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TEXAS

TEXAS I-l I G M WAY DEPARTMENT

COOPERATIVE KESEARCH

PRELIMINARY INVESTIGATION OF HAULING STRESSES IN PRESTRESSED CONCRETE PILES

1 -- - -- - -.. - -- - -- - - . .- - - - -- - -- -- - -- in cooperation with the

. - . .. . . -. - . . - -- -. . . Department of Commerce

- -. - - . - -. . . . . . - Bureau of Public Roads . - . . . . .- -. . . .- - -- cz

. . -. . .. .- - . . .- . .-

RESEARCH REPORT 33-6 STUDY 2-5-62-33 PILING BEHAVIOR

PRELIMINARY INVESTIGATION OF HAULING STRESSES

IN PRESTRESSED CONCRETE PILES

Thomas C . Edw'ards ~ s s i s t a n t Research Engineer

and

T. J. Hirsch Associate Research Engineer

Research Report 33-6

Piling Behavior Research Study Number 2-5-62-33

Sponsored by

The Texas Highway Department In Cooperation with the

U. S . Department of Commerce, Bureau of Public Roads

September, 1966

TEXAS TRANSPORTATION INSTITUTE Texas A&M University

College Station, Texas

ACKNOWLEDGEMENTS

The authors wish t o acknowledge the cooperation of the personnel of ~ l s t r i c t 17 of the Texas Highway Department who furnished the pole trailer used in the t e s t s . Appreciation is expressed a l s o t o Mr. A. L. Jones and Mr. Albert Ball of the Texas Engineering Extension Service who furnished the tractor and much needed advice concerning the hauling of the pile.

The opinions, f indings, and conclusions expressed in this publication a re those of the authors and not necessar i ly those of the Bureau of Public Roads.

TABLE OF CONTENTS

Chapter I -- Field Investigation. ................................ 1

Introduction ............................................... 1

Objectives ................................................ 1

Test Program. ............................................. 2

General .............................................. 2 Test Si te Layout.. .................................... 2

Test Procedure ........................................ 2

Chapter I1 -- Mathematical simulation.. ......................... 9

j Chapter I11 -- Conclusions. . ................................... 16

References ...........,........................................17

Appendix B..................................................*lg

LIST OF FIGURES

FIG. NO.

1

2

3

CAPTION

Test Hauling Configuration

Controlled Bumps

Strain Gage Location and Electrical Schematic

Dead Load Stress Condition

Cracked Pile After Passage Over Bump No. 3

Pile and Wheel Simulation

Stress Correlation for Passage of Wheel No. 1 Over Bump No. 2

Stress Correlation for Passage of Wheel No. 2 Over Bump No. 2

S t resses Produced During 15 mph Travel on Smooth Pavement

'LIST OF TABLES

TABLE NO. TITLE

1 Maximum Fiber S t resses and Stress Ratios

' PAGE

3

PAG E

8

CHAPTER I

FIELD INVESTIGATION

Since 1961, the staff of the Texas Transportation Insti tute has been engaged i n piling research. A part of this research h a s been concerned with pile s t r e s se s during driving. I t has been shown that under certain driving conditions, i t is possible to introduce large tens i le . s t resses into the pile. The magnitude of these s t r e s se s is dependent upon the so i l res i s tance , the s t i f fness of the cushion block, the unit weight and elas t ic i ty of the pi le , and the weight and velocity of the pile driver 's ram.

L The tensi le strengt,h of a prestressed concrete pile is dependent upon the final

prest ress and the inherent t ens i le strength of the pile concrete. If a pile is initially cracked, by handling or hauling, the avai lable tensi le capacity of the pile wil l b e lowered by a n amount equal to the inherent tensi le strength of the pile concrete. The remaining tensi le capacity of the pile wil l depend solely on the final prest ress . It should b e noted that in the c a s e of a cracked pi le , repeated passage of tensi le

0, s t r e s s waves , in exces s of the avai lable tensi le capacity of the pi le , could b e very detrimental to the structural integrity of the pile. Repeated opening and closing of

I t he crack (v i s ib l e by dusting a t the c r ack ) wil l gradually disintegrate the adjacent concrete. Furthermore, there is a l s o the danger of corrosive agents gaining entrance to the reinforcing s t ee l by way of a crack. This possibi l i tyalone makes the u s e of cracked piles questionable in marine environments.

I Obi ec t ives

In the course of a laboratory investigation of longitudinal s t ra in waves in 5000 psi concrete p i les , the question of pre-cracked pi les a rose . The investigators were concerned that the piles to b e used in this investigation had been subjected to dynamic overload due to the method used i n transporting the pi les from the cast ing yard to the laboratory. The pi les were transportedapproximately 150 miles by truck (tractor and pole t ra i ler) . It was noted upon receipt of the pi les that the dis tance

I between supports was greater than that recommended by the Bridge ~ i v i s i o n ' o f the Texas Highway ~ e ~ a r t m e n t . It was decided, therefore, that a very limited tes t program would b e conducted to get some quali tat ive and quantitative data on the behavior of piles under certain controlled hauling conditions. Field data on pile s t ra ins or s t r e s s e s were a l s o desired in order to es tab l i sh the feasibil i ty of develop- ing a mathematical model to predict the s t r e s s e s generated during hauling.

It is the object of this report to give the resu l t s of th i s limited tes t program and i) explore the feasibility of using a mathematical simulation to predict dynamic

s t ress levels induced by hauling.

Test Proqram

General--One solid prest ressed concrete pi le , 65 ft in length and 15 in . square . was instrumented with strain gages a t two locations and a n accelerometer was placed

a t one location. The pile was placed on a pole trailer and hauled over a known bump a t a predetermined speed . The subsequent s t ra ins and accelerations were recorded on a recording oscil lograph. Figure 1 is a schematic of the pile and hauling configu- ration. The location of the strain gages and accelerometer a re shown. The pile was hauled i n the manner shown to simulate the hauling conditions used when the pile was originally received. The THD Bridge Division recommends that a 65 ft x 15 in . square pile be hauled with minimum end overhangs of 14.4 ft and 11.7 f t . Accord- ing to these recommendations, the maximum length of pile that can be hauled safely , with the overhangs shown in Figure 1 , is 53 ft . These recommendations assume that the pile wil l not b e damaged a s long a s the calculated dead load s t a t i c s t r e s s ( n o impact factor) for the support conditions, is no more than 60% of the ini t ia l prestress This assumption is predicated on a 1.50 impact factor for hauling with zero allowable tensi le s t r e s s in the concrete (assuming 90% ini t ia l prest ress a t hauling ) .

Test Si te Layout--Three s e t s of controlled bumps were used in this program. yese bumps were constructed of standard dimensioned lumber and a r e shown i n

rigure 2 . The tes t s i t e w a s located on the concrete parking apron a t the Texas A&M University Research Annex (old Bryan AFB ) . Ample space was available for acceler- ation and safe deceleration of the hauling vehicle. The power suppl ies , s t ra in gage amplifiers and recorder were transported i n a sa te l l i t e vehicle which followed to the s ide and rear of the hauling vehicle . Electrical connections were made through a n extension cable . Appendix A gives a n equipment l i s t and Figure 3 gives a schematic of the instrumentation.

Test Procedure--The tes t program consis ted of running the vehicle and pile over a single bump (one of the three controlled bumps ) . During each run the s t ra ins and accelerations were recorded continuously for the passage of the entire vehicle over the bump. A traffic radar unit recorded the speed of the hauling vehicle. After each run, the pile was inspected for cracks or other damage.

Test Results--The strain gages were located a t the points of maximum posit ive and negative dead load movement for the pile supported a s shown in Figure 1. This was done s o a s to b e ab l e to compare these s t r e s s e s with the dynamic s t r e s s e s induced by hauling.

The gages were positioned on the pile and wired a s shown in Figure 3 . w i t h this arrangement, the s t ra ins measured were the average of the tensi le and compressive

ra ins . Consequently, this average s t ra in is a direct measure of the curvature of the

- 2 -

I

p- - . 9 . - --.--, , ,,.. , :. :. - n ,, , - ,', , ,

' . . * . i . . > :, -----vp, . . . ..,< i-:::: r.3.:. , i. , '

, *'("( , . ;I I * , '"%4' L ' .-U

. , .. (7, .:: *

1 I '

- , ? . . .. , , !

,, , , ? b..:i, * ,. -

" .:I . , I .

1. : , "4 "?-+ L . , < - ,

. , 1 . :. : . ! ////////////// /' ! - : . ". -1.. . , , , J i . . " . , " k c - , , , -7 , i ,: . I ' : , ,

I. 1 -":* 1 BUMP NO. I

i

-. ,.

: . . \, , ,,, ,, .

, , .,,,,* I

..I , . I

//////////////// 1

,. . . < " ', .-- * _ . . BUMP NO. 2

,. x - , ; y ; z - k ,

- \ \ F,,.::' .. f;?

--L (2"x 6")

. . . ,

. .. - . 2 " ' ' . . . - ., ,y,m. 7. F.11 5 . . 8 +. -:

BUMP NO. 3

'FIGURE 2. CONTROLLED BUMPS . .

. * TYPICAL SECTION THRU 15" SQ.

P l L E A T GAGE L O C A T I O N \ A,/(: * - . . * . . \ c

' * v . . . . , -.. 2. . - -

* . A * . . *A. 0- . . -

' f 6.75"

f 6.76"

el \ r I

* * .

"'DpML-60 S T R A I N GAGE

GAGE LOCATION ON PlLE

- AMPLIFIER - RECORDER

ELECTRICAL SCHEMATIC

FIGURE 3, STRAIN GAGE LOCATION AND E L E C T R I C A L

SCHEMATIC I

Jn each t e s t the s t ra in gage bridge was ze ro balanced with the pile supporting t he dead load. Therefore the s t ra ins recorded ref lec t only t he dynamic response .

eam and not necessa r i ly a measure of true s t r e s s when multiplied by t he modulus of 4 I

Figure 4 shows the s t a t i c dead load bending s t r e s s for t he support condit ion shown. Note the maximum t ens i l e f lexure s t r e s s is 1260 ps i . Allowing for t he f ina l pres t ress of 703 p s i , the pile concrete is required t o r e s i s t 557 ps i . Appendix B shows that the t en s i l e capaci ty of the pile concrete was 455 psi ba sed on direct t ens ion t e s t s on 3 i n . x 3 in. . x 22 in . prisms. The modulus of rupture was 1220 ps i based on f lexural t e s t s on 3 in . x 4 i n , x 16 i n . beams center point loaded on a 1 4 i n . span . The true flexural t en s i l e capac i ty of the pile concrete is somewhat between t he se two extremes. This ind ica tes tha t the pile could have cracked under a dead load condi t ion, A careful examination of t he pile before the t e s t s revealed that the pile was c racked . Hairline c racks were vis ib i le in the t en s i l e f ibers a t the b i n t of maximum moment. These c racks were observed t o c l o s e when t he pile was

supported a t the recommended pickup points .

Table I g ives t he va lues of t he maximum dynamic s t r e s s during t he pa s s age of t he hauling vehic le over the controlled bumps. A s shown in Figure 1 , wheel 1 is the drive wheel of t he tractor and wheel 2 is tha t of the s ingle ax le pole t ra i ler . The maximum t ens i l e flexure s t r e s s (dead load s t r e s s plus dynamic s t r e s s ) is given for the passage of e ach wheel over the particular bump. Also shown are the ra t ios of naximum dynamic s t r e s s t o dead load s t r e s s a t e ach gage locat ion, This ra t io is normally referred t o a s the impact fac tor . It should be kept in mind that the pile u sed in t h e s e t e s t s had been previously cracked during hauling and , consequent ly t he s t ra ins and s t r e s s e s recorded a re influenced by t he presence of c racks and t he reduced flexural s t i f f ne s s ,

The damage done t o the pi le during the passage over bump No. 3 is shown in Figure 5 Note that in severa l p laces the pile is cracked ac ro s s the entire c ro s s sec t ion lndlcating complete reversa l of s t r e s s . Also note the widespread cracking throughout the length of the pile.

6

e las t i c i ty of the pile concrete . The average s t ra in was not a lways proportional t o the s t r e s s s ince c racks in the pile a t the gage locat ions were open a t t imes and c losed j l

a t t imes , Calcula ted s t a t i c dead load s t r e s s e s were determined using a n uncracked sect ion. Dynamic s t r e s s e s were determined by multiplying the measured s t ra in by the modulus of e las t i c i ty of the pile concrete . These s t r e s s e s are f ic t ic ious in t he

I ins tances when t he pile c racks a t the gage locat ions . In t h i s c a s e the s t ra in gages give the average s t ra in over the c racks .

I i

15 IN. SQ. SOLID PRESTRESSED CONC. PILE

3'- 0'' -t

n I n

a 4 8'- 4"

65'-0" .c' -

SHEAR

BENDING MOMENT

1260 PSI

D.L. BENDING

INITIAL PRESTRESS = 8 7 9 PSI

FINAL PRESTRESS = 7 0 3 PSI

FIGURE 4. DEAD L O A D STRESS CONDITION

.

TABLE I : MAXIMUM TENSILE FlBER STRESSES

AND IMPACT FACTORS

NOTE : f max = MAXIMUM FIBER STRESS (BASED ON ~ ~ 7 . 1 5 x 10' PSI)

f d l =DEAD LOAD FIBER STRESS, GAGE 1 +472 PSI GAGE 2 + I 2 6 0 PSI

f r = INITIAL PRESTRESS = 879 PSI

-

WHEEL AT BUMP

I

2

GAGE LOCATION

I

2

I

2

BUMP NO. I k

VEHICLE VELOCITY = 19 MPH

fmax (PSI)

1922

6 7 3

1556

10 0 0

f max - f dl

1.5 3

1.43

1.23

2.12

BUMP NO. 2 VEHICLE VELOCITY

= 21 MPH f max (PSI)

2 9 3 8

1089

2184

1833

BUMP NO. 3 VEHICLE VELOCITY

= I I MPH f mar - f dl

2.33

2.31

1.73

3.88

f max

(PSI)

3 0 3 1

1449

2519

1295

f max f dl

2.4 1

3.07

1.99

2.7 4

FIGURE 5. PI$..€ D A M A G E

CHAPTER I1 -----

MATHEMATICAL SIMULATION - - - - - - _ _

An attempt was made t o simulate the behavior of the pile by using a lumped mass representat ion of the p i l e , Figure 6 (a) shows the lumped mass representa t ion used in the simulation. Springs a t the supports s imulate t he physical springs of t he rear trcctor wheels and t h e dolly on the pole t rac tor , The formulation of the dynamical equations of motion for the multidegree of freedom system assumes smal l deflectiorrs a c d neglects any shea r or rotary inert ia e f f ec t s . Plas t ic ef fects are included by a s s u m i ~ g e l a s t i c , perfectly p las t ic behavior, This assumption is i n error i n that a cracked concrete sec t ion does not exhibit th i s type behavior. However, s i nce t he sirnulatior was intended only t o be preliminary i n nature , i t was fel t tha t t h i s effort t o include accclrate mornert curvature behavior would not be done a t t h i s s t age .

The forcing funct ion, for a wheel pass ing over a bump, was simulated a s a rigid d i s c pivoting about a surface discont inui ty , This behavior is shown i n Figure 6 (b) . The fo rce , which wil l vary with tzme, is dependent on the deformation of t he carriage spring. The deformation i s t he net movement of, the pile mas s (at the support point) with respec t t o the d l s c or wheel . The carriage spring is a simulation of t h e combined sprip.9 of t he dolly and t i re , , For t he purpose of t h i s simulation, a spring cons tan t of 5000 Ib/in. was u sed for springs K 1 and K 2 . Note t ha t no dashpot ( s h o c k absorber) was included i~ the simulation, The inclus ion of a dashpot a t th i s s t age of t he inves- "gation would introduce complications not warranted i n t h i s preliminary invest igat ion. - 1 a reasonable e n g i n e e r k g correlat ion c a n be shown using a simplified model, t hen sophis t ica t ion c a n only b r i ~ g improvement.

Bl~mp 2 was chosen t o be correlated with t h e r e su l t s of the mathematical simula- t ion, , Figure 7 shows a time v s , s t r e s s correlat ion plot for gage locat ions 1 and 2 for the pa s sage of wheel Number 1 (drive wheel of tractor) over the bump. The s t r e s s va lue s shown a r e plct ted from the equilibrum posit ion and hence reflect only t he dynamic r e s p o n s e , Note tha t the s t r e s s va lues of the simulation d o not correlate well wlth the measured v a l u e s , In both c a s e s , t h e s t r e s s va lues a r e t o o large and t he r e spocse i s quicker . This i s probably due t o t h e fac t that no dashpot (shock absorber) was i nc luded , The fac t that flexural damping was ignored is of s ignif icance a l s o . The presence of c racks wovld a l s o ter,d t o dec r ea se t he natural frequency of the r e a l sys tem somewhat ,

Figure 8 shows the ccrrelat lon for the pa s sage of wheel Number 2 over the bump. Note that the s t r e s s va lues for gaae locatlon 1 a re again large and not in phase . The reasoning glveri above appl les here a s we l l , The s t r e s s for gage 2 however is in very good agreement wlth the measured s t ra in v a l u e s , It should be noted that t h i s gage was lccated dl rsc t ly over the wheel which was on the bump and hence would not be influenced a s greatly by damping.

k I

f DISCRETE PlLE MASS

(a> 0

PlLE DISCRETE MASS

SPRING COMPRESSED BY

DIFFERENTIAL MOVEMENT

BETWEEN WHEEL 8 MASS

U

BUMP (SURFACE DISCONTINUITY)

(b)

FIGURE 6. PlLE AND WHEEL SIMULATION

I I

Figure 9 shows t h e s t r e s s e s produced when t h e hauling veh ic le t r ave led over a smooth pavement a t 15 mph, The maximum s t r e s s is 150 ps i a t g a g e locat ion 1

) and 105 ps i a t g a g e locat ion 2 ,, These s t r e s s e s were genera ted by a random forcing t

func t ion . (due t o wheel out of b a l a n c e , s u r f a c e i r regular i t ies , e t c , ) , therefore , they were present throughout t h e t e s t and h e n c e were superimposed on t h e s t r e s s e s d u e t o the bump, This could exp la in some smal l s t r e s s and phase var ia t ions i n t h e

i corre la t ion ,

t I f

I I

I

I I

6 I

., CHAPTER I11

CONCLUSIONS

The data presented in Chapter I1 i l lustrate that the magnitude of s t r e s se s involved in hauling may be much larger than normally specified. In severe con- di t ions , that may be encountered during hauling, dynamic impact factors for maximum s t r e s s can be a s high a s 2 , ,25 (based on dead load s t r e s se s ) . This in- dicates that for piles which are supported in a cri t ical or near cri t ical manner, th6 probability of cracking during hauling i s high,

The attempt t o mathematically simulate pile hauling s t r e s se s was only partially success fu l , A good correlation w a s obtained for s t r e s s e s in the pile a t the gage location near the wheel being bumped. Very poor correlation was obtained for gage locations remote t o t'he bumped wheel.

The simulation does give the indication however, that closer correlation can be obtained by added sophistication t o the model by developing a more rea l i s t i c approach t o the routine used t o simulate the carriage response; by including flexural and other damping factors; by using a rea l i s t i c moment-curvature relation for the pile that would accurately simulate the flexural behavior of the cracked sect ion.

A model that h a s been thoroughly validated is of inestimable value in sett ing the ?roper values on the behavior factors which dicta te the design of a prestressed con- crete member. With a tool such a s t h i s , methods of hauling and handling could be fully explored and appropriate specifications written. It could a l so be used i n spec ia l c a s e s where unusually long piles or beams must be handled and transported.

REFERENCES

1 1

1 . H i r s ch , T . J , and Samson, C , H . "Driving Practices for Pres t ressed Concrete .Piles. " Texas Transportation Ins t i tu te Research Report 33-3. Project 2-5-62-

I 3 3 , April, 1966.

2 . Handling Controls for Concrete Piling. Bridge Division of t he Texas Highway 1 Department, July, 1 9 6 4 .

1 I 3 . - Ibid. F 1 1 4 . Edwards, T. C . "An Analytical Solution of the Impact Behavior of Sign Posts . "

Doctoral Disse r ta t ion , Texas A&M Univers i ty , May , 19 66. S

') Equipment Test,

. l o Strain gage:

., APPENDIX A

a , Manufacturer - Tokyo Sokki Henkajujo C o . , Ltd.

b , Specif ica t ions

Type - PML-60 Res i s tance - 120 ohm k 0 , 5 % Gage Factor - 2.19 Gage Length - 60 millimeters

2 . Strain Gage Power Supply and Amplifier:

a . Power Supply - Honeywell Model 12 1 (5000 c p s carrier frequency)

b . Amplifier - Honeywell Model 119 Carrier

3 . Recorder:

a . Honeywell Model 1508 Visicorder (opt ica l galvanometer type M 1650)

' b. Recording Paper - Kodak Lineograph Direct Print, Spec. 2 .

APPENDIX B .. LABORATORY TEST PILES

CAST ON FEBRUARY 5 , 1964

C l a s s "F!' Concrete (quant i t ies per cy )

Cement Type I11 Gravel Sand Water Slump Pla stiment Sika Air Air Content Unit Weight

Properties a t 14 days of a g e

Compressive Strength 6" x 12" cy l . Modulus of Rupture 3 " x 4" x 16" prism Tensi le Strength 3" x 4" x 22" prism Modulus of Elasticity ( s t a t i c ) Modulus of Elasticity (dynamic ) Poisson ' s Ratio (dynamic ) Concrete Steam Cured a t 145' for 14 hours

610 lbs or 6 1/2 s a c k s 1896 lbs 1360 l b s 217 l b s or 26 gal lons 2-3 in 12 .8 o z 6 .4 o z 4% 4083 l b s l c y 151.1 lbs/cy

6655 ps i 1220 ps i 455 ps i 7 .15 x 106 ps i 7.56 x 106 ps i 0.2 1