Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

Downloaded From : www.EasyEngineering.net
Design of Wood Members for Formwork 99
For a beam continuous over more than two equally spaced supports,
with a uniform load distributed over its full length, the maximum
total shear is frequently determined as an approximate value from the
following equation:
V = 5wL/8
= 5wl/96 lb (5-15)
For a simple beam with one or more concentrated loads acting
between the supports, the maximum total shear will occur at one end
of the beam, and it will be the reaction at the end of the beam. If the
two reactions are not equal, the larger one should be used.
Example 5-4
Consider the 2 × 6 beam selected in Example 5-1 that supports three
equally concentrated loads of 190 lb each. Check this beam for the
unit stress in horizontal shear.
ww.EasyEngineering.n
The maximum shear force can be calculated as:
V = [3 × 190 lb]/2
= 285 lb
From Eq. (5-13), the applied horizontal shear stress is
f v
= 3V/2bd
= [3 × 285 lb]/[2 × 1.5-in. × 5.5-in.]
= 51.8 lb per sq in.
Table 4-2 indicates an allowable horizontal shear stress for a 2 × 6
No. 2 grade Southern Pine as 175 lb per sq in., which is greater than
the applied shear stress of 51.8 lb per sq in. Therefore, the 2 × 6 beam
will be satisfactory in horizontal shear.
Example 5-5
Consider the beam previously selected to resist the bending moment
in Example 5-2. It was determined that a 3 × 8 S4S beam of No. 2
Downloaded From : www.EasyEngineering.net