Load Testing of Temporary Structural Platforms

Transcription

1 1820 Loa Testing of Temporary Structural Platforms Ralph E. Bennett III, PE, Harvey Abramowitz, John H. Bennett, Rick J. Henrickson, Carris Koultouries, Walter Kucharski an Branon W. Treway Purue University Calumet Hammon, IN Abstract Scaffoling planks are wiely use in the construction fiel as temporary platform support for workmen an materials. These scaffoling planks unergo ynamic impacts aily ue to the weight of people an materials roppe onto them. These ynamic loas are not taken into account by professional an trae organizations, engineers, contractors an workers. This inclues the Occupational Safety an Health Aministration (OSHA) Hanbook, which only lists the static loa limits. Therefore, a stanarize ynamic loa proceure was evelope. Base on this proceure, an experiment is escribe, which can be use as a laboratory exercise for a course in strength of materials. Introuction In recent years, attention has been irecte to the effects of ynamic loaing on soli-sawn an composite woo planks. Interest has multiplie because the scaffol platform material is the weakest link of any temporary structure. Because current platform esign is base on static loaing, 1 this proceure oes not consier normal platform usage. Common usage always inclues application of ynamic loaing, such as workers jumping, materials being roppe, or loa hanling equipment hitting the platform from a higher level. To reuce the current esign factors of safety against failure, each occurrence must be aresse an evaluate to get a realistic esign factor of safety. A stanarize ynamic loa proceure nees 2 to be evelope to estimate the potential effect of the ynamic loaing. Therefore, an investigation was unertaken to (1) esign an buil a testing apparatus; (2) evelop a stanarize testing proceure; (3) etermine the theoretical an actual ynamic results for soli sawn an manufacture woo platform; (4) compare theoretical an actual static an ynamic loaing results; an (5) evelop ynamic loaing criteria. The results from this investigation have recently been reporte. 3 The experiment escribe here is base on these results, which are a combination of the work one by two engineering senior esign teams an a senior project in the Department of Construction Technology. The Page

2 objectives of this experiment are to: (1) introuce the stuent to the concept of ynamic loaing, as compare to static loaing, (2) emonstrate a metho for etermining ynamic loaing, (3) compare theoretical an actual static an ynamic loaing results, an (4) evelop an recommen ynamic loaing criteria. The faculty avisors involve with this work inten to introuce this experiment as part of the laboratory exercises in a strength of materials course. Theory Static Loaing Three Point Ben Test For a rectangular soli material such as a scaffoling plank, a three point ben test can be use to etermine the flexural moulus, or moulus of elasticity (MOE) in bening. The results from this test are plotte similarly to a stress-strain curve, with the eflection replacing strain. The MOE in bening is etermine in the following manner. The maximum eflection of a plank for a given loa is: 4 Y max = PS 3 / 48EI Eq. 1 where Y max = maximum eflection of plank at a given loa P = applie static loa S = supporte span length E = moulus of elasticity (MOE) I = moment of inertia The moment of inertia for a rectangular cross section is: 5 where w = plank with t = plank thickness Substituting into Eq. 1, gives: I = wt 3 /12 Eq. 2 Y max = PS 3 /4Ewt 3 Eq. 3 Solving for E: E = PS 3 /4wt 3 Y max Eq. 4 This final relationship can be use to etermine the static MOE. The above is the most typical metho for fining the flexural moulus. Another way of etermining this moulus is a vibration technique. 6 In this metho the plank is simply supporte at its ens an is loae at mi-span. The natural frequency of vibration is use to fin the bening MOE. Page

3 Vibration Analysis The Static Testing Apparatus, Vibration Analysis Machine, mae by DynaMOE tm works as follows. 7 A boar or panel is lai on a knife-ege support at one en, an a loa cell at the other. By pressing the member own at mi-span an releasing it, the member vibrates vertically. The loa cell senses the changing reaction force an the A/D car acquires the electronic signal. DynaMOE tm software recors the signals an analyses the ecaying sine wave. The average recore force is the average reaction force on the loa cell, an hence is half the weight on the member. The frequency of the ecaying sine wave is f. Given the member geometry, weight an frequency, the ynamic moulus of elasticity E is calculate. This measure E is a simpler way of etermining bening MOE for graing purposes, or for quality assurance. The relationship between ynamic moulus of elasticity, E, an frequency, f, of a transversely vibrating prismatic member, simply supporte at its ens, is: E = f 2 W L 3 / 2.46 g I Eq. 5 where an W = weight of the member L = length of the member I = moment of inertia about the axis perpenicular to vibrating irection g = acceleration of gravity 2.46 = constant for simple en supports Eq. 5 is vali for a prismatic member of uniform cross section, such as rectangular beams or panels, or I-joists. Due to the fact that there is some overhang to support the prismatic member, L in Eq. 5 is replace by the span, S, between supports an W is multiplie by S/L to give the weight of the member between supports. This gives: E = f 2 W S 4 / 2.46 g I L Eq. 6 If the member has a rectangular cross-section, with w = with an t = thickness or height, then: E = f 2 r S 4 / g t 2 Eq. 7 where r = weight per unit volume? If E is in psi, S, t an L in inches, f in Hz an r in lb/in 3, then g is 386 in / sec 2. While E is calle the ynamic moulus of elasticity, its value shoul be similar to the usual three point ben test iscusse previously. The wor ynamic refers to the fact that the member is being vibrate. The initial loa applie was one so in a static manner. Dynamic Loaing During ynamic loaing the beam will be consiere to unergo impact loaing. The following is taken from Jensen an Chenoweth. 8 Consier the simple beam in Fig. 1. The impact loa W Page

4 is allowe to fall freely onto the mi-span of the beam. The beam eflects an amount uner this conition, as inicate in Fig. 1. This same eflection coul be prouce by a graually applie loa of sufficient magnitue. This loa will be calle the equivalent static loa, P EQ. Because the eflections an stresses uner the equivalent loa P EQ are the same as those uner the impact loa W, the internal strain energies are the same. The external work one by the loas must also be the same. PEQ W(h + ) = Eq. 8 2 Since the eflection of the beam is proportional to the loa that prouces that eflection: PEQ W = Eq. 9 where is the static eflection uner a graually applie loa W (Fig. 1). Substituting the value of P EQ from Eq. 8 gives: W W(h + ) = or h + = Eq Rearranging gives: Completing the square yiels: 2-2 = 2h Eq = 2 + 2h Eq h or ( - ) = (1+ ) Eq. 13 Taking the square root of each sie gives: - = 1 + 2h 2h or = ( ) Eq. 14 Thus the eflection uner impact loaing is consierably greater than the static eflection for the same loa graually applie. The factor (2h/ can be thought of as an impact factor, which always excees unity. Page

5 Since the stresses in a beam are proportional to the loa that prouces them: s /W = s/p EQ Eq. 15 where s is the stress ue to the graually applie loa W an s is the stress ue to the equivalent loa P EQ. From Eq. 9, P EQ = W(/ ). Substituting for P EQ in Eq. 15: s /W = s /W or s = s (/ ) Eq. 16 2h From Eq. 14, / = ( ). Hence: 2h s = (1+ 1+ )s Eq. 17 The stress s uner impact loaing is, then, the impact factor multiplie by the stress s uner the same loaing graually applie, that is, the static loa. Equipment an Supplies 1. Static Testing Apparatus Compression Tester In this case a Tinius Olsen machine was use. 2. Three point Test Assembly (use in conjunction with compression tester) 3. Static Testing Apparatus Vibration Analysis Machine mae by DynaMOE tm 4. PC, equippe with a A/D boar 5. Dynamic Testing Apparatus 6. Scaffoling Planks a. Soli Sawn Woo Plank i. Surface Finish - Rough an Dresse (surface 4 sies) ii. Types Southern Yellow Pine an Douglas Fir b. Manufacture Woo Plank i. Veneer Type (a) Horizontal (b) Vertical ii. Suppliers (a) 3 Manufacturers Testing Apparatus Static: A DynaMOE tm test apparatus (Figs. 2-6), a vibration analysis machine, was use to etermine plank weight an Moulus of Elasticity (MOE). This metho allows etermination of MOE using ynamic methos. A conventional three-point assembly (Fig. 7) was usually use to Page

6 confirm the static MOE. A Tinius Olsen compression tester with a specially esigne apparatus was use for the ben test (Fig. 8). After ynamic testing was performe the three-point assembly was occasionally use to measure a change in MOE. Dynamic: A newly esigne an built apparatus was use to measure the ynamic force (Fig. 9). The apparatus esigne for plank testing consiste of a steel A-frame, a 250 lb steel weight, a ½ ton hoist, a quick release mechanism, plank supports, a fine fixe weight rop height ajuster, an weight guies. The quick release mechanism for releasing a fixe weight consiste of a 500 lb electromagnet an a DC power supply. The weight guies were mae from two linear bearings running on two col-rolle 1-inch iameter rouns. The fine height ajustment fixture was mae from a 1-ton turnbuckle. The ½ ton hoist an the electromagnet were use to elevate the 250 lb weight. The electromagnet an the weight guies allowe for a free-fall impact at the center of the selecte scaffoling plank. The plank was supporte at the base of the A-frame. A loa cell was installe at one en of the frame supports to measure the ynamic force applie to the plank. The actual ynamic loa was capture on an attache igital peak meter. Stanarize Testing Proceures Static Nonestructive Proceures: 1. set up an calibrate DynaMOE tm test apparatus 2. measure plank imensions an water content 3. vibrate plank; an 4. rea an recor MOE, plank weight an frequency. Static estructive proceures: 1. set up an calibrate the combine Tinius Olsen test frame apparatus 2. measure plank imensions 3. rea an recor plank applie loas at prescribe eflections; an 4. calculate MOE. Dynamic proceures: 1. set up an calibrate the ynamic test apparatus 2. raise test weight 3. set test specimen in place 4. place an release weight at prescribe heights 5. rea an recor peak loas; an prescribe heights 6. repeat steps 2 thru 5 until failure of the specimen occurs. Page

7 Samples Teste Test specimens inclue soli-sawn (rough an resse), horizontally an vertically veneere planks. Soli-sawn planks were Southern Yellow Pine an Douglas Fir. Veneere planks were supplie by three manufacturers (Fig. 10). Sample variables were plank span, height, with, moisture content, stiffness an specific gravity. Results an Discussion Typical results are shown in Fig. 11, where the experimental ynamic loas are compare with the theoretical equivalent static loas. Table I summarizes the range of values obtaine for ifferent planks an shows that the ratios of experimental to theoretical loas varie from 1.1 to 2.4. In aition, the following were foun: (1) Manufacture woo planks have less variable stiffness characteristics then o soli-sawn scaffol planks; (2) Manufacture woo planks have a more preictable ultimate failure loa range than o soli sawn planks; an (3) Laminate joints or laminate veneer placement in manufacture planks, like annular rings spacing, knot size an placement of soli sawn scaffol planks, prove to affect specimen ynamic resistance. Dynamic Loaing Criteria Base on ratios of actual peak to theoretical loa a safety factor of three is recommene to prevent failure. Implementation This newly esigne experiment will be introuce into future materials testing laboratories in the Departments of Engineering an Construction Engineering Technology. Summary 1. The stuent is introuce to the concept of ynamic loaing. 2. A metho for etermining ynamic loaing was emonstrate. 3. A comparison of equivalent static an actual ynamic loas was mae. 4. A safety factor of at least 3 is recommene as the ynamic loaing criteria. 5. This experiment will be incorporate into the laboratory component of a strength of materials course. References 1. Occupational Safety an Health Aministration, OSHA Safety an Heath Stanars Page

8 for the Construction Inustry, Commerce Clearing House Inc., Chicago, IL, June W. C. Duncan an R. Bennett, Dynamic Woo Plank Testing, Duncan, McCormick an Associates, Costa Mesa, CA, Oct R. E. Bennett III, H. Abramowitz, J. H. Bennett, R. J. Henrickson, C. Koultouries, W. Kucharski, an B. W. Treway, The Static an Dynamic Loaing on Scaffoling Planks, Eighth International Conference on Composites, Tenerife, Spain, August 2001, pp E. F. Byars, R. D. Snyer, an H. L. Plants, Engineering Mechanics of Deformable Boies, New York, NY, Harper an Row, 1983, p Ibi., p M. F. Ashby an D. R. H. Jones, Engineering Materials 1, An Introuction to their Properties an Applications, Oxfor, Englan, 1987, pp DynaMOE tm OperationManual, Maison, WI, Structural Reliability Consultants, 1991, pp A. Jensen an H.H. Chenoweth, Statics an Strength of Materials 3 r e., New York, NY, McGraw Hill, 1975, pp Biographical Information Harvey Abramowitz receive a B.S. in Materials Science from the Henry Krumb School of Mines, School of Engineering an Applie Science, Columbia University. He complete M.S. an D.Eng.Sc. egrees from the same school in extractive metallurgy/mineral engineering. Prior to coming to Purue University Calumet, he was a Research Engineer at Inlan Steel Research Laboratories an Visiting Professor at the University of Missouri, Rolla. He recently was a Visiting Scholar at Northwestern University. Dr. Abramowitz is an Associate Professor of Mechanical Engineering an is responsible for the materials sciences, soli waste management, an freshman engineering esign an orientation courses. Major areas of research inclue the treatment of waste streams for metal recovery, the cryogenic treatment of steels, an strength of composite structural materials. Ralph Bennett III receive his B.S. in Civil Engineering. from the Univ. of Wisconsin. He receive an M.S. in Civil Eng. from the Illinois Inst. of Tech. He is a license PE in IN an IL. Prior to joining Purue Univ. Calumet, he ha a civil engineering career at Inlan Steel Co. Prof. Bennett has hel aministrative an teaching positions at Purue. While at Purue, he has been a civil eng. consultant an an expert witness within the international community. Page

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10 Fig. 1 Derivation of Impact Factor. (a) Impact loa. (b) Deflection uner impact loa. (c) Deflection uner graually applie loa W. () Equivalent graually applie loa to give some eflection as impact loa W. Fig. 2 Static Testing Apparatus - DynaMOE TM Fig. 3 Plank resting on support an loa cell of DynaMOE TM Page

11 Fig. 4 DynaMOE TM Loa Cell Fig. 5 DynaMOE TM Support Fig. 6 Output from DynaMOE TM Test Page

12 Fig. 7 Conventional Three Point Test Assembly Fig. 8 Static Testing Apparatus Tinius Olsen Testing Machine Fig. 9 Dynamic Testing Apparatus Fig. 10 Manufacture Woo Plank Page

13 Fig. 11 Typical Dynamic Loaing Results TABLE I: DYNAMIC LOADING RESULTS Plank MOE MOE Density Moisture Ratio Dynamic Static Content Experimental/ (x10 6 psi) (x10 6 psi) (lb m /in 3 ) (%) Theoretical Southern Pine Douglas Fir ND ND Horizontal Veneer A ND Horizontal Veneer B Vertical Veneer ND: Not Determine Thickness in With in Length in Page