The Importance of Allowing Room for Expansion and Contraction

Posted by in Case Studies, Structural Principles | September 10, 2018

While waiting at an exit on I-71 for a traffic light to change, I noticed a large vertical split in one of the posts for the guardrail (see feature photo).  Being of the forensic persuasion, I gave a quick thought as to “why did that happened?”  (Yes, that’s what we forensic engineers do when we see something that is not right.  Well, at least that is what I do.)

I could see the problem immediately, so, I pulled out my cell phone and photographed the condition for later consideration.

The split occurred at a thru-bolt (Figure 1).  The thru-bolt caused the split by shifting to the right (again see Figure 1).

Note thru-bolt tight to left side of slot

Figure 1 Note thru-bolt tight to left side of slot.

What would cause the bolt to shift to the right?  Notice that the left edge of the slot is against the left side of the bolt (Figure 1).  Thus, guardrailing is providing the driving force.  Had the slot in the guardrailing been placed such that the bolt was near the center of the slot (as the guardrailing engineer designed it) the guardrailing could have expanded without pushing the bolt to the right.

Why does a guardrailing expand?  Guardrailings are usually made of steel and steel has an expansion coefficient of 0.00065 for every 100 degrees Fahrenheit of temperature change.  That means a 100 foot long steel guardrailing will expand to 100 feet, 3/8 inches when the temperature rises from 30 degrees to 80 degrees:

∆ (increase in length) = 100 feet x 12 inches/foot x 0.00065 x (50 degrees/100 degrees)

Now, with the sun beating down on the railing in July, the temperature could get up to 120 degrees or even more.  And, if a 100 foot long the railing was installed on cool fall or spring day, say 50 degrees, the increase in length becomes over ½ inch.

And now you can understand why the bolts are set in a slot, preferably near the CENTER of the slot – to allow the guardrailing to expand in the heat and contract in the cold without pushing the thru-bolts out the sides of the timber posts (Figure 1).

This principle of expansion and contract is true for concrete, masonry, timber, vinyl, plastic, etc. as well.  We see this principle in action when a concrete roadway buckles in the summer.  That is why structural engineers and architects specify joints at a particular spacing in long building or paving components.

But, there is more!  Notice that the post to the left does not have a vertical split (see the feature photo graph).  Apparently, if the bolt is set nearer the center of the timber post, the timber material is better able to resist splitting.  The bolt location in the post with the split is visibly nearer to the side of the post (see the feature photograph).

I think I will monitor the situation for a few years to see if and when the timber material weakens enough to split.