How did bridge builders of old ensure both ends of the bridge would perfect meet in the middle before laser measuring was around?



How did bridge builders of old ensure both ends of the bridge would perfect meet in the middle before laser measuring was around?

In: Engineering

They used other measuring techniques. Things like string/rope and anything to measure an angle could be used to draw two parallel lines, then you just stay within those lines.

They had things like levels and surveyors tools. With a surveyor’s kit you could get quite accurate – like to within an inch or two or better. Read up on how they built cathedrals. Iron right angles, bubble levels, chalk lines, plumb bobs… the tools of stone masonry and carpentry really haven’t changed much in over a thousand years.

Furthermore, they couldn’t quite build out from the bridge pylons the way they do now with cantilevere’d tower supports etc.. They’d build a pier out in the middle of the river and build it up to a certain level with stone (using my level and surveyors scope from the shoreline to measure the right height). Let it set and remove the wooden caisson they used to dig down to the river bed. Then they’d construct a wodden trestle/framework that spanned from the pier to one shore, while the foundation on that shore is being built. Make both sides of the trestle level is easy enough, then the trestle framework supports the stone until the mortar sets. Rinse/ repeat on the other side, or until you reach the other bank.

Bridges aren’t as precisely sized as you think even today, physics won’t let them be

The longest span on the new Tappan Zee is 370 meters long and can change length by 20 cm between a hot summer day and a cold winter day. Bridges are built with expansion joints which let them expand/contract as temperatures change and also take up some slack during construction

With old stone bridges a lot of stones were adjusted for their spots. Have a smaller gap than expected when you get to the middle? Smack the stone with a hammer until it fits! Then add some mortar to take up all the slack. Voila! A bridge that “meets perfectly” in the middle

Our precision measurements let us better understand the load capabilities of structures and make parts in advance so we can build them faster, but every engineering design supports some slop in the final measurements

The original Huey P Long bridge in New Orleans has a 1.5′ “jog” in the roadway about 1/3 the way up the bridge. I suspect it was due to measurement errors. The lanes were super narrow, at 9′ wide, with no shoulders and most driver had to grip the steering wheel tightly while traveling the bridge, Especially if an 18 wheeler was next to you.

I’d ask the same question about *tunnels*. You can’t even see where you’re headed.

IIRC there’s some tunnel connecting Manhattan with either New Jersey under the Hudson River or Brooklyn under the East River (I don’t remember what side) that was built in the late 1800s with two excavation teams from both sides that met in the middle with an error of less than 1 inch. I can’t get through my skull how this could be done at the time.

Even more impressive is the transcontinental railroad, one of the tunnels they dug out from both sides and met up in the middle. They were within an inch I think

To blow your mind, the middle of this bridge washed out and they rebuilt it. It has a bit of a kink now. Steel beam suspension bridge.

String. They lined up string measured by levels and squares. Also stone masons back then were absolutely ace when it came to construction. Roman masonry work still functions in some places after 2020 years.

They did pretty much what I did when I built a fence around my house:

* Put a stick at either end.
* Have someone at one of the sticks, looking so that both sticks line up.
* Have someone else put remaining sticks in a line between them, lining up the top of the sticks with that sight line.

Accurate to within a millimeter or so, if done carefully.

Sure, they probably didn’t line up sticks, but the principle remains. With two set points, one at each end, it’s very easy to accurately put stuff on a straight line between them.

laser measuring? I can build you a bridge with 100 feet of rope, a hatchet, and a few* pitchers of lemonade

There’s a lot of talk about protractors and string here … that’s only partly right. Traditional survey instruments (same principles often used today) measured angles, horizontally and vertically, extremely precisely. Quick precise distance measurement came later, but one carefully measured baseline and a series of intersecting angles can coordinate points better than you’d expect.

GPS isn’t as useful as you’d think for surveying in closed-in or covered areas, so we still use similar instruments almost every day. Behind all the software, modern terrestrial survey instruments still just measure horizontal angles, vertical angles, and slope distances.

Leveling is even easier! In fact, if you want to be really basic, you can even do it with water.

Source: professional land surveyor.

There’s a misconception that ancient and even recent past is super primitive compared to our modern (as of 2019) knowledge and tech, but people have been super innovative and resourceful for several thousand years. Just the last 10-20 years have probably doubled our knowledge of the total so far…

Today we seem puzzled that people could have built anything without digital technology and other gizmos.

But, we tend to forget that great builders, like the Romans for example, constructed roads, sewers, aqueducts, bridges, structures, without lasers and computers. Their works have stood for centuries. And, imagine doing math with Roman numerals! They even had rudimentary cranes and understood gearing. They could do a lot of things we do now, but just didn’t have power machinery to do it.

For a great look at ancient building techniques, read the Roman author and architect, Vitruvius, who wrote [De architectura](

Modern measuring tools are more practical and resistant to human error, but precision isn’t necessarily a modern thing. The [theodolite]( has been around since 1600 and along with other precise distance measuring tools it allowed for incredible precision in building.

Antique topography instruments were much more mechanical and human dependent, but topography isn’t a new science and the old instruments weren’t necessarily much less precise than modern instruments.