How to Build a Howe Truss
A truss bridge is a bridge that is made up of connected straight elements which are stressed from compression, tension or sometimes both to support an active bridge. A Howe truss is when the diagonals of a truss are under balanced loading and are under compression.
Step 1 – Prepare the Blueprint
Prepare your blueprint. Trusses, in general, are series of triangles formed to spread out the compression load across the bridge. A Howe truss is a type of truss and has upper and lower horizontal members where you will find, in between, vertical and horizontal members. The vertical members of the web of steel take the tension and the diagonal members take the compression. The truss extends over a distance and this is called the span. Web members connect the upper and lower lines or beams.
The Howe truss height is measured from the centre to the centre of the chords. The chord is the upper beam connected by the members. For a span that is 36 feet, the height should not be less than one-ninth of the span. For a span 20 feet to 80 feet, the height should be less than one-tenth of the span. Generally, a height of one-seventh to one-sixth of the span and is mostly used because it’s the most economical. The amount of panels should be such that the braces will have an incline of from 36 degrees to 60 degrees. The best inclination is about 45 degrees. Take note that when the span exceeds 36 feet and the height is one-twelfth of the span, braces having an inclination of 30 degrees will greatly increase the stresses and will require larger timbers and rods. As a guide, though it’s not really necessary, it’s better to make the Howe truss symmetrical about the center.
As added information, the panels don’t need to be uniform in width. A bearing wall or post is a point where the centre lines of the end brace and of the tie-beam meet each other and should always cove over the support. Generally, the post should come over the brace by at least 6-inches beyond the inner face of the wall.
Step 2 – Prepare the Materials
Prepare the materials and connections that you need to build your Howe truss. Decide on the steel and connectors that you plan to use. Weld connectors are used for tubular steel. The “I” beam steel will need bolt connectors. Trusses can be built using wood.
Do the necessary preparations for abutments which include excavating to bedrock and pouring concrete. Anchor bolts and re-bar are to be placed into the concrete because this is where the steel is attached to.
Step 3 – Build the Truss
Start building the Howe trusses following your blueprint. Remember that you need two identical Howe trusses that are to be placed on both sides of the bridgeway or roadway.
Step 4 – Place the Trusses
Now that the identical Howe trusses are done; use a crane to put the trusses in position. In popsome cases the Howe trusses are not built in place. In this case, there’s a need to install the cross members binding the trusses together.
Step 5 – Make the Deck and Paving
Once the trusses are put in place, it’s time to begin with the bridge decking and paving. The material to be used for decking and paving are dependent on the use of the Howe trusses.