Timber for Lintels

[Crazy Cutter]

 

So, anyone had any good results hollowing out a beam to cover an RSJ?

 

Yes and look good in place!

[agg221]

With respect, stop and think for a moment vs. large structures made of reinforced concrete or wire/rod pre-stressed concrete, etc, etc, if it was the case of what you say it would be bloody stupid to put steel in concrete wouldn’t it if they were dissimilar in expansion and contraction property under normal temperatures

 

b101uk - Pete Bannister is right on this one. The property in question is the coefficient of thermal expansion (CTE), expressed in mm expansion per mm length per degree Kelvin in temperature. The number quoted is usually multiplied by 10 to the power minus 6. Expansion is non-linear over a wide temperature range, so CTE is a temperature dependent property, but within normal environments it's pretty linear so a value to 2 significant figures is reasonable.

 

Ordinary construction steels are about 14, concrete ranges from 8 to 12 (depending on aggregate used) and wood is about 4 to 5.

 

Absolute expansion doesn't matter so much, it's differential expansion that matters (subtract one from the other). For concrete, this gives variation differential thermal expansion from 2 to 6. To put this in context, in a 10m length of reinforced concrete, increasing from 0degC to 30degC the steel could increase in length by up to 2mm more than the concrete.

 

In pre-stressed concrete, the clue is in the name. The steel is put in tension so that it puts compressive stress in the concrete. As it warms up, the stress reduces, however the initial imposed stress is deliberately high enough that the stress is not relaxed below the design limit within the anticipated temperature range. As temperature decreases, the stress increases.

 

These factors impose practical design limits on the maximum component size you can make without expansion joints. Coincidentally they have a similar effect on train tracks, which are also pre-stressed, so there's a maximum length of welded rail you can make and in the record heat of summer about 5yrs ago (when it went over 100) there were some trains that couldn't run as it was outside design specification for the UK.

 

Hope this clarifies!

 

Alec