# Problem 710 | Two simple beams at 90 degree to each other

**Problem 710**

Two timber beams are mounted at right angles and in contact with each other at their midpoints. The upper beam A is 2 in wide by 4 in deep and simply supported on an 8-ft span; the lower beam B is 3 in wide by 8 in deep and simply supported on a 10-ft span. At their cross-over point, they jointly support a load P = 2000 lb. Determine the contact force between the beams.

**Solution**

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R = contact force between beams A and B

Subscript (

_{A}) = for upper beam

Subscript (

_{B}) = for lower beam

Moment of inertia

$I_A = \dfrac{2(4^3)}{12} = \dfrac{32}{3} \, \text{ in}^4$

$I_B = \dfrac{3(8^3)}{12} = 128 \, \text{ in}^4$

Note:

The midspan deflection of a simple beam loaded with concentrated force at the midpoint is given by

See Case No. 6 in the Summary of Beam Loadings.

The midspan of upper beam A is under 2000 lb applied load and contact force R. R will act upward at beam A.

$\delta_A = \dfrac{2000(8^3)(12^3)}{48E(\frac{32}{3})} - \dfrac{R(8^3)(12^3)}{48E(\frac{32}{3})}$

$\delta_A = \dfrac{3\,456\,000}{E} - \dfrac{1728R}{E}$

The lower beam B is subjected by the contact force R at midspan.

$\delta_B = \dfrac{R(10^3)(12^3)}{48E(128)}$

$\delta_B = \dfrac{1125R}{4E}$

The deflections of upper beam A and lower beam B are obviously equal. R will act downward at beam B.

$\delta_A = \delta_B$

$\dfrac{3\,456\,000}{E} - \dfrac{1728R}{E} = \dfrac{1125R}{4E}$

$3\,456\,000 = \dfrac{8037R}{4}$

$R = 1720.04 \, \text{ lb} \,$ *answer*

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