girder1.html

Chapter 1

Supports and Vertical Reinforcing Bar

Build the girder on the ground or in place. Support is required for a girder built in place. The support can be a wood scaffold, or, angle iron which spans the distance. Finished girders built on the ground are lifted into position by a crane. Girders built on the ground or on wood scaffold need 1/2 inch (1.27 cm) concrete support blocks for the steel, this is a size made on site before the girder is built.

If angle iron is the chosen method, and the length requires welding, be sure the resulting length of steel angle iron is straight. 1 1/2 to 2 inch (3.8 to 5 cm) angle iron is sufficient. It should be 1/8 to 3/16 inches thick (30 to 50 mm). Lift the angle iron into place, use care to avoid bending it. loosely attach the ends. Allow for 1/2 inch (1.25 cm) of plaster under the angle iron.

Support the angle iron with a 2x4 post so that the span is divided in half (5 x 10 cm post). Use a stretched string to make sure the angle iron span is flat and straight. Measure the 2x4 support carefully before cutting. Burn a hole in the bottom of the angle iron and drive a nail through the hole and into the 2x4 post. Secure the ends of the angle iron.

Arc Welding Reinforcing Bar

Arc welding reinforcing bar adds another dimension to ferro cement strength. Use low hydrogen welding rod. Weld one side only to flat steel surfaces. A weld on both sides of a round shape causes the flat shape to bend around the circular shape: when the welds cool and shrink. Weld rapidly. Minimize cutting into reinforcing bar with arc blow, add dimension with the weld. Clean and paint welds, especially on larger projects which take longer to complete.

Vertical Reinforcing Bars

Start at the center. Mark angle iron every 9 to 10 inches (23 to 25 cm). Use # 4 bar. Start with one for the middle. Bend a right angle. Use a torch or make a jig to cold bend as tight as possible. Make it four feet (1.2 m) vertical and 6 3/4 inches horizontal (17 cm). This horizontal size is very minimal and assumes such a small size is chosen so it will fit on top of 2x8 or 2x10 framing material. Weld the vertical in place at the center. Put one weld on the horizontal portion of the angle iron, and another weld on the vertical. Cut two 15 foot (4.5m) lengths of reinforcing bar. Use straight ones. Tie togeather with two foot overlap (60 cm). Temporarily tie the 26 foot (8 m) to the center vertical at a designed height or one that looks about right. Pull the ends down to position and temporarily tie. Make two more verticals to divide the span into quarters. Make them longer than necessary for arc adjustments. View the curve and make final adjustments. Tie light lines to the center vertical to keep the girder armature stable. Fill in verticals.

Horizontal Reinforcing Bars and Start Welded Wire

Put a layer of welded wire on the outside of the girder (the side away from the bottom bend). Position the bottom of the welded wire 1/4 to 3/8 inch (6 to 9 mm) above the bottom of the angle iron. Put a horizontal bar on the outside 18 to 20 inches up (45 to 50 cm). Follow this with layer of welded wire inside the verticals. Stagger the welded wire so the horizontals and verticals are 1 1/2 inches apart (3.8 cm). The girder has four layers of welded wire with an optimum pattern of 3.8 cm squares. Now put a horizontal bar on the inside at 9 to 10 inches (23 to 26 cm) up , then put another inside horizontal 18 to 20 inches (45 to 50 cm) above the first. Fill in on the outside to create a 9 to 10 inch (23 to 26 cm) staggered grid of reinforcing bar. Put a #3 bar at the bottom of the outside welded wire. Put another arc at the top on the other side of the verticals from the first.

Adjust grid space at the arc line according to what looks okay. The grid does not need to be perfect squares at the arc line. Quality work at the top curve is determined by what the girder is for and what connects to it.

Bottom Angle of Girder

If this is a freespan top loaded girder (no walls underneath) the bottom horizontal should be a a larger "L" than the dimension used above, or, a "T" rather than an "L" (built on a scaffold, rather than with angle iron). If this is a free span bottom loaded girder, the top should be a "T". In these cases, all vertical to horizontal bends can be a larger radius because they need not fit into angle iron.

Chapter 2