DECO - Maintenance Technology Reprint

MAINTENANCE LOG
Helpful maintenance information and problem solving case studies from suppliers of plant equipment and services.
Foundation
Anchor Bolt
Design

I. Establishing Design Criteria

Foundation bolts that connect plant equipment to concrete foundations must be properly selected and installed to assure machinery and foundation perform as expected. Here is an outline of how to approach the design problem.
     Work associated with the design of an anchoring system can be divided into three parts: Establishing anchor bolt design criteria, calculating anchor bolt design strength, and planning anchor bolt installation. This article covers design criteria.
     Anchor bolt layout. The first design step is to determine the anchor bolt layout pattern: circular, square, or rectangular. If a machine is purchased, the anchor bolt pattern will be determined by the equipment supplier.
     Anchor bolt pattern is determined by plant personnel if they design a machine. The best anchor bolt layout is symmetrical because it will help eliminate installation errors. When a symmetrical layout is not possible, the pattern should be designed to make the lack of symmetry obvious. For example, the anchor bolt group could easily be rotated 90 deg during installation and remain unnoticed if the spacing in one direction from the axis is only a 1/2 in. less than in the other direction from the axis.
     Calculating design loads. Most machinery installations are subject to a combination of forces which must be summarized, see diagram.

Vertical forces: Vertical forces (F_v) are summed with downward forces (F⁽⁺⁾) being positive and upward forces (F^(-)) being negative, using the following equation:
F_v = F⁽⁺⁾ + F^(-)
Equation symbols are further defined in the diagram.
Horizontal forces: Horizontal forces (F_H) must be summed about a particular axis (x or y). With the summation of horizontal forces about each axis, the vector sum of horizontal forces acting on the anchor bolt group can be found using the following equation:
F_H = F_HX² + F_HY²
Overturning moments: Overturning moments (M_R) are caused by moments applied by the machine or by horizontal forces applied above the machine base. When there are multiple overturning moments, the moments must be defined to a particular axis (x or y). With a summation of each moment about an axis, the resulting moment about the anchor bolt group can be found using the following equation:
M_R = M_x² + M_y²
Torques: Torque applied to the anchor bolt group is the sum of all torque applied by the machine or horizontal forces applied off center from the center of gravity of the anchor bolt group. Total torque can be found using the following equation:
T_T = T + (F_H x D₁)

TYP. Foundation Forces

M (x or y) = Overturning Moments

T = Torque

F (- or +) = Vertical forces

F_H (x or y) = Horizontal forces

C.G. = Center of gravity of anchor bolt group

D₁ = Distance off center of anchor bolt group

D₂ = Distance above machine base

a or b = Distance of anchor bolts from X or Y axis

Resulting vertical force, horizontal force, overturning moment, and torque must be calculated for the foundation bolt pattern before bolt strength can be specified.

Maximum design load. After all forces, moments, and torques have been calculated, the maximum load must be determined when all forces are applied to the anchor bolt group.

Maximum Axial Force =
(F_V/N) ą (M_RC/I)
where
F_V = vertical forces
N = number of anchor bolts
M_R = resulting moment
C = distance of anchor bolt from neutral axis
I = moment of inertia of anchor bolt group

A positive value for the axial force is a compressive force acting on the anchor bolts; a negative value is a tensile force (uplift).

Maximum Shear Force =
(F_H/N) + (T_T/NR)
where
F_H = horizontal forces
N = number of anchor bolts
T_T = total torque
R = radius of anchor bolt from centroid

     Standard handbooks on static and strength of materials provide further details on the formulas involved in calculating anchor bolt design loads.
     Factor of safety. Because of the many elements involved in an anchor bolt system design, a factor of safety must be applied to protect the anchors and foundation from an overload situation or ultimate failure.
     An acceptable factor of safety for the ultimate strength of an anchor bolt would be 4 for a pure static loading condition and 8 for a loading condition involving dynamic effects (such as those from vibratory or reciprocating equipment with stress reversal of the anchor bolt group). The above factors of safety would apply when the loads or forces of the machine are well defined.
     When the forces of a machine cannot be well defined because of lack of information from the supplier or because the machine is complex, a load multiplier should be applied to the forces, moments, and torques used to calculate bolt requirements. The load multiplier need only be applied to the loads on the machine from the live-load forces because dead-load forces can be easily defined in machinery foundations.
     The load multiplier used is determined by the designer and would vary between 1.5 to 2, depending on the degree of uncertainty.
     Another situation justifying a load multiplier would be a complex foundation, which would have catastrophic effects or high breakdown cost if an overload situation occurred. Again, the designer determines the magnitude of the multiplier used. However, the magnitude of the multiplier should never be greater than 5, because this value would lead to unrealistic designs.
     A future article will examine anchor bolt design strength and anchor installation.

Material furnished by DECO Manufacturing Co., Decatur, IL

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