Fall Protection Systems

Fall Arrest Anchor Analysis Aaron Randal

Andy Cash Tiruneh Haile

December 7, 2005

Introduction

Existing Roof and Anchor Conditions

Anchor Protruding Through Roof Sheeting and Weather Proofing

Interior View of Beam Rafters, Roof Purlins and Existing Anchor

Close-up View of Anchor and Attachment Loop

Goal Statement

To evaluate the condition of the existing roof anchor

and verify compliance with OSHA regulations for Fall

Arrest Systems.

Analysis Action Plan

Identify Parameters of Interest

Deflection

Maximum Stress

Verify Elastic Range Model

Supporting Conditions

Eliminate Unnecessary Part Features

Attachment Loop

Roof Sheeting

Purlins

Determine Applicable Loads – OSHA Summary

OSHA Summary Working heights of 6 FT or more require fall protection

Fall Protection Methods

o Engineering Control

Platforms and Railing

o Fall Protection

Fall Arrest

Fall Arrest System

o System Components

Full Body Harness

Connection Devices

• Dee-rings

• Lifelines (Steel Cable)

• Lanyards

Tie-off Point (Steel Pipe Anchor and Underlying

Structure)

Fall Arrest System Requirements

o Lanyard (Shock Absorbing)

5,000 lb minimum breaking strength

1800 lb maximum arrest force during deployment

o Lifeline

5,000 lb minimum breaking strength

o Tie off Point

Rigid and not deflect greater than 0.04 IN (1mm) for an

applied force of 2,250 lb (10 kN)

Theoretical Background

Linear Stress Strain Relation Ship

εσ *E=

Stress, Force and Area Relation

AF /=σ

Strain Relation

ll /∆=δ

Force and Deformation Relation

ll

AEF ∆= *)(

Load Matrix

{ } [ ]{ }UKF =

Shape Function for Solid92

Analysis Details Material Properties Modulus = 29e6 10^6 psi

Yield Strength = 36 ksi (tension), 21 ksi (shear)

Ultimate Strength = 58 ksi (tension)

Static Analysis Single Pipe Run

Pipe Material and Size Verification 4” Nominal Pipe Size Deflection < 0.04”

Existing anchor pipe fixed to 1 ft square steel plate. The plate is an extruded with a 1 in thickness. The volume was free meshed at the default setting. All nodes in the four side planes and bottom plane were then fixed with zero displacement in all degrees of freedom. A 2250 lbf was attached to a single node at approximately the center of the top of

the anchor. The analysis was run and the results were reviewed. The pipe section was found to satisfy the displacement limitations. Boundary Conditions, Mesh and Load Application

Post Processing Results

Pipe and 36” Beam Section Run – Displacement/Yield Test

36” W12x50 Wide Flange Beam – 2250 lbf Load in Positive X Direction

Displacement at Tip Exceeds Allowable 0.04” – Reinforcement or Redesign Required

36” W12x50 Wide Flange Beam – 2250 lbf Load in Positive X Direction

Von Mises Stress Not Exceeding σY = 36,000 lbf/in2

Reinforced Pipe/Beam System – Displacement/Yield Test

Reinforced W12x50 Wide Flange Beam with 2250 lbf

Maximum Allowable Tip Displacement (0.04”) Exceeded by Factor of 2

Reinforced W12x50 Wide Flange Beam with 2250 lbf Load -

Von Mises Stress Not Exceeding σY = 36,000 lbf/in2

Reinforced Pipe/Beam System – Fracture Test

W12x50 Wide Flange Beam

Von Mises Stress 5000 lbf Static Force – Ultimate Strength (50 ksi) Exceeded at Pipe and Beam Connection

W12x50 Wide Flange Beam

Tip Displacement - 5000 lbf Static Force Summary The pipe anchor alone complies with OSHA requirements. When connected to the standard wide flanged beam the tip displacement of the anchor exceeds the allowable dimension. After reinforcing the sides of the beam by welding quarter inch plating the anchor system no longer deflects excessively at the tip. With an applied load of 5000 lbf the stresses in the anchor system exceed the ultimate strength for this material. Further reinforcement of the anchor system is required or more appropriately the system should be redesigned with higher grade materials.

Acknowledgements

I. Finite Element Analsys, Moaveni 2003

II. Mechanics of Materials, Beer & Johnston 2002

III. ANSYS Manual

IV. www.machinedesign.com

V. U.S. Department of Labor

VI. Lab Safety Supply