Whipple Bowstring Truss

Whipple claims a bowstring iron truss — but the real breakthrough is that he could calculate the force in every member. Note both the shape and the analysis.

Bend a strip into an arch, feet apart, and press the crown. Feel the feet push outward — the arch is in compression and wants to spread. Note where the thrust goes.

Run a string between the arch's feet and press the crown. The string goes taut and stops the feet spreading. That is the bowstring: arch above, tie below.

Make a curved top chord from stiff wood strips — the compression member. In Whipple's bridge this was cast iron, strong when squeezed.

Run a straight tension member along the bottom, foot to foot — the "string." Whipple used wrought iron, strong when stretched. Model it with wire.

Connect the bow to the string with vertical hangers and diagonals so a load on the deck is carried up into the arch. Now it is a working truss.

Load the deck and check each member: the bow is squeezed (compression), the string is pulled (tension), hangers pulled, diagonals as the geometry dictates. Mark C or T on all of them.

Put the spring scale in line with the bottom string and load the deck. Read the pull. The heavier the load, the tighter the string — the number Whipple learned to predict.

Rebuild with a shallower arch and re-measure the string tension for the same load. A flatter bow pulls the string far harder. Rise controls force — this is the kind of relationship Whipple could calculate.

Cast iron is strong pushed, brittle pulled; wrought iron is tough pulled. Confirm your model has the stiff member in the bow and the tough one in the string — the wrong way round would snap.

Imagine cast iron in the string (tension). It would crack — cast iron is weak in tension. Matching material to force is the safety of the whole bridge.

Overload the truss and watch which member fails first. A well-designed truss fails at the member you predicted, at the load you predicted. Design by analysis means no surprises.

The patent. US 2,064, granted 24 April 1841 to Squire Whipple of Utica, New York. Whipple is often called the father of iron bridge engineering, and his bowstring truss is widely regarded as the first scientifically designed truss bridge — the first to be laid out from a correct analysis of the forces in every member rather than by tradition and guesswork.

Why "scientifically designed" is the whole point. Before Whipple, a truss was proportioned by experience and precedent, which is why bridges of the 1830s failed with grim regularity — nobody could say how hard any member was actually being pulled or pushed, so builders could not know which parts were dangerously weak. Whipple worked out the statics: he treated the truss as a system of members meeting at pin joints and calculated the tension or compression in each, publishing it in 1847 in A Work on Bridge Building, the first American engineering text to do so. Steps 8 and 9 hint at the power of this — once you can compute that a flatter arch multiplies the tie force, you can size every member to its actual load instead of guessing, which is both safer and lighter.

The bowstring resolves the load beautifully. A curved top chord carries the load as an arch — pure compression, thrusting outward at the ends (step 2) — and instead of needing massive abutments to resist that thrust, the straight bottom chord ties the two ends together and takes the thrust as tension, like the string of a bow (step 3). Whipple then made each chord of the right iron: cast iron for the compression bow (cheap, stiff, strong when squeezed, brittle when pulled) and wrought iron for the tension string and diagonals (tough, strong when stretched). Match the material to the force and every pound of iron does its proper job.

What it led to. Whipple's method — analyse the forces, size each member, choose the material to suit — is simply how structural engineering is done today; the arithmetic got more sophisticated, but the idea that you calculate a structure before you build it starts here. His bowstring trusses carried the Erie Canal and early railroads, and the broader family of iron and then steel trusses he made calculable went on to span the rivers and carry the railways of the industrial world. A number on a page, it turns out, is the strongest thing you can put in a bridge.