Calculo metalico

Beam-Column Base Plate Design— LRFD Method
RICHARD M. DRAKE and SHARON J. ELKIN

INTRODUCTION It is common design practice to design a building or structure beam-column with a moment-resisting or fixed base. Therefore the base plate and anchor rods must be capable of transferring shear loads, axial loads, and bending moments to the supporting foundation. Typically, these beam-column base plateshave been designed and/or analyzed by using service loads1 or by approximating the stress relationship assuming the compression bearing location.2 The authors present another approach, using factored loads directly in a method consistent with the equations of static equilibrium and the LRFD Specification.3 The moment-resisting base plate must have design strengths in excess of the requiredstrengths, flexural (Mu ), axial ( Pu ), and shear (V u ) for all load combinations. A typical beam-column base plate geometry is shown in Figure 1, which is consistent with that shown on page 11-61 of the LRFD Manual.4

where: B N bf d f m base plate width perpendicular to moment direction, in. base plate length parallel to moment direction, in. column flange width, in. overall column depth, in. anchorrod distance from column and base plate centerline parallel to moment direction, in. base plate bearing interface cantilever direction parallel to moment direction, in. m n N 0.95d 2 (1)

base plate bearing interface cantilever perpendicular to moment direction, in. n B 0.80b f 2 (2)

x

base plate tension interface cantilever parallel to moment direction, in. x f d 2 tf 2 (3)

tfcolumn flange thickness, in.

Fig. 1. Base Plate Design Variables

Richard M. Drake is Principal Structural Engineer, Fluor Daniel, Irvine, CA. Sharon J. Elkin is Structural Engineer, Fluor Daniel, Irvine, CA.

The progression of beam-column loadings, in order of increasing moments, is presented in four load cases. Case A is a load case with axial compression and shear, without bending moment.This case results in a full length uniform pressure distribution between the base plate and the supporting concrete. This case is summarized in the LRFD Manual4 beginning on page 11-54 and is summarized herein for completeness. Case B evolves from Case A by the addition of a small bending moment. The moment changes the full length uniform pressure distribution to a partial length uniform pressuredistribution, but is not large enough to cause separation between the base plate and the supporting concrete. Case C evolves from Case B by the addition of a specific bending moment such that the uniform pressure distribution is the smallest possible length without separation

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between the base plate and the supporting concrete. This corresponds tothe common elastic limit where any additional moment would initiate separation between the base plate and the supporting concrete. Case D evolves from Case C by the addition of sufficient bending moment to require anchor rods to prevent separation between the base plate and the supporting concrete. This is a common situation for fixed base plates in structural office practice. That is, a rigid framewith a fixed base plate will usually attract enough bending moment to require anchor rods to prevent uplift of the base plate from the supporting concrete. CASE A: NO MOMENT—NO UPLIFT If there is no bending moment or axial tension at the base of a beam-column, the anchor rods resist shear loads but are not required to prevent uplift or separation of the base plate from the foundation. Case A, abeam-column with no moment or uplift at the base plate elevation, is shown in Figure 2.

1. Assume that the resultant compressive bearing stress is directly under the column flange. 2. Assume a linear strain distribution such that the anchor rod strain is dependent on the bearing area strain. 3. Assume independent strain distribution. All three methods summarized by AISC5 assume a linear...