3. Calculate the limiting compressive stress in column at zero slenderness ratio F* from the equation:

F* — Fc (Cd)(Cm)(C)(Cf)

where Cd, Cm, Ct, Cf are defined tables (see Tables 3.4a, b, 3.7, and 3.8)

4. Calculate the column stability factor Cp from the formula:

C — 1 + Fce/F*E //1 + FcE/F*; V Fce/F*e

p 2 x 0.8 2 x 0.8 / 0.8

5. The allowable compressive stress F’c in the strut is given by

Fc — F* (Cp)

6. If F’c < FcE, this means that the selected cross section is not enough to resist buckling. So increase the size of the cross section and go iteratively through steps 1 to 6 until you get Fce < F’c.

7. The maximum load that can be carried by the strut is the product of FC and the actual (not the nominal) cross­sectional area of the selected strut.

8. The maximum spacing of struts in feet that can be carried by one strut is obtained by dividing the maximum load by strut load per foot.

It should be noted that the strut usually carries compression or tension force depending on the direction of the horizontal load applied to the form. Those two forces are equal in magnitude but differ in their sign. Designing struts as compression members usu­ally ensures that they are safe also in tension because we are con­sidering an additional precaution against buckling associated with compression.

EXAMPLE 1

Design formwork for a 15-ft-high concrete wall, which will be placed at a rate of 4 ft/h, internally vibrated. Anticipated tempera­ture of the concrete at placing is 68°F. Sheathing will be 1 in. thick (nominal) lumber and 3000-lb ties are to be used. Framing lumber is specified to be of construction grade Douglas Fir No. 2.

SOLUTION: