Bridge Tester Design

Here are the main design considerations when building your own bridge tester.

I used 2x2's for the main frame of the tester. One problem that I ran into was using 1/4" x 5" lag bolts. I tried to use two of these at each of the frame joints to make the connections. Be careful when placing these bolts or you will break through the sides of the wood or drive the lag bolts into one another. However, the ability to use a hex driver on the lag bolts outweighed the problems- it greatly speeded the construction of the tester.

2x2's seemed plently strong (my tester held 400 lbs with no problems). If you wanted to further strengthen the unit any method of bracing the corners would work well. You can see I put a corner bracket into the front right corner (see picture below) for added strength on a corner where my lag bolting was imperfect.

The scissor jack worked wonderfully for this application. Turning the crank causes the jack to extend and exert a force downwards. This downward force is applied to a small (2cm x 2cm) block that spans both trusses of the bridge at the center. This block transfers force to the bridge until it breaks. NOTE: I did not purchase my scissor jack at the above link. I got mine at a local auto parts store (~20 dollars) but rumor has it that these can be obtained extremely inexpensively at a junk yard.

The most difficult aspect of construction was the 1/4" rod that does the pressing. A simple solution is to mate a 1/4" carriage bolt to the scissor jack- but these have a tendency to bend at forces > 150 lbs. I had the welding shop at school build a stronger piece using 1/4" welding steel. This held up to 400 lbs at which point my arm was tired.

This tester will not work on the floor. If the tester is not tightly secured to the surface on which it rests the entire frame will lift up. I solved this problem by placing the tester on a table and using 4 large C-Clamps to secure the tester to the table. This worked with no problems