Analytical and Experimental Investigation of a Polycarbonate Raised Floor System

Kenneth M. (Mac) Will, Ph.D.Larry Daniels, P.E.Larry Kahn, Ph.D., P.E.Fred MeyerAdam Slapkus

Outline

Description of StructureObjectivesPreliminary Model & ResultsRevised Models & ResultsExperiment & ResultsConclusionsGTSTRUDL Features that Proved Useful & New Features Needed to Improve Productivity

Description of Structure

Raised floor in a military facilityElectrical and ???? under floorLocated in a high bay areaPanel descriptions 9 inch x 9 inch x 2.625 inch high polycarbonate

pedestal panels Carpet panel on top of four panels but no

connection between the panels Panels have ribs, stiffeners, and tubular legs

Sample will be passed around

Objectives

A lift vehicle was needed in high bay area in order to perform maintenance in ceiling (approximate 40 feet above floor)Could vehicle be placed on raised floor? If so, are there any restrictions on vehicle?

Manufacturer’s Data on Panel

Concentrated load performance 600 lbs can be applied to any 1 inch square area

Uniform load 75,000 lbs applied uniformly over entire panel

Ultimate load 2,000 lbs on any 1 inch square area without

failure

Preliminary Model

Simple model with smeared thickness for the ribs revealed that deflections were excessive. Was the model accurate? Was the manufacturer’s data in error?Decision was made to more accurately model the panel.

Refined model

Model one-quarter of model assuming symmetric BC on edges and 600 lb load applied over 1 inch square area at center of panel.Material properties for the polycarbonate were not available from the manufacturer so values were obtained another source: E 334,000 psi Poisson’s ratio 0.38

Refined model (quarter panel)Assumed boundary conditions at base of legs

Two different sets of boundary conditions were used at the base of the leg(s) in order to “bracket” the results Assume the base of the legs completely restrained

against vertical and horizontal translations (pinned).

Assume the base of the legs completely restrained vertically but allowed to move horizontally. Note: the symmetric BC’s prevented instabilities.

Refined model (quarter panel)

SBHQ6 and SBHT6 elements used 1924 joints 1935 elements

Mesh shown on next slides

Quarter panel model

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Quarter panel model(another view)

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Displacement Contour

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Conclusions from these analyses

Displacements reasonableStresses okReactions invalid – hold down reactions which can’t occur in real structure since panel is just resting on the floor.Model supports at base of legs using nonlinear springs. Results on next slides.

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Displacement contour with nonlinear springs at bottom of leg

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SXX Contour for plate with NL springs

Half Panel Model needed

Most critical location of wheels is with the wheels near the edge of panel and half-way between the legsHalf – panel model developed as shown on next slides.

Half Panel Model

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Half Panel Model Results

Deflections of approximately 0.25 inches with 600 lb load and a stress of 15,000 psi when load near edge.A method was needed to distribute the load more directly to legs.First proposed steel plate but it was decided that maintenance personnel would not use steel. Structural plywood was then proposed as maintenance personnel have it readily available.

Modeling Plywood

Add ¾” plywood panel on top of floor panel Assume plywood is isotropic and assume value of

E = 1,000,000 psi

Connect plywood to top of floor panel using compression only membersHow to create all of these compression only members? This will be discussed near end of presentation.Half panel model with plywood and compression only members shown on next slide

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Plywood panel

Compression Only Members

Results Using Plywood

Deflections reduced to appx. 0.1 inches when 600 lbs near edge and stresses reduced to appx. 2,200 psi.Decision made to test panel in lab with actual wheels from scissor lifts with and without plywood.

Experimental Program

Decision was made to test panels with wheels from two scissor lift vehicles: A 10 inch wheel simulating a lift with a 600 lb

service wheel load. This wheel was made of a hard plastic and was 2½ inches wide. This wheel was designated the “small” wheel.

A 16 inch wheel simulating a lift with a 2000 lb service wheel load. This wheel had a rubber tire and was 5 inches wide. This wheel was designated the “large” wheel.

Experimental Program (cont)

Wheels were located at the center of the panel and at the edge of the panelTesting was performed with and without a ¾ inch plywood panel on the top of the floor panel

Experimental Program (cont)“small” wheel

Without plywood Load at center

Deflection of 0.151 inches at center with load of 623 lbs. No permanent deflection when load was removed

Load at edge Deflection of 0.268 inches at edge with load of 620 lbs. Slippage at the bottom of the legs noted. Permanent deflection of 0.018 inches under load

Experimental Program (cont)“small” wheel without plywood

Ultimate test Load applied at center of panel and

increased up to a failure load of 1,573 lbs. Deflection at center was 0.4 inches. Legs exhibited considerable liftoff and rotation.

Experimental Program (cont)“small” wheel

With plywood Load at center

Deflection of 0.047 inches at center with load of 611 lbs.

No permanent deflection when load was removed

Load at edge Deflection of 0.086 inches at edge with load of

618 lbs. No permanent deflection

Experimental Program (cont)“large” wheel

Without plywood Load at center

Deflection of 0.368 inches at center with load of 2000 lbs.

A permanent deflection of 0.18 inches at center when load was removed.

Load at edge Deflection of 0.84 inches at edge with load of 1963 lbs. Significant rotation and the rib located under the load

between the legs failed.

Experimental Program (cont)“large” wheel (cont)

With plywood Load at center

Deflection of 0.036 inches at center with load of 2000 lbs.

No permanent deflection when load was removed.

Load at edge Deflection of 0.148 inches at edge with load of

2008 lbs. No permanent deflection.

Experimental Program (cont)“large” wheel (cont)

Ultimate test on two different panels with plywood: Load at center

Ultimate load of 10,650 pounds Load at edge

Ultimate load of 4,303 pounds

Conclusions from Experimental Program

Plywood panel successfully distributed the load more directly to the legsBoth proposed scissor lifts (large and small wheels) would work with ¾ inch plywood overlay.Manufacturer’s data is suspect. Need data on experiment’s performed by manufacturer. They may have used plate on top of panel that distributed load directly to legs.

Deflection Comparison betweenExperiment and Analysis

0.0850.1482000edge

yesLarge

0.0610.1222000center

yesLarge

0.330.248600edge

noSmall

0.1810.151600center

noSmall

0.0970.086600edge

yesSmall

0.0210.047600center

yesSmall

Analytical(in)

Experiment(in)

Load(lbs)

PlywoodWheel

Conclusions

Comparison between analytical and experimental results “good” for panels without plywoodAnalytical model “too stiff” in most cases when plywood added Why?

Assumed isotropic properties for plywood and assumed properties for panel

Assumed linear behavior for panel. Unknown friction at bottom of panel legs. This was

neglected in the analysis. Load distribution from wheels was assumed to be

uniform over the contact area between the wheel and the floor panel or the wheel and the plywood

GTSTRUDL Features that Proved Useful

Construction Points and Lines useful for generating the various regions of the model (ribs, plate, legs). Defining the spacing using Curve defined Spacing was very useful to ensure that meshed regions matched. See next slide.Copy model useful when going from quarter to half plate

Construction Points & Lines

GTSTRUDL Features that Proved Useful (cont)

Resequencing ID’s of top plate of panel and then using commands to create compression only members proved useful.Again Boundary Outline in Scope Environment helped identify regions where model was connected incorrectly. See next slide.

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Inches Pounds

Boundary Outline from Scope EnvironmentFormat Boundary OutlineAgain Boundary

GTSTRUDL Features that Proved Useful (cont)

Check Model Load Summation verified load was applied correctly Check duplicates & floating joints View feature was indispensable in being able to isolate parts of model and then returning later.

Views created

New Features Needed to Improve Productivity

Create a view of the Current Display (this will be added in Version 26)Boundary Outline Display in GTMenuMirror option in Copy Model Nonlinear Springs & Compression Only Effects needed to be preserved in GTMenu & created in the Input File generated in GTMenu.