Recently a large Crane fell in Tampa and caused a lot of damage. I heard people say it was “rated” for wind speeds that were less than the wind speeds of the recent hurricane. How can it be measured down to certain winds speed, as in, 100 MPH is safe, but 130 MPH is not safe?
In: Engineering
You can calculate aerodynamic drag at a given air speed for an object with a given surface area and shape(using a wind tunnel to verify your assumptions), and you can build your structure strong enough to withstand those forces. The higher up, the more forces, both because winds are faster and because there’s a longer lever arm acting against the base of the tower.
force is proportional to velocity squared, so a 30% increase in wind speed is more like a 70% increase in forces.
We do wind load calculations. You can think of the crane as a sail that catches the wind. We calculate the approximate area of the crane, estimate wind speeds at different elevations (e.x. the wind gets faster the farther up from the surface of the earth), and determine the forces created at different wind speeds. The size and shape of the load (what the crane is lifting) is also included. Safe wind speeds are based around what the crane can withstand (its structure) and that the bearing surface will support (like a pile foundation, a barge, tracks on dirt, etc).
You can read about the Big Blue engineering disaster at Miller Park for a good lesson on wind loads. A 400+ ton section of roof fell as it was lifted by a GIANT crane because bad management pushed for an unsafe lift and ironworkers paid in blood.
In really simple terms, there’s math to both know how much force is applied at each wind speed and how much force the steel can take before breaking. Wherever those two lines cross is the rating
In addition to the way the limits are calculated, they will also over engineer and under estimate the minimums. It will be designed to handle 150 mph winds, but will be advertised as having a 100mph limit.
beyond any answers you get here, Practical Engineering on youtube has a couple videos talking specifically about this.
I like that the governor said most people took their cranes down. No, they didn’t. Take DAYS.
I’m an engineer but not an elevated crane one.
However if I was asked to I have a few tools. One is too look at the published charts on the drag ( forced) vs wind speed made by other engineers test data. I can the estimate the bending moment applied to the tower vs the strength of the material is made from.
If want to be really accurate I can put a cad model of the crane into a software called FEA (finite elemental analysis) and run an analysis called CFD (computational fluid dynamics) the simulate the wind blowing over the tower along with the force and stress it creates.
And don’t forget the anchoring, or the ground under the crane supports that has to withstand the loads calculated in the above explanations.
so basically engineers use a combo of math physics and materials science to figure this out. they do wind tunnel tests and analyze how cranes bend or sway in high winds. often they have a safety factor built in meaning they test for higher winds than normal. hurricane winds are like nature’s bullies though so sometimes things go wrong. it’s important they keep it safe so the cranes don’t end up being the real wrecking balls
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