Prestressed Concrete Piles in Jointless Bridges

56

Mounir R. Kamel Research Associate Department of Civil Engineering University of Nebraska-Lincoln Omaha, Nebraska

Joseph V. Benak Ph.D., P.E.

Professor and Vice Chairman Department of Civil Engineering

University of Nebraska-Lincoln Omaha, Nebraska

Maher K. Tadros Ph.D., P.E. Cheryl Prewett Professor of Civil Engineering University of Nebraska-Lincoln Omaha, Nebraska

Mostafa Jamshidi Assistant Bridge Engineer

Nebraska Department of Roads Omaha, Nebraska

The purpose of this investigation is to determine the feasibility of using precast, prestressed concrete piles in jointless bridges and to develop design criteria and pile-abutment joint details that can be used in practice. This phase of the project comprised an experimental and analytical study of the load-deflection behavior of HP10-42 and 12 x 12 in. (305 x 305 mm) precast, pre-stressed piles, and a computer analysis of the pile-soil interaction. The test results show that the proposed joint would allow the use of concrete piles in integral abutment bridges of lengths comparable to those with steel piles. A field investigation to study the performance of two prestressed concrete piles in a full-scale integral abutment bridge is currently being carried out.

I n jointless and integral abutment bridge superstructures, thermally induced movements in the superstructures must be absorbed by the substructures. Most states in the

United States use steel piles for these bridges. However, be-sides being susceptible to corrosion, steel piles have a rela-tively small cross section, depending on skin friction and/or bedrock bearing for their capacity. For this reason , steel piles are more suitable in stiff soils consisting of significant amounts of clay or where the bedrock is close to the surface.

On the other hand, precast concrete piles, besides their skin friction capacity, have relatively large cross sections. Thus, they densify the soil as they are driven. In various parts of Nebraska (and other parts of North America) where

PCI JOURNAL

sand/gravel soils prevail, required steel pile depths may exceed 100 ft (30.5 m), which is not only expensive but can also be impractical. Concrete piles on the order of 50 to 60 ft (15.3 to 18.3 m) in length may be satisfac-tory in these situations.

The objective of this research was to determine the feasibility of using pre-stressed concrete piles in jointless bridges, and, if feasible , to develop acceptable design criteria and pile-abutment joint details.

The project consists of: (a) Laboratory and analytical studies

of load-deflection behavior of HPl0-42 and 12 x 12 in. (305 x 305 mm) precast, prestressed concrete piles

(b) Computer analysis of pile-soil interaction

(c) Field investigation of both types of piles installed in a full-scale bridge in Omaha, Nebraska

This paper covers the laboratory and analytical studies of load-deflection behavior of one steel pile and two pre-stressed concrete piles, and the com-puter analysis of pile-soil interaction . It also provides a brief report on the field investigation , which is still un-derway and will be the subject of a separate paper.

BACKGROUND Historically , a system of expansion

joints, roller supports and other struc-tural releases was provided in long bridges to permit thermal expansion and contraction [see Fig. l(a)]. Providing expansion joints in a bridge, however, leads to a substantial increase in initial cost. In addition, expansion joints are sources of deterioration and frequently do not operate as intended, thus result-ing in high maintenance costs.

Jointle ss or integral abutment bridges provide an attractive design al-ternative [see Fig. l (b)]. These are de-fined as bridges with no movement joints at the abutments. The support-ing foundation, therefore, has to be flexible enough to accommodate su-perstructure deformation due to vol-ume changes caused by temperature, creep and shrinkage. The maximum deformation that can be allowed by the piles without significantly decreas-ing their vertical load capacity or in-

March-April 1996

Expansion Joint Deck Approach Slab

(a) Bridge with expansion joints

Fig. 1. Types of bridge abutments.

tegrity is of primary importance. Currently, most states, including

Nebraska, use steel H-piles in integral abutment bridges. When used as fric-tion piles, steel H-piles must be driven deeper than concrete piles to attain the required vertical load capacity. Load-deflection tests on one steel pile and two concrete pi les were conducted to evaluate and compare their stiffness. The computer program LPILE was used to analyze both concrete and steel piles in different types of soil.

The results of the analysis and tests showed that concrete piles have lim-ited flexibility for lateral loads with current pile-abutment details, so they can be used only in short span integral abutment bridges. For concrete piles, a new pile-abutment joint was investi-gated consisting of a neoprene bearing pad with a teflon layer that allows for controlled movement and/or rotation of the pile relative to the abutment. Laboratory tests were conducted to study the behavior of the proposed joint under axial and lateral loads.

The test results showed that the pro-posed joint would allow the use of concrete piles in integral abutment bridges of lengths comparable to those with steel piles.

CURRENT PRACTICE A survey was conducted among

highway agencies in the United States to identify those agencies that use precast, prestressed concrete piles in integral abutment bridges. The sur-

Integral Abulment

Aexible Piling

(b) Jointless bridge with integral abutments

vey showed that steel, timber, con-crete cast in drilled holes , concrete cast in thin steel shells, and precast concrete piles are all used in integral abutment bridges. However, most of the states prefer to use steel H-piles (see Fig. 2). States that use integral abutments rely on their own experi-ence, empirical formulas, and simpli-fied design assumptions to place span limits, rather than depending on theo-retical calculations.

Many states use predrilled over-sized holes filled with granular soil, ass uming that pile stresses are re-lieved and allowable lengths of inte-gral abutment bridges are increased accordingly. Various depths of these holes are required by different states. An additional feat ure of predrilled oversized holes is the reduction of downdrag forces when compressible soi I is present and/or the minimiza-tion of the effects of elastic shorten-ing when prestressed concrete super-structures are used.

Where stee l H-piles are used, most states do not consider the effect of thermal movement of the superstruc-ture on the piles as long as bridges are designed according to their span limits and details. From a review of the li ter-ature, it was found that there has been no reported spec ific research con-ducted on prestressed concrete piles in integral abutment bridges. The review included research on piles in integral abutment bridges, 1.2.3 laterally loaded piles: ·56 and seismic design and ductil-ity of prestressed concrete piles. 7•8

57

D No response l!l@li:]:l ·l Steel HP piles only II Steel and CJ.P piles mm Integral Abutments II Steel and timber piles II Steel, C.I.P and P.P.C. piles WfB are not used

Note: C.I.P = Cast-in-place concrete P.P.C = Prestressed Pn:east Concrete

Fig. 2. Types of piles in integral abutment bridges in various states.

LABORATORY TESTING

Laboratory tests were conducted on three pile-to-pile-cap specimens tested as cantilevers to obtain the load-deflection relationship. The objectives of the tests were to compare the stiff-ness of concrete piles with the stiffness of steel piles and to evaluate the flexu-ral rigidity, equivalent stiffness (EI) vs. bending moment for concrete piles.

Three pile specimens were tested, one steel H-pile and two prestressed concrete piles. The steel H-pile (Spec-imen 1) was 10 in. x 42 lbs/ft A36 steel (25.4 mm x 0.61 kN/m). The two concrete piles were 12 in . (305 mm) square, one with 9 in. (228 mm) pitch spiral reinforcement (Specimen 2) and the other with 3 in. (76 mm) pitch spi-ral reinforcement (Specimen 3) . Pile Unit 3 represents the standard detail

58

used in Nebraska for 12 in. (305 mm) concrete piles. The concrete pile cap dimensions and details were prepared with the same standards currently used by the Nebraska Department of Roads (NDOR) so as to simulate the bridge abutment. The embedded length of both the concrete and the steel piles was 24 in. (610 mm). Test results are shown in Fig. 3.

STIFFNESS OF PRESTRESSED

CONCRETE PILES

To predict the behavior of the con-crete pile at different loading stages, a nonlinear analysis was used to calcu-late the moment vs. stiffness of the piles. The stiffness of a cracked sec-tion varies along the pile length ac-

cording to the magnitude of the bend-ing moment the section is subjected to, and hence, it varies according to the stage of loading.

The basis of the nonlinear solution is to calculate the proper depth of the compression zone for a section at a given concrete strain value. Once both the depth of compression zone and the strain distribution are obtained, all forces at the cross section can be cal-culated. These forces are the forces in the concrete and in the top and bottom strands as well as an applied axial load, if it exists .

In this analysis, the axial applied load is zero. With a trial and error procedure, the proper depth of the compression zone can be obtained and all strains and stresses at the section are known. The flexural rigidity, R, of the section is de-fined by the following equation:

PCI JOURNAL

10 20 15.00

10.00

5.00

0.00

0.0 0.5

Deflection (mm)

30 40 50 60

Steel HP Pile

12" Concrete Pile with 9" Pitch Spiral Reinforcement

12" Concrete Pile with 3" Pitch Spiral Reinforcement

1.0 1.5 2.0

Deflection (in)

60.0

40.0

20.0

2.5

Fig . 3. Combined load-deflection test results for steel and concrete piles.

R =M e ( 1)

where e is the curvature of the section, which is equ a l to th e s lope of the strain diagram, and M is the moment at the section. At load levels causing linear elastic stresses, R is equal to Ef where E is the modulus elasticity and I is the transformed section moment of in erti a. Fig. 4 shows the ass um ed stress and strain distribution diagrams of a pile section in a cracked stage.

The nonlinear concrete stress-strain relationship is represented by Eq. (2). Thi s analyti cal approximation of the concrete stress-strain relationship was given by Hognestad9· 10 as a res ult of his experimental study on the behavior of concrete members under combined bending and axial loads:

The integration of Eq. (2) over the d e pth of th e co mpress io n zo ne,

March-April1996

zl f c

1---t X 1---l a. Cracked section b. Strain distribution c. Stress distribution

Fig. 4. Properties of prestressed cracked pile section.

shown in Fig. 4, gives the total com-press io n fo rce Fe in th e co ncre te bl ock. By ano the r integrati o n, the moment of Fe about the neutral ax is can be obtained . The strand stress-strain relationship developed by De-valapura and Tadros 11 was used in the analys is. More detail s on the analysis are given in Appendix B. A spread-sheet was used for the calculations. Fig. 5 shows the calculated moment vs . R (equi valent E f) relationship.

The predicted test deflections were calculated using the variable fl exural rig idity by nonlinear analys is. The method of virtual work was used to cal-culate the deflections by di viding the span into 16 equal segments. A step-by-step method of calculating both the fl exural rigidity by nonlinear analysis and the defl ections corresponding to each inc re ment of load is given by Kamel. 12 Fig. 6 compares the calculated and the experimental deflection curves.

59

Moment (kN-m)

W W ~ ~ ~ W M W 8.00

~ 20.0 6.00 Flexural Rigidity, R \ IS.O Flexural Rigidity, R

(kN-m2 ) x 103

I\ ~

(k-in2

) x 106 4.00

w.o

2.00

~~

' 5.0

0.00 0.0

0 100 200 300 400 sao liOO 100 wo

Moment (k-in)

Fig. 5. Calculated bending moment vs. R of 12 in . (305 mm) concrete pile.

ANALYSIS OF PILE/SOIL SYSTEM BEHAVIOR

The LPILE computer prog ram of Reese and Wang6 was used to solve the problem of laterally loaded piles using the method of fi nite differe nces. Reese ' s soil p-y curves were used in

10

15.00 -

-

--

10.00

the program and were included as a subroutine. The program provides de-flec ti on, moment diagrams, so il re-sponse, and p-y curves for laterally loaded piles. It was used in this study to determine the maximum allowable horizontal defl ection that a single pile can undergo wi thout exceeding its

Deflection (mm)

20 30 40 I I

• ~-::: · -· - --------=,_----- .... .. ------ ,,., - ,,-- ~ ,;'

,' ,·""' - I ... : ,/ - I,. ,.

II I A ; : 5.00

service moment capacity. A total of 13 cases were run with

the LPILE program for various types of soil and parameters. Parameters of the first six cases are presented in Ap-pendix C as an example. Cases 1 to 5 represent 12 in. (305 mm) square pre-stressed concrete pi les in loose sand , loose sand followed by dense sand , dense sand, loose sand followed by soft clay, and loose sand followed by stiff clay, respectively. Case 6 repre-sents 10 in. (254 mm) steel H-piles in loose sand.

Cases 6 to 9 represent 10 in. (254 mm) steel H-piles with the same soils as Cases 1, 2, 4 and 5. Cases 10 to 13 represent concrete pi les with a re -duced modulus of elasticity of 50 per-cent with the same soi ls as Cases 1, 2, 4 and 5. The concept of using a re-duced modulu s of elastici ty to ac-count fo r creep actio n in la terally loaded concrete piles is being investi-ga ted by seve ra l resea rc hers .3·5

Reese's p-y curves were used to rep-resent soil stiffness for different types of so il.

50 60 I

- 60.0

·-· 1-·-- 40.0

- 20.0 - • I • calculated deflection by nonlinear analysis IJe - 'I I I -·-·- 12" Concrete Pile with 9" Pitch Spiral Reinforcement I - I I 12" Concrete Pile with 3" Pitch Spiral Reinforcement I -------

I I I I I I I I I 0.0 0.00

0.0 0.5 l.O 1.5 2.0 2.5

Deflection (in)

Fig. 6. Evaluation of deflection calculation by nonlinear analysis.

60 PCI JOURNAL

Maximum moment (kN-m) 10 20 30 40 50

8.0

6.0 ..--.. "' 0.. g ~ .9 4.0 ] j

2.0

60 70

60' loose sand 40.0 case2

case 3

10' loose sand and 50' dense sand 60' dense sand

h = hinged pile top f = fixed pile top

30.0

20.0

10.0

~ '-'

~ .9 -; ... ~ t

Table 1. Summary of analytical results.

Lateral load Deflection corresponding corresponding to maximum to maximum

moment load Case (kips) (in.)

I 1-h 7.8 0.34 1-f 7.2 0. 11

2-h 7.8 0.3 1 2-f 7.2 0.10

3-h 10.5 0.20 3-f 10.5 0.05

4-h 7.8 0.34 4-f 7.2 0.12

5-h 7.7 0.32 5-f 7 .2 0.10

6-h 7.0 0.65 6-f 8.0 0.25

7-h 7.0 0.64 7-f 8.0 0.24

8-h 7.0 0.70 8-f 8.0 0.22

9-h 7.0 0.65 9-f 8.0 0.2 1

10-h 7.8 0.5 1 10-f 8. 1 0.18

11 -h 7.8 0.48 11 -f 8.1 0.17

--12-h 7.8 0.53 12-f 8. 1 0.18

-

T -

13-h 7.8 0.51 13-f 8.1

I Note: I kip= 4.448 kN; I in . = 25.4 mm. f = fi xed joint h =hinged joint

0 .18

predrilled hole filled with loose sand was studied by introducing a layer of loose sand at the top 10 ft (3.05 m) of embedment. Table 1 and the resulting moment and deflection curves show that the loose sandy layer has a signifi-cant effect on the behavior of the ex-amined cases. Because most of the de-flections and moments occur within the top 10 to 12 ft (3.05 to 3.66 m) of the pile, the type of soil in this region will have a significant influence on the behavior of the pile under lateral loads.

This result is clearly shown in Case 3 when loose sand was not used. In this case, a very small amount of deflection was sufficient to cause the maximum allowable lateral load. In addition, very comparable results were obtained in all cases when using an upper 10 ft (3 .05 m) loose sand layer, regardless of the type of soil below this depth .

62

PROPOSED PILE-ABUTMENT JOINT

The feasibility of using a sliding joint for pile/abutment connection was investigated. A joint capable of allow-ing the abutment to slide and rotate over the top of the pile would allow for more lateral expansion than a rota-tionally restrained pile top connection. Fig. 9 shows a proposed joint detail.

A bearing pad was used at the top of the pile consisting of a layer of ran-dom-oriented-reinforced, fiber neo-prene coated with a teflon layer. The teflon layer allows lateral movement against an embedded steel plate that is connected to the cast-in-place concrete abutment by welded studs or reinforc-ing bars. The four sides of the pile top were covered by a compressible mate-rial such as expanded polystyrene or urethane styrofoam.

The compressible material at the two sides, in the direction of deflection, al-lows lateral movements . On the other two faces (front and rear), the com-pressible material was used to break the bond between the abutment poured concrete and the pile head when the abutment moves laterally . The joint could be manufactured in one piece and placed on the pile top once the pile is driven to its final position.

One possible concern about this pro-posed detail is that piles are some-times significantly damaged due to driving impact, or are not of the 2 ft (620 mm) proper embedment length into the abutments . This issue was considered by the authors and assur-ance was given by a number of con-tractors and precast concrete produc-ers that it can be practically and economically resolved. The pile can be easily cut to proper length. Also, grouting materials are readily avail-able to restore damaged ends to the original pile dimensions.

TESTING OF PROPOSED JOINT

A prototype of the pile-abutment joint was tested in the laboratory to verify its behavior. The bearing elas-tomer pad should have the following capabilities:

1. Carry an applied axial load of the pile of about 90 kips (400 kN) for a 12 in . (305 mm) concrete pile.

2. Allow some rotation between the upper and lower faces of the pad.

3. Have a minimum coefficient of friction to allow sliding against the upper steel plate.

Any commercially available bearing pad that meets the above requirements could be used. The chosen pad for the test was manufactured by JVI Inc ., Skokie, Illinois, and designed accord-ing to their Design Guide Handbook.7

A sliding pad was chosen with a thick-ness of 1h in. (12.7 mm) and dimen-sions of 8 x 8 in. (203 x 203 mm). The JVI MASTICORD pad was coated with a 3/ 32 in . (2.4 mm) teflon layer. The pad has an allowable compressive stress of 2.50 ksi (17.23 MPa) and a coefficient of friction of less than 5 percent against a smooth stainless steel surface.

Fig . 10 shows the schematic test setup as well as the test results . Verti-cal loads were applied to the joint as well as lateral loads . Lateral deflec-tions were measured for various lateral loads acting under constant vertical loads. The vertical loads were 30, 60, and 90 kips (133, 266, and 400 kN) . Lateral loads were applied until the cap moved against the pile head unit for about 1 in. (25.4 mm), then the load was reversed.

The results show the capability of the proposed joint to allow lateral movements of about 1 in. (25.4 mm) in each direction under sustained verti-cal loads of up to 90 kips (400 kN). The lateral force that overcomes the friction and the resistance of the com-pressible material is about 5 kips (22.3 kN). The observed maximum vertical deflections due to the compressibility of the joint system were 0 .10, 0.17, and 0.20 in. (2.5, 4.3, 5.1 mm) corre-sponding to 30, 60, and 90 kips (133, 266, and 400 kN) vertical loads.

The total material cost of the pro-posed joint was $25.00. The cost of the bearing pad was $18.00 and the rest of the cost was for the steel plate and the compressible material. When the proposed joint is used, it will not be necessary to use the predrilled holes fi lled with loose sand unless needed for other purposes.

PCI JOURNAL

6.0

g ~ .9 ~

!i 4.0

j

2.0

0.0 5.0

Horizontal deflection (nun)

10.0

case 1

case2

15.0

(fJ' loose sand

10' loose sand and 50' dense sand

•.. ~.~... 60' dense sand h = hinged pile top f = fixed pile top

40.0

30.0

~ 20.0 ~

.9

] 10.0

0.0 ~----+----t------+-------je-----+------t----~ 0.0 0.1 0.2 0.3 0.4 0.5 0.6

Horizontal deflection (in.)

Fig. 8. Horizontal force vs. horizontal deflection for 12 in. {305 mm) concrete pile in sand .

cast in place concrete abutment

8" X 8" X 1/2" bearing pad with teflon coating

l{l" expanded polystyrene

0.7

/ 2" expanded .Jt" polystyrene

bearing pad

3/16" steel plate

headed studs

expanded polystyrene

precast concrete pile

Fig. 9. Proposed concrete pile-abutment joint.

March-April 1996

3-d view of joint material

63

Lateral deflection (mm)

-30.0 -20.0 -10.0 00.0 10.0 20.0 30.0 I I I 1

8.0

7.0

6.0

s.o 4.0

3.0

2.0

1.0 Lateral

0.0 force (k) -1.0

-20

-3.0

-4.0

-5.0

p p-it ·····;-»•r ph ---- X :6()Jt

,/'"""" ~ r--.... _P_=901t ) ""'---,..... __ X t--- ~"--- ..... l} .. . ..... -- _, ·- - 1--- ...... r--......... _ ----·-· --- .. !--__... t_,-

fl ··-·· -·-·· - -r-\ -~· 1

./ J l I

11 ! i V! 1 l'f 1-

l \ 1'---- ····- 1-- f--- -··-· -·-· I ·-···· ······ 1-·-· · r--

I- :.- 1- ·- 1- ·- - !-"'. j t-- _..

30.0

20.0

10.0

Lateral o.o force (kN)

-10.0

-20.0

-6.0

-7.0 -30.0

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.00 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Lateral deflection (in.)

Fig. 1 0. Horizontal force vs. horizontal deflection for 12 in. (305 mm) concrete pile head with the proposed pile-to-pile cap joint.

APPLICATION OF PROPOSED JOINT DETAIL

The proposed joint allows relative lateral movement up to 1 in. (25.4 mrn) with a maximum generated lateral load of 5 kips (135 kN). In most soil types, the concrete piles deflect elasticall y about 0.15 in. (3 .8 rnm) under this lat-eral load. Thus, the proposed joint de-tail is applicable in alJ cases where the longitudinal volume change deforma-tions of the superstructure require up to 2.3 in . (58.5 mm) of total movement at the abutments.

It is difficult to set a maximum total bridge limit, as temperature variations depend on the bridge locatio n and state practices. However, the experi-ence in a number of states including Nebraska, where temperature varia-tions are severe, would indicate ac-ceptability of this proposed system for concrete bridges up to about 600 ft (183 m) long and steel bridges up to 350ft (107m) long.

64

According to AASHT0,13 the brak-ing force is estimated as 5 percent of the bridge li ve load. In bridges, live load normally does not exceed 40 per-cent of the total vertical load. There-fore, the maximum horizontal longitu-dinal breaking force on the pile is esti mated to be 1.8 kips (8 .0 kN). Testing of the proposed joint showed that it could resist up to 5 kips (22 kN) of load before sliding occurs.

FIELD INVESTIGATION

The objective of Phase 2 of this re-search is to study the performance of two prestressed concrete piles in a full-scale integral abutment bridge. The bridge is a four-Jane , two-span continuous composite steel girder bridge located in southwest Omaha, Nebraska.

The structure has eight girders with spans, from east to west, of 160 and 164 ft (49 and 50 m). Each abutment

is supported on fifteen HPlO steel piles . Two prestressed concrete test piles are located between the steel piles, one at each abutment.

Primary instrumentation consists of an inclinometer to measure horizontal deflections at various depths within the test piles. Bridge deck length changes between abutments are moni-tored by means of a 66 ft (20 m) long extensometer tape . Temperatures were read for each measured incre-ment of distance using a mercury-in-glass thermometer in order to correct tape length.

In fro nt of each abutment at the test pile locations, observation pits were installed. They provided an exposure of about 9.0 in. (230 mm) below the bottom of the abutment. Direct obser-vations of pile distress within this depth could be made using an inspec-tion mirror and flashlight. Details of the field investigation will be pre-sented in a separate paper as soon as the study is completed.

PCI JOURNAL

CONCLUSIONS

The following conclusions are drawn from the results of this investigation:

1. The detail of the pile-abutment connection that is currently used in Nebraska and other states for steel piles needs to be modified for possible use with concrete piles. Allowable lat-eral deflections of concrete piles with rotationally restrained pile/abutment joints would be very small.

2. The capacity of steel piles to ac-commodate lateral deflections is greater than that of concrete piles. However, the difference is not signifi-cant when compression stresses are limited to their allowable values.

3. The common practice of using a

1. Greimann, L. F., Abendroth, R. E., Johnson, D. E., and Ebner, P. B., "Pile Design and Tests for Integral Abut-ment Bridges," Final Report, Iowa DOT Project HR-273, ISU-ERI-Ames 88060, December 1987.

2. Greimann, L. F., Girton, D. D. , and Hawkinson, T. R., "Validation of Design Recommendations for Integral Abut-ment Piles," Report, Iowa DOT Project HR-292, Ames, IA, September 1989.

3. Wasserman, E. P., "Jointless Bridge Decks," Engineering Journal, Ameri-can Institute of Steel Construction, Third Quarter, 1987.

4. Evans, L. T., Jr., and Duncan, J. M., "Simplified Analysis of Laterally Loaded Piles," Report No. UCB/ GT/82-04, Department of Civil Engi-neering, University of California at Berkeley, Berkeley, CA, July 1982.

March-April 1996

predrilled hole filled with loose sand has a significant effect on the behavior of laterally loaded piles. Because most of the deflections and moments occur within the top 10 ft (3.05 m) of the pile, soil type in this region will always con-trol the behavior of the pile regardless of the type of soil below this depth.

4. A new type of joint is proposed for use at the top of concrete piles. The pro-posed joint would allow the prestressed concrete piles to be used in integral abutment bridges with a total allowable movement of at least 2.3 in. (58.4 mm). The new joint would also allow con-crete piles to be used in concrete bridges up to 600ft (183.0 m) long and steel bridges up to 350 ft ( 107.0 m) long. Therefore, concrete piles could

REFERENCES

5. Reese, L. C., "Behavior of Piles and Pile Groups Under Lateral Load," Re-search Report, FHWA/RD-851106, U.S. Department of Transportation, Washington, D.C., 1985.

6. Reese, L. C., and Shin-Tower Wang, "Documentation of Computer Program LPILE, Version 3," University of Texas, Austin, TX, 1989.

7. Masticord Structural Bearing Pad De-sign Guide, Third Edition, JVI Inc., Skokie, IL.

8. Sheppard, D. A., "Seismic Design of Prestressed Concrete Piling," PCI JOURNAL, V. 28, No. 2, March-April 1983, pp. 20-49.

9. Hognestad, E., "A Study of Combined Bending and Axial Load in Reinforced Concrete Members." Bulletin 399, Uni-versity of Illinois Engineering Experi-

provide an alternative design solution in integral abutment bridges where steel piles are currently used.

ACKNOWLEDGMENT The authors wish to thank the

Nebraska Department of Roads, the Pre-cast Concrete Association of Nebraska, and the Center for Infrastructure Re-search, University of Nebraska, for spon-soring this project. Special thanks are due to Mr. Morrie Workman, Plant Manager, Wilson Concrete Company, and his staff for their assistance during the pile testing. Thanks also go to Dr. Amin Einea for his valuable input in reviewing this paper and to Ms. Deborah Derrick for proofreading and providing editorial input.

ment Station, Urbana, IL, November 1951.

10. Lin, T. Y., Design of Prestressed Con-crete Structures, Third Edition, 1981, p. 5.29.

11. Devalapura, R., and Tadros, M. K., "Stress-Strain Modeling of 270 ksi Low-Relaxation Prestressing Strands," PCI JOURNAL, V. 37, No.2, March-April1992, pp. 100-106.

12. Kamel, Mounir, "Precast Prestressed Concrete Piles in Integral Abutment Bridges," Master's Thesis, Department of Civil Engineering, University of Nebraska, Omaha, NE, 1992.

13. AASHTO, Standard Specifications for Highway Bridges, 13th Edition, Amer-ican Assocation of State Highway and Transportation Officials, Washington, D.C., 1992.

65

A, B, C, D = constants presented with details of equation by Devalapura11

Aps = area of prestressing steel

Ac = cross section area

Eps =modulus of elasticity of prestressing steel

Ec =concrete modulus of elasticity

fc' =concrete compressive strength at 28 days

fc =concrete compressive stress

fps = stress in the strands

/pu =ultimate stress of pre-stressing steel

APPENDIX A- NOTATION

Fe =compressive force in concrete block

Fpsl• Fps2 =forces in prestressing steel

MeNA =moment at neutral axis

Pse =effective prestressing force

R, EI =flexural rigidity of con-crete pile cross section

t = width of section

.1 =horizontal or lateral movement

Lit:psl =compression strains in-duced to upper strands

Lit:ps2 = tension strains induced to lower strands

e = concrete strain

eei =initial strain in concrete before applying bending moment

£0 = strain in concrete at con-crete stress equal to J:

eps =strain in prestressing strands

ee,dec =decompression strain in concrete due to effective prestressing force

eps,dec =decompression strain in strands due to effective prestressing force

8 =curvature or slope of strain diagram

APPENDIX B- ANALYSIS OF STIFFNESS OF PRESTRESSED CONCRETE PILES

The flexural rigidity, R, of the section is defined by the following equation:

R=M 8

(Bl)

where 8 is the curvature of the section, which is equal to the slope of the strain diagram, and M is the moment at the section (see Fig. 4). The nonlin-ear concrete stress-strain relationship is represented by Eq. (B2) as follows:

The integration of Eq. (B2) over the depth of compression zone (see Fig. 4) gives the total compression force Fe in the concrete block as follows:

x=c F',; = t J fcd.x (B3)

x=O

Substituting £ by ( 8x) in Eq. (B2), where 8 = t:clc, and integrating gives the total compression force Fe:

The moment of Fe about the neutral

66

axis can be given by the following equation:

x=c

McNA = t J fcxd.x (B5) x=O

Using the same substitutions as in Eq. (B4), the moment of the concrete force about the neutral axis is given by the following equation:

,28318 4 [ ( ) ( )

2 l MeNA =tfe 3 eo c -4 eo c (B6)

The stress in the strands fps can be calculated from their strain values using the following equation:

(B7)

where eps is the strain in the prestress-ing steel and fpu is the ultimate stress of the prestressing steel. A, B, C, and D are constants presented with the de-tails of the equation by Devalapura and Tadros. 11 After all losses, the strain in the strands, eps,dec• the decom-pression strain due to the effective

prestressing, is calculated from the following equation:

where

p e ___ se_ ps,dee- A E

ps ps

Pse = effective prestressing force Aps = area of prestressing steel

(B8)

Eps = modulus of elasticity of pre-stressing steel

The initial strain in the concrete, eei• before applying any bending mo-ment, is calculated from the following equation:

p £ . = £ = ____§!!__

e1 e,dee A E c c

(B9)

When the section is subjected to the applied moment causing compression in the concrete top fibers and tension in the bottom fibers, the top strands will be subjected to a certain compres-sion strain, Lit:psl• and the bottom strand to a tension strain, Lit:ps2. Flexu-ral moment decreases the tension strain in the top strands and increases the strain in the bottom strands. The final strain in the strands is calculated by the summation of eps,dee and Lieps· Fig. 5 shows the calculated moment vs. R (equivalent EI) relationship.

PCI JOURNAL

Case

1 12 in. concrete pile in loose

sand

2 12 in. concrete pile in dense sand with 1 0' predrilled hole filled with loose sand

3 12 in. concrete pile in dense sand without predrilled hole

APPENDIX C- CASES OF PILE/SOIL SYSTEMS RUN BY LPILE PROGRAM

Properties

Pile properties and Axial Load Concrete strength = 5.50 ksi Modulus of elastisity = 4230 ksi Effective prestressing Force= 100 kips Applied axial load = 90 kips

Soil Properties:

Loose sand soil Modulus of subgrade = 25 pci Density = 0.063lb/in.3 0 Angle of internal friction = 30

Soil Layers

190 kips

1-f FixedJoint 1-h HingedJoint

Pile fumerties and Axial Load

as Case 1

Soil PrQperties:

Loose sand properties as Case 1 Dense sand properties: Modulus of subgrade = 225 pci Density = 0.075 lbtin.l

0 Angle of internal friction = 40

Pile Properties and Axial Load

as Case 1

Soil PrQperties: Loose sand properties as Case 1 Dense sand properties as Case 2

190kips

2-f Fixed Joint 2-h Hinged Joint

190kips 190kips

3-f Fixed Joint 3-h Hinged Joint

Case

4 12 in. concrete pile in soft clay with 10 ft predrilled hole filled with loose sand

5 12 in. concrete pile in stiff clay with 10' predrilled hole filled with loose sand

6

10x42 steel pile in loose sand

Properties

Pile fmlzerties and Axial Load

as Case 1

Soil Pmperties:

Loose sand properties: as Case 1 soft clay properties: Modulus of subgrade = 30 pci Density = 0.063 lb/in~ Cohesion = 3.0 lb/in. Strain at 50% = 0.02

Pile Prolmies and Axial Load

as Case 1

Soil Properties:

Loose sand properties: as Case 1 Stiff clay properties: Modulus of subgrade = 200 pci Density = 0.069 lb/in~ Cohesion= 13.0 lb(m. Strain at 50%= 0.007

Pile Pro.perties and Axial Load Section 1 Ox42 HP steel pile Cross-sectional area= 12.4 in? Section modulus = 14.2 in? Steel A 36 ksi

Soil properties:

Loose sand properties: as Case 1

Soil Layers

190kips 190kips

+ •

4-f Fixed Joint 4-h Hinged Joint

190kips

+ 190kips

•

5-f Fixed Joint 5-h Hinged Joint

llOkips llOkips

6-f Fixed Joint 6-h Hinged Joint