Thermal Expansion of aluminium

It seems there are really 2 issues here discussed ( or asked as one). 1) The Thermal Expansion of a material is solely dependent on its own temperature. 2) What temperature a material is, or will be under various conditions is dependent on many factors, and these are independent of Thermal Expansion.

Considering - Actual Aluminum Alloy used
Finish ( clean aluminum or black anodized)
Ambient Air temp
Air flow
Altitude and Humidity
Strength of Sun (eg. latitude)
Incident angle of the sun to the staff

There are engineers who's full time job is dealing with this... IMO assume ambient air temp - the sun does not shine on both sides of the staff and aluminum has good thermal conductivity - so one side may be heated by the sun the other side will radiate and conduct heat to the air.

For a 2 M aluminum staff - it will not be centimeters.

However if you are asked to place anchors in one material (say cement CTE 10m/mK 10^-6)) to which a long beam will be set (say aluminum 22) -- and the temperature in the location may change dramatically - the effect of the CTE should be in the back of your mind. Esp when the customer calls screaming that the beam does not fit!

Also why bridges are not hard bolted ( typically left floating on a locating pin / bolt, or rollers) to their mounts and they have expansion joints. That is all part of the job of the bridge engineer. The methods of construction ( suspension and trestle for example) also help minimize the effect of the CTE. You see the same thing in long pipe runs - the mounts are on rollers and every no and then a jog or "u" is added to allow some flex - to absorb the changes in length.

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