Simplicity of design is a modern tendency of engineering in practice. Such structures as aircraft hangars and bridges can now be efficiently and economically built from sections or units of a standard pattern
TESTING A UNIT-BUILT BRIDGE of the Callender-Hamilton type with a 10-tons wagon. This bridge, the first of its kind in Great Britain, was erected in 1936 at Dulas, Montgomeryshire. Further pictures of its construction appear below.
THERE is a tendency in engineering practice at the present day towards simplicity of design, so that structures may be easily and quickly erected by comparatively unskilled labour. This is particularly true of the building industry, and an excellent example of this simplicity and ease of erection is afforded by steel scaffolding.
British engineers must be continually alive to the needs of the export market, and in this the need for simple and robust structures is even more vital than in Great Britain. Perhaps this is one of the chief reasons why unit construction has made such rapid headway in recent years. To-day it is accepted as standard practice for many different purposes.
An interesting and simple method of unit construction has recently been introduced to the market. Known as the “Bellman” hangar, it has been designed with a view to the rapid and simple erection of aircraft hangars and similar buildings in all parts of the world.
Each unit is built up of standard steel sections, the joints being welded together, a light and strong component being thus made. Hangars built on this system in England have withstood extremely severe weather conditions. This type of hangar can be built up to a span of at least 95 feet and to a height of 25 feet, which is sufficient to house an aeroplane of exceptional size. The units have been designed with ease of transport in view, and they can be loaded into railway trucks or carried by mules to their destination. The units can be set up with unskilled labour, and no foundations or guys are required, nor is any timber used.
Another important point to be borne in mind is that varying sizes of hangars can be built from the same units as the standard hangar. The units can be used also for buildings of many different types, from garages and factories to movable workshops. Not only are the units suitable for temporary structures, but they can also be used for permanent buildings.
The rapid building of large-span roofs is a matter of considerable importance in the building industry. A roof must be strong and light, able at the same time to support glazing or covering and to resist wind loads and heavy falls of snow. A simple and ingenious type of roofing is built on what is known as the “Lamella” system.
This comprises the building of an arched roof by means of a series of standard units erected in the form of a number of diamond-shaped panels. Any type of roof covering can be easily laid upon this foundation. The roof is entirely self-supporting, so that no trusses are required. It is so rigid that it can be built in cantilever fashion, so that the two halves meet in the centre of the span.
The diamond-shaped units consist of four main parts. These are the “Lamella” bars, top and bottom stays, or purlins, and a pair of angle pieces to give a good strong joint at each point of intersection. Each component is of pressed steel, and the shape of the “Lamella” bars is such that it gives considerable strength without undue weight. Another feature of this system is its resistance to fire, and it has been proved that it will withstand local damage without danger to the whole structure. The damage is confined to the immediate vicinity of the fire, and the affected parts can easily be replaced.
This system has been successfully used for aircraft hangars, and at Heston Airport, Middlesex, there is a hangar of this type having a span of 82 feet and a length of 149 feet. A hangar has been supplied to the Air Ministry with a span of 120 feet, a length of 240 feet and a height in the centre of 40 feet. The units nest together conveniently for transport. With this form of roof it is possible to build flat or steep arched roofs. It is adaptable to any span between 30 and 200 feet.
HANGAR AT HESTON AIRPORT, Middlesex, built of “Lamella” units. The hangar has a length of 149 feet and its roof a span of 82 feet. Buildings of this type offer strong resistance to fire, and local damage will not endanger the strength of the whole structure.
There are many works of construction carried out in Great Britain and abroad where the use of temporary bridges is essential. In rough and mountainous country it is often desirable to have bridges which can be easily transported and quickly erected.
At first sight it might appear that the design of such a bridge would present few difficulties, but the structure must be composed of light component parts, easily and quickly erected by unskilled labour and must have exceptional strength and rigidity when erected. Much thought has been devoted to this problem in the past, and it is only comparatively recently that an ingenious solution has been achieved.
Cross-Bracing Unnecessary
For work in remote places many other factors must be taken into consideration. The maximum weight of any single component must be such that the component can be carried on mule or donkey, and in many instances it may be possible to use only manual labour. In addition to this, all the parts must be able to withstand rough handling without suffering distortion, so that light weight must be combined with exceptional strength.
There is also the necessity for protecting the parts against the effects of atmospheric corrosion. Nowadays this can generally be done by galvanizing the parts, and this must be carefully carried out to give the best results. Bridges erected in certain districts will probably be subjected to flooding, and a structure which is not well protected will be particularly liable to corrosion from this source.
Perhaps the most important point, however, is that the structure should be absolutely simple in design. There should be no complicated connexions, and small nuts and bolts should be avoided. These are often troublesome to fit in position, and large bolts have the advantage that they are simple to fit and not liable to sustain damage or to be bent during transport and handling. It must be possible to carry out erection from simple instructions, which can be clearly followed, even by those who have never had any previous experience of structural work.
After much patient research a unit construction type of bridge has been devised which appears to meet the requirements. This is known as the Callender-Hamilton bridge and is patented in Great Britain and foreign countries. The basic idea of the design is that a number of different spans can be built up from the standard parts. The heaviest single component of the trusses weighs only 164 lb. This component is a special gusset plate and is the key to the success of the design. Plates are welded on to the gusset plate in such a way that the floor beams of the bridge can be bolted on to them. As there are pairs of diagonal members at each joint, connected to corresponding pairs of these gusset plates, the necessity for putting in cross-bracing over the top of the bridge is avoided. This is of great importance, as it means that the height of the vehicle passing over the bridge is not in any way limited.
AFTER THE FLOODS which followed a severe storm in June 1936, it was found that the masonry arch which carried the road to Llanidloes at Dulas, Montgomeryshire, was badly damaged. The road had to be closed to traffic and it was decided to erect a temporary bridge of the Callender-Hamilton unit type. The standardized steel members were first assembled a the site on the bank of the river. A condition of the contract was that the work was to be completed in two and a half weeks.
In 1936 a masonry arch bridge on the Llanidloes road at Dulas, Montgomeryshire, was washed away by floods after a severe storm. It was decided to replace the bridge by a Callender-Hamilton bridge to provide a temporary crossing for traffic while a new bridge was under construction.
The temporary bridge has a span of 50 feet and the order was placed with the contractors on the understanding that the bridge was to be in service within two and a half weeks of placing the order. All the steelwork for the bridge was delivered on the site in fourteen days, and only three days later the completed span was in position.
The completed span was launched by simple tackle and a pair of light derricks. The concrete supports on the river banks were prepared beforehand, so that the span was soon in position. If it had been desired to build a larger span, it would merely have been necessary to use a larger number of the standard members. By this means spans up to 200 feet can be built up from the components without difficulty.
TWO LIGHT DERRICKS and simple tackle served to launch the completed span in position. The concrete supports on either bank had been prepared beforehand. The steelwork used in the bridge consisted of standard steel members, jointed together by bolts of large diameter. All the steelwork was galvanized after fabrication. The use of large bolts for connexions facilitated the assembly of the bridge.
A test load was sent over the completed bridge. When a 10-tons lorry was sent across the deflection at the centre of the span amounted to about half an inch. The total weight of the bridge is only about seven tons, and the total labour involved, including that required for the launching operation, was about 360 man-hours. This gives a good idea of the labour cost of this type of structure. A great deal depends on local conditions and on the class of labour employed.
After Faulty Assembly
The bridge at Dulas was the first Callender-Hamilton bridge in Great Britain. Recently numerous bridges of this type were supplied to the Anglo-Iranian Oil Company for shipment abroad. Their utility in wild and desolate country has been proved, as they have been erected largely by unskilled labour. This type of bridge lends itself to colonial and overseas work.
Some stringent tests were recently undertaken on a span of 140 feet. This was erected on level concrete abutments. It was provided with a timber decking, and the total weight of the bridge amounted to-93½ tons.
The span was submitted to a total moving load of more than sixty tons, which was slightly in excess of the designed load. The span was then repeatedly submitted to some heavy impact loads, which imposed much greater stresses upon the bridge than any which would be met with in practice. After the span had been dismantled, it was found that no damage whatever had occurred. This proved that the span could be loaded and reloaded many times without any ill effect.
THE SPAN IN POSITION. Timber decking was laid across the span to serve as a roadway. Within fourteen days of the order being placed for the bridge all the steelwork had been collected at the site. Three days later the bridge had been assembled and was in position. The total labour involved amounted to about 360 man-hours. The deflection on the first loading was about half an inch a the centre of the bridge; later the deflection was only about one-eight of an inch.
Interesting experiments were also carried out to determine the result of faulty assembly, and the way in which it would affect the strength of the bridge. Such tests are particularly important where the overseas application of this type of bridge is concerned. To imitate this condition the connecting bolts were left fingertight, and the span was raised about 2½ in. at one corner.
The stresses measured, with the same loading as in the previous test, were less than those recorded when the bridge had been properly erected. It is believed that the stresses would have been greater if the bolts had been tightened, but even then it is considered that the stresses would not have been dangerous unless the difference in level had been much greater than 2½ in. It is therefore safe to assume that the results of faulty assembly would not be serious, a point of paramount importance where unskilled labour is employed. Examination of the bolts after this test showed little damage.
Each of these various systems is exactly suited to the purpose which it has been designed to fulfil. Each is a remarkable example of this age of specialization, and shows what can be done by the application of scientific principles to structural work. Considerable reduction in weight has been achieved by the use of welding wherever suitable. The technique of unit construction also shows that the best ideas in engineering are often of a simple character.
UNIT CONSTRUCTION IN ASIA MINOR. This building was composed of “Lamella” diamond-shaped steel units. They are of light weight and are compact for transportation. All the units for this building are of the same size. They were transported for a distance of 125 miles on pack mules. The units can be quickly erected or taken down.
