WATIOXAL ADVISORY COMMITTEE FOR AEROEAUTICS---I- TECH1JICAL EOTE NO. 530 3i3iuDING STRESSES DUE TO TPORSION 113 GIMTILJZVER BOX BEAMS

The paper begins with a brief discussion on the ori-gin of the bending stresses in cantflever box beams under torsion. A orltical survey of existing theory is followed by a summary of design formulas; this summary is based on the most complete solution publfshed but omits all refinec ments consfdered unnecessary at the present state of de-velopment. Strain-gage tests made by the X‘A.C.A. to ob-tafn some experimental verification of the formulas are described next. Finally, the formulas are applied to a series of box beams previously static-tested by the U.S. Army Air Corps; the results show that the bending stresses due to torsion are responsible to a large extent for the free-edge type of failure frequently experfenced in these tests,-IYTRODUCTION The problem of designing a box beam in torsion is common in aircraft construction. If all cro$s sections of the beam are free to warp out of their-plane, the walls will be in pure shear, which can be easily calculated. If, however, the cross sections are partly or complete_lg restrained from warping, which is the case if any vayia-tion of cross seotion or of loadfng occurs along the span, then bending stresses will arise in addition to the shear stresses. These bending stresses may be very large at the root of a cantilever box attached to a rigfd support; since direct bending stresses usually exist also, the calm culation of bending stresses due to torsion is important. The theoretical analysis of this problem has been made fafrly recently,- and it is the purpose of this gaper to give a critical survey of existing literature and a summa-ry of design formulas. The reliability of these formulas was checked by some strain-gage tests, whfch are described and discussed. Finally, ' the paper shows the results of applying the formulas to a series of duralumin box beams...----:; i-a...-...a