot bridges, some of which are upwards of sixty feet span, and which did not cost more than L.40, including mason work ; and these not only carry as many people as can go on them at once, but as many horses. Bridges of the same span, for carriages, will cost L.8O more, independent of mason work. These bridges are adapted to every situation, but some situations will cost much less than others.

'I may also observe, that in wooded dens and ravines, where there is rock on both sides, which is often the case, these bridges could be erected at an expense of little more than twenty shillings for mason work, and put up by any ordinary mechanic, as they are all put together and proven at the factory.'

1847 'TREMENDOUS FLOOD IN THE ISLA, ERICHT, AND TRIBUTARIES. .... The bridge over the Ericht at Blairgowrie is ruin. Its most western arch and pillar gave way, and now there is no passage by it; and indeed, we may say, no passage across the river at present near Blairgowrie for carriages of any description. Sir William Chalmers' suspension bridge at Glenericht, the stone bridge at Kirkmichael, the bridge at Kindragan, and several other bridges on the Ardle and its tributaries have been either swept away or are useless for transit.

1847 'On the Ericht, which is continuation of the Ardle and Blackwater, after their junction at the Bridge of Cally, Sir William Chalmers’ fine iron suspension bridge was borne down by the flood ;...'

1847 'The ravages on the Ericht have been disastrous in the extreme. The beauitiful chain bridge at Glenericht is destroyed. ...'

Details of Construction

An excellent series of pencil sketches showing details of the bridge were produced by Geoffrey D. Hay and reproduced in 'Monuments of History' published by the RCAHMS in 1986. These sketches, together with numerous photographs, are available online here.

Reference to the simpler Kirkton of Glenisla Footbridge provides a starting point for studying the construction of this bridge. There, the wooden deck is supported on a simple 'ladder' frame comprised of a pair of long, thin wrought iron plates pierced to accommodate wrought iron bar cross members. Four vertical wrought iron pylons are embedded in the masonry abutments, and these are braced by diagonal bars, themselves embedded in the abutments. From these pylons a series of wrought iron rods connect with the deck/cross members, in the manner of a cable-stayed bridge.

These 'Glenisla' principles are also found on the Haugh of Drimmie/Glenericht Bridge, but with the important addition of a system of underbracing, in the form of a pair of slender tensioned suspension rods below the deck, supporting the deck by a series of props. Peters describes this arrangement as an 'underspanned suspension bridge', and credits Robert Stevenson as the first to publish proposals for an underspanned suspension bridge, in 1821. Underbracing is also applied to Crathie Suspension Bridge, originally constructed by Messrs Justice.

Like the Glenisla bridge, the Glenericht Bridge deck has very slender side bars, nominally 3" deep and 7/16" thick, but locally swelled to accommodate the projecting ends of the cross members (photo 3).

The cross members supporting the deck are of 'lenticular' form, having curved upper and lower members (the upper members being curved to provide the camber for the wooden deck). See photos 5, 6 & 7. The upper member is formed from angle iron, while the lower member is flat bar iron. The use of rolled angle iron bars is perhaps more likely to be consistent with the post-1847 rebuild than with the original 1823 construction.

Vertical struts support the cross members from the tensioned suspension rods (photo 7). The bottom of these struts is provided with cottered straps to secure them to the tension rods, while the top ends are forked and pinned to the lower cross members. The forged struts are extended above the fork to make contact with the upper curved angle iron member. Each strut would need to be forged to suit its specific location, so as to make good contact at three points - the upper cross member, the lower cross member, and the suspension chain.

The tension (suspension) rods below the deck can be described as chains, as each is assembled from several (four?) long lengths of 1 1/16" diameter wrought iron bar with pinned forged eyes at each end. Geoffrey D. Hay's sketches show that the extreme end of each chain is connected to a pair of rods which are presumably embedded in the masonry abutments.

The four pylons which serve a vital role in linking the bracing rods with the back stays are forged from 3" square wrought iron bar. They are embedded in the masonry abutments, surprisingly close to the edge (photos 13 & 14). Ideally the pylons would be subjected only to compression loading, with no bending moments, but in reality the method of connecting and tensioning the stays is far from ideal.

The diagonal stays are fanned out from the pylons (photo 3), and are attached to the pylons via eye-bolts (photos 9 - 12). Five of each set of seven stays pass through oblique holes in the pylons. The nuts for the eye-bolts are provided with tapered washers to accommodate the obliquety, with various degrees of success (some of the eye bolt shanks are bent - see photo 12). This differs from the arrangement at Glenisla. Most of the eye bolt screw threads are of an unusually coarse pitch, suggesting that they are original. Several have a finer pitch, and are probably replacements (photo 12).

The pylons are 'upset' by forging to swell the bar at the positions of the eyebolt holes.

Different arrangements are made for attaching the back stays to the pylons. The ends of the stays are forged to provide bridles (stirrups). Two of each set of three embrace the pylon, but, curiously, the third is attached by a pin which passes through the pylon (top of photo 12), requiring a hole in the pylon which must weaken it. There is no provision for adjusting the tension of the back stays. The back stays are made from wrought iron bar, 1.5" square.

The horizontal handrailing on each side constitutes a tensioned structure in its own right. On each side there are four horizontal 3/4" diameter wrought iron bars, and one 1.5" wide flat bar at the top, all tensioned from braced pylons located beyond the main bridge pylons. See photo 15. The tensioned round bars are clamped to vertical bars whose lower end is connected to the cross members which support the deck. Alternate upright posts are provided with diagonal bracing which also terminate at the deck cross members. See photo 16. Lengths of the round horizontal bars are joined with pinned forged eyes (photo 17), similar to the connections on the suspension chains.

The vertical posts of the handrails are connected to the ends of the cross members, and are clamped to the tensioned horizontal bars. They are also supported by diagonal bars, which are joined at the bottom end to the vertical bars. The ends of the diagonal bars appear to be joined by forge-welding. See photo 18, which also shows a small horizontal bar passing through the vertical post. This serves to support the wooden walkway.

The practical difficulties faced by the blacksmiths and by the site erectors were immense. The shaping of the 3" by 7/16" side members, 104 feet long would have challenging, and even handling them on site would have been interesting, in view of their flexibility and unweildiness. Then, it was necessary insert the ends of the 29 cross members through the holes in both side members.

This bridge is classified in this guide under 'Suspension Bridges - Unusual Type'. To describe it as a suspension bridge is convenient, but not totally accurate, as the deck is not fully 'suspended' as it would be in a 'pure' suspension bridge. If there is a definition of a 'pure' suspension bridge, it will probably specify catenary chains or cables, to which are attached near-vertical suspension rods or cables supporting the deck. The deck, entirely supported by these suspenders, will receive only limited constraint from the abutments, as can be appreciated by standing at the end of the deck of a moderately-sized suspension bridge when a vehicle passes over. The Haugh of Drimmie bridge does have catenary chains, but these are underneath the deck, providing partial support for the deck by struts. The main, or at least the most obvious support for the deck, comes from the array of stays connected to the vertical pylons. In this respect it might be regarded as a rod-stayed cantilever bridge.

Now, if we consider the the assembly stresses in a cable or rod-stayed bridge, the stays will be in tension, the vertical pylons will ideally be in compression, and as a result the deck's horizontal members will be in compression. Clearly the 105 ft long horizontal wrought iron members, with a section of just 3" by 7/16" have limited compressive capability, so it follows that the wooden deck structure must make an important contribution. G. D. Hay's sketches show that the deck's cambered roadway is made of 3 1/4" thick planks bolted to the cross members, superimposed with thinner diagonal planks. The wooden walkways at the sides are probably of similar thickness and are supported on substantial wooden beams. These can be seen in phot 5. These wooden components must provide an appreciable amount of stiffening to the structure. It is not clear how compression forces in the deck are transmitted to the abutments.

Comparisons

Like the Haugh of Drimmie Bridge, Crathie Suspension Bridge was made by Messrs Justice of Dundee, and was underbraced by chains, and had trussed cross members (albeit flat-topped, not cambered). Both of these bridges were rebuilt (Crathie was originally built in 1834 and rebuilt in 1885), and it is not clear whether the same features were present originally.

Linlathen East Iron Bridge will be mentioned here, as it also had trussed cross members, unusual in an early 19thC bridge, and it is located near Dundee, but there the resemblance ends. Its maker and actual date are not known.

In 1823 George Buchanan proposed a suspension bridge for Montrose which was to have a heavily-cambered 30 ft wide iron deck with trussed 'lenticular' cross members. Each cross member was to have curved top and bottom members with a series of 11 vertical struts between. The bridge was not built. The superficial appearance of the cross members resembled the small main spans of George Stephenson's Gaunless Viaduct, built in 1823. Wooden lenticular trusses may have been made in Europe centuries earlier.

Going further back, a wire-braced bridge with many similarities to the Haugh of Drimmie Bridge was built at King's Meadows, Peebles, in 1817. It was designed by John Stevenson Brown and built by Redpath, Brown and Co. It had a span of about 110 ft. Five diagonal braces, 0.3" diameter, fanned out from each of four pylons. As at Drimmie, the handrails were separately tensioned from straining posts, but these posts together with the main pylons were braced to posts set further back. Remarkably, the bridge survived until c.1960, by which time catenary cables had been added.

Access

Currently (June 2022) there are 'Do not cross' signs at the bridge.

A discrete winding road leads to the bridge from the A93 road. It is a private road, but it serves the Glenericht House Arboretum. Parking is available for 2 or 3 cars, and there is a charge of £4 cash per adult visitor (2022).

See Also

Loading...

• [2] Canmore