Program Brantac uses information supplied by you to calculate the data you need in order to make a model suspension bridge.

The required information is as follows –

Length of main span between centre line of towers

Clearance above water line at centre of main span

Height of towers above water line

Camber of deck from ends to centre

Gap between deck and cable at centre of main span

Number of deck segments in main span

Width of deck

The program works out the information needed for the main cables and the hangers for a suspension bridge which has side spans equal to half the main span.  The information can be printed directly on paper or stored in a disc file.  The program has been made very simple to create a small download file.

The lengths of the hangers are printed twice.  The first set of numbers is for the case where the cables go only from the hangers to the deck.  The second set is for the case where the hangers go under the deck and are connected at each end to the main cable.  The second method offers several advantages, including the possibility of making a model that can be taken apart for transport.

It is a good idea to use a small amount of camber, even if you want a flat deck, to allow for stretching of the cables.

Here is a diagram showing two ways of attaching a deck to hangers.  The upper part shows a single hanger going under the deck.  The lower part shows two separate hangers.  The advantages of the undersling method are –

Half as many strings to cut up.

Strings not attached to deck, so bridge can be taken apart for transport.

Main cables and hangers can be built as a complete assembly, and rolled up around a mailing tube for storage or transport.

The next picture is a sketch showing how a set of cables and hangers might look.

The main cables and the hangers should be cut longer than the calculated lengths to allow for attachments.  

Each hanger should be stretched out straight, and the two attachment points should be marked with a pen or a sharp felt-pen.  It is very important to mark the hangers very accurately, more so than for the main cables.  Also, increasing the number of hangers or decreasing the sag of the main cables requires greater accuracy.

The main cables can be folded in half to save time.  The cable should be stretched out straight, and taped to a measuring tape, also straight.  The attachment points can be marked as for the hangers.

Now each hanger can be tied to the main cables, making sure of getting exact alignment of the markers.  Once all the knots have been tied and checked for position, a drop of glue can be placed on each one.  That completes the most exacting and tedious part of the work.

Here is a diagram showing how the main cables may be attached to the top of the towers.  By avoiding glue, the possibility of dismantling is retained.

Make sure that the deck is neither too stiff nor liable to develop kinks.

To anchor the main cables at the ends, hollow wooden anchor blocks can be used, containing heavy weights.  The diagram, partly in section, suggests how this can be done.

The picture below shows how the assembly is begun.  The cable assembly is placed on the ground around the towers.  The deck is threaded through both towers, and then the four attachment points of the main cables are lifted on to the towers.  After that, everything can be adjusted into its correct position.