Discussion- Design Concepts for Jib Cranes
DISCUSSION Design Concepts for Jib Cranes
Paper by JAMES FISHER and STEVEN THOMAS (2nd Q. 2002) Discussion by MICHAEL HEMSTAD
I
thank the authors for a very thorough and informative article on jib cranes. To this, I would add the following regarding maximizing the crane’s effect for column design. For a single jib crane on a column, the position of the jib crane resulting in the maximum effect on the column can be calculated as follows. The position of the jib crane is described by the angle θ, defined as 0° for a wide-flange column when the jib is turned so as to maximize column minor axis bending and torsion, and 90° when the jib is oriented to maximize column major axis bending (i.e. parallel to the column web). For an HSS column, the major axis is defined (for the purposes of this discussion) as the axis bent by the jib positioned at θ = 90°. I hope the reader will forgive the circular definition. To clarify, if the jib brackets are mounted on the 10 in. face of an HSS 8 × 10 × ½, the major axis modulus Sx as used herein will be less than the minor axis modulus Sy. Summarizing the notation of the article (to which reference should be made): RH = Horizontal reaction of the jib crane at the hinges. P = Vertical load (lifted load and weight of hoist). Some allowance should be made by the designer for the jib selfweight as well. A, B, C = Vertical dimensions along the column from upper hinge to roof framing, distance between hinges, and from floor to lower hinge respectively. X = Maximum distance from the hoist to the hinge axis of the jib (essentially, the length of the jib). D = Distance from the hinge axis to the face of the column.
dc = Column depth. tf = Column flange thickness. For an HSS column, torsion manifests itself as a shear stress in the walls (fs = T/2bht) and thus is not directly additive to axial and bending stresses. This shear stress should be evaluated separately by the designer and, if significant, should be checked along with bending
Paper by JAMES FISHER and STEVEN THOMAS (2nd Q. 2002) Discussion by MICHAEL HEMSTAD
I
thank the authors for a very thorough and informative article on jib cranes. To this, I would add the following regarding maximizing the crane’s effect for column design. For a single jib crane on a column, the position of the jib crane resulting in the maximum effect on the column can be calculated as follows. The position of the jib crane is described by the angle θ, defined as 0° for a wide-flange column when the jib is turned so as to maximize column minor axis bending and torsion, and 90° when the jib is oriented to maximize column major axis bending (i.e. parallel to the column web). For an HSS column, the major axis is defined (for the purposes of this discussion) as the axis bent by the jib positioned at θ = 90°. I hope the reader will forgive the circular definition. To clarify, if the jib brackets are mounted on the 10 in. face of an HSS 8 × 10 × ½, the major axis modulus Sx as used herein will be less than the minor axis modulus Sy. Summarizing the notation of the article (to which reference should be made): RH = Horizontal reaction of the jib crane at the hinges. P = Vertical load (lifted load and weight of hoist). Some allowance should be made by the designer for the jib selfweight as well. A, B, C = Vertical dimensions along the column from upper hinge to roof framing, distance between hinges, and from floor to lower hinge respectively. X = Maximum distance from the hoist to the hinge axis of the jib (essentially, the length of the jib). D = Distance from the hinge axis to the face of the column.
dc = Column depth. tf = Column flange thickness. For an HSS column, torsion manifests itself as a shear stress in the walls (fs = T/2bht) and thus is not directly additive to axial and bending stresses. This shear stress should be evaluated separately by the designer and, if significant, should be checked along with bending