How to Locate the Centroid of a Composite Beam

Place the beam on the floor and work with the top-facing rectangular face. Use the tape measure to measure each side of the rectangle, even if you already know the dimensions. Mark the measurements down on a piece of paper.

Divide each of the measurements by two to find the midway point. Go back to the rectangular face with the tape measure, measure each side and place a mark on the beam at each of the four midway points. Try to be as precise as you can with the mark; make it thin, marking the exact midway.

Connect the corresponding midway points with a tape measure, T-square, ruler or string. Connect the two lengthwise midway points, and the two midway points marked on the width. Draw a straight line along the edge of the tape measure to connect the corresponding midway points.

Run string or a tape measure connecting the lower left-hand corner of the rectangle with the upper right-hand corner. Have the edge of the string or tape measure connect the corners as precisely as you can manage. Draw a line down the string or tape measure to connect the corners with a straight line. Repeat with the lower right-hand and the upper left hand corner.

Look at the criss-cross pattern of lines you've drawn on the rectangle face. The intersection of all the lines, in the middle, is the centroid of the rectangle.

Repeat this process on all six sides of the beam, even the stub ends. If you could draw lines through the beam, connecting all six two-dimensional centroids, you'd find the centroid of the three-dimensional object. The beam should balance on any of the four lengthwise centroid marks.