Why is thermal expansion not visible?
Thermal expansion isn't usually visible because of the small amount of lengthening, compared to the original length.
The longest steel arch bridge in the US is 517 m long. If this bridge undergoes a temperature change from
(Let us use only the coefficient of linear expansion for steel, to simplify this question.)
That means that, overall, the bridge actually increases in length a total of 86.6 cm from winter to summer. If you divide this up, there is only 1.6 millimeters of expansion for each meter of length of bridge. This is why we don't notice it.
One notable exception to this is with power lines. In the summer, thermal expansion causes power lines to sag very low to the ground. In wintertime, in climates where extended periods of cold can be expected, these lines can actually get quite tight.