ce 163 - 4
DESCRIPTION
steel design 4TRANSCRIPT
Compression Members
COMPRESSION MEMBERS
Compression members are structural elements that are subjected only to axial compressive forces. The most common type of compression member occurring in buildings and bridges is the column. Compression members are also used in trusses and as components of bracing systems.
COLUMN THEORY • CRITICAL BUCKLING LOAD • Leonhard Euler • Failure may occur by
compressive yielding rather than buckling.
• Prior to failure, the compressive stress will be uniform over the cross secMon at any point along the length, whether the failure is by yielding or by buckling.
• Euler Load (Euler buckling load)
A: cross-‐secMonal area r: radius of gyraMon with respect to the axis of buckling L/r: the slenderness raMo and is the measure of a member’s slenderness, with large values corresponding to slender members. At this compressive stress, buckling will occur about the axis corresponding to r.
For less slender members:
• Friedrich Engesser (1889) • Small slenderness raMo for
members, the stress at which buckling occurs is greater than the proporMonal limit of the material, the relaMon between stress and strain is not linear.
Effec<ve Length Euler and tangent modulus equaMons assumpMons: 1. The column is perfectly straight, with no
iniMal crookedness. 2. The load is axial, with no eccentricity. 3. The column is pinned at both ends.
KL: effecMve length K: effecMve length factor
CODE REQUIREMENT The nominal compressive strength is:
LRFD:
ASD:
NSCP Note: For members designed on the basis of compression, the slenderness raMo KL/r preferably should not exceed 200.
LOCAL STABILITY
LOCAL BUCKLING Two types of elements must be considered: UnsMffened elements, which are unsupported along one edge parallel to the direcMon of load, and sMffened elements which are supported along both edges. For compression members, shapes are classified as slender or nonslender. If a shape is slender, its strength limit state is local buckling, and the corresponding reduced strength must be computed.