ii i ill ii i ii i i ii i i ii ii i 111111111111111

II I Ill II I II I I II I I II II I 111111111111111 ':-c : :S "a"'·:: "

I. Report No.

FHW A!I'X-00/1856-1 I 2. Government Accession No.

4. Title and Subtitle

LATERAL CONNECTION METHODS FOR DOUBLE TEE BRIDGES

7. Author(s)

Harry L. Jones 9. Performing Organization Name and Address

Texas Transportation Institute The Texas A&M University System College Station, Texas 77843-3135 12. Sponsoring Agency Name and Address

Texas Department of Transportation Research and Technology Transfer Office P. 0 . Box 5080 Austin, Texas 78763-5080

15. Supplementary Notes

z_~c;q f i t'Q \ \Je<..s\()r)

Technical Renart Documentation Page

3. Recipient's Catalog No.

5. Report Date

August 1999 6. Performing Organiz.ation Code

8. Performing Organiz.ation Report No.

Report 1856-1 10. Work Unit No. (TRAIS)

11. Contract or Grant No.

Project No. 0-1856 13. Type of Report and Period Covered

Letter: September 1998-August 1999 14. Sponsoring Agency Code

Research performed in cooperation with the Texas Department of Transportation and the U.S. Department of Transportation, Federal Highway Administration. Research Project Title: Lateral Connection Methods for Double Tee Bridges 16. Abstract

This report presents the results of a study of potential means for connecting the adjacent flanges in double tee bridges. The research team reviewed connection details found in the literature, analyzed the current TxDOT connection, and recommend a detail for use with composite deck slabs.

17. Key Words 18. Distribution Statement

Prestressed, Concrete, Bridge, Multi-beam, Double Tee, Connection

No restrictions. This document is available to the public through NTIS: National Technical Information Service,

19. Security Classif.(ofthis report)

Unclassified

_Form DOT F l 700~ 7 (8-72)

5285 Port Royal Road, Springfield, Virginia 22161

I 20. Security Classif.(ofthis page)

Unclassified - · - ·

Reproduction of completed pace authorized ·

.. 21. No. of Pages

38

LATERAL CONNECTION METHODS FOR DOUBLE TEE BRIDGES

by

Dr. Harry L. Jones Associate Research Engineer

Department of Civil Engineering and

Texas Transportation Institute

Report 1856-1 Project Number 0-1856

Research Project Title: Lateral Connection Methods for Double Tee Bridges

Sponsored by the Texas Department of Transportation

In Cooperation with the U.S. Department of Transportation

Federal Highway Administration

August 1999

TEXAS TRANSPORTATION INSTITUTE The Texas A&M University System College Station, Texas 77843-3135

DISCLAIMER

The contents of this report reflect the views of the author, who is solely responsible for the facts and accuracy of the data, and the opinions, findings, and conclusions presented herein. The contents do not necessarily reflect the official views or policies of the Texas Department of Transportation (TxDOT), Federal Highway Administration (FHW A), The Texas A&M University System, or the Texas Transportation Institute. This report does not constitute a standard, specification, or regulation, and its contents are not intended for construction, bidding, or pennit purposes. In addition, the above listed agencies assume no liability for its contents or use thereof The use of names of specific products or manufacturers listed herein does not imply endorsement of those products or manufacturers. The engineer in charge of the project was Dr. Harry L. Jones, P.E.# 35066.

V

ACKNOWLEDGMENTS

This research project is being conducted under a cooperative program between the Texas Transportation Institute, the Texas Department of Transportation, and the U.S. Department of Transportation, Federal Highway Administration. The TxDOT project director for this research is Mr. Jeff Cotham. His assistance is acknowledged and appreciated.

VI

TABLE OF CONTENTS

Page

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

INTRODUCTION .. . ...... ... ..... ...... ..... . . ... .... . .. .. . ....... .. ..... . 1

SURVEY OF CONNECTION DETAILS .. . .. ........ . . . .... .. .. .. . .. . .. .. . . .. . 1 Parking Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 State DOT Bridges .. ........ .. . . .... ... .. . .. . . .. ... . .... . ....... .. ..... 4

TxDOT STANDARD CONNECTION ..... . ... . ... . ... . . ... . ......... ... .. ... 16

LIVE LOAD DISTRIBUTION FACTOR .. .. . . . . .. ...... .. ..... . . .. . ... .. . .... 23

CONNECTION FORCES .. . .................... ... ... . .. .. . . . . . .. . . .... . . .. 23

NEW CONNECTION RECOMMENDATIONS-TEES WITHOUT COMPOSITE DECK ... .... ..... .. ........ .. .. .. . .. ..... . ... 24

NEW CONNECTION RECOMMENDATIONS-TEES WITH COMPOSITE DECK . . ... . . . .. . ..... . .. ....... .... .... . . .. .. . . .. 25

BIBLIOGRAPHY . .... . . . .... ..... . . .. .. .. . .. .. .... .... ... .. . . . .. ... .. .... 27

vu

LIST OF FIGURES

Figure Page

1 Connection Detail Developed by Martin et al . (First of Two Versions) .. . . . ... .. . 2 2 Connection Detail Developed by Martin et al. (Second of Two Versions) . ... ... .. 2 3 Connection Detail Developed by PCI ... . ... . . . .. .. . . .. . . .. .. . .. ... . . .. .. 3 4 Connection Detail Used by Nebraska DOT .. . . ... ... . . . .. . . . .. ... . .... . . . . 4 5 Connection Detail Developed by Florida DOT . ... ... ..... . .. . . . . .. . . . .... . 5 6 Welded Plate Detail by Martin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7 Connection Details for Joining of Flanges in Double Tees .. . .. . .. ........ .. .. . 7 8 Methodologies for Connecting Multi-Stemmed Precast Members (48 in crs) . . . . .. 9 9 Methodologies for Connecting Multi-Stemmed Precast Members (96 in crs) .. . . . 10

10 Methodologies for Connecting Multi-Stemmed Precast Members (60 in crs) . .. . . 11 11 Methodologies for Connecting Multi-Stemmed Precast Members (55 in crs) . .. . . 12 12 Connector Details of Specimens IA, lB, and lC . . .. .. . .... . .. ......... . . . 13 13 Connector Details of Specimen 2A . ... . .. .. . . .... .. . . . .. . . . . . . . . ..... . . 14 14 Connector Details of Specimens 3 A and 3B . . . . . .. .... ... . ... . . . . . . .. .. . . 15 15 Connection Used by Tx:DOT for Double Tee Bridges . . ... ... . .. . .. . .. . . .. .. 17 16 Four Components ofForce Acting on a Connection . . . ...... . . . . . . ... . . . . .. 19 17 Suggested Joint Detail with Composite Deck Slab . .. . . . . . . .... . .. . .. . . . . .. 26

Vlll

:;:.

Table

1 2 3

4 5

6

7

LIST OF TABLES

Page

Total Shear Strength . . . . .. . . . . . . . . . . .... . . .. .. . .. ... . .. . .. . . . . . ... . 16 Beam Arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Analysis of Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Details of Connection Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (22 in Tee) .. .. . . . . ... .. . . .. .. . . . . . . 20 Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (28 in Tee) . . . .. . . . .. . . . . . . . . . . . . . . . 21 Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (36 in Tee) . .. .... . . . . .. . . . ... . . .. . . 22

IX

INTRODUCTION

This interim report presents a summary of information developed on methods of

connecting the edges of flanges in prestressed concrete double tee bridges to ensure that adequate

lateral transfer of wheel loads takes place. The data presented comes primarily from a survey of

literature on connections for precast concrete elements and transportation structures, along with

information gathered through telephone conversations with transportation officials in various

states concerning their experiences with double tee bridges. After discussion of the merits of

various connection types, recommendations are presented for connections that should be

investigated further for possible use in TxDOT bridges. These recommendations are intended as a

starting point for discussions with TxDOT design and construction personnel as we seek to

?evelop the best possible connection detail.

SURVEY OF CONNECTION DETAILS

Parking Structures

Prestressed concrete double tees are most widely used in building structures. Among

these, parking garages have conditions which loosely approximate those found in bridges in that

vehicular wheel loads are to be transferred between adjacent units. In order to accomplish this,

and also in some cases for seismic considerations, various schemes have been developed for tying

adjacent flanges together. Figures 1 and 2 show two details cited by Martin et al. [1983]. Each of

these is typically spaced at 4--0 ft centers along the common edge between adjacent tee units and

involves no grouting. While relatively inexpensive to fabricate and install, there is little or no data

on their performance under long-term HS-20 truck traffic. Figure 3 shows another detail,

developed by the Prestressed Concrete Institute (PCI) and described in PCI [1998] which offers a

calculation procedure for sizing the 12 in anchor bars and determining the spacing of connections

along the edge. The spacing calculation is somewhat dubious in that it is based on the shear

strength of the flange concrete and is not related to the vertical wheel force which must be

transmitted across the joint. All three connections cited in this section are used without grouting

of the joint and may involve the use of an asphalt wearing surface placed on the tees.

1

c:,0

c:,O

3/4"

l c;o

c:,0

t c::,O

c:,0

11 /4 II --, ~ c:,O

Joint Sealant

Bar to Fit: Weld at Top

~ Weld Plate and Anchor

Figure l. Connection Detail Developed by Martin et al. (First of Two Versions) .

Flat Bar 3/4"

f 1" Clearance

c::;o

- . - c7 -o,"""?:r" - · -

·_c:,Q

L 3/8"

Figure 2. Connection Detail Developed by Martin et al. (Second of Two Versions) .

· 2

1 2"

T

10°

welded wire mesh 4 x 4, w4 x w4

I ◄ 4 II

6"

►I

Figure 3. Connection Detail Developed by PCI.

3

State DOT Bridges

Martin et al. (1983] reports the experience of the Nebraska DOT with the connection

shown in Figure 4 on 27 in tees having a 5 in flange thickness and no composite deck slab. These

structures were built as replacement bridges on relatively low-volume roads and at the time of the

report, had served with no reported problems. Conventional grout was used in the shear keys and

the plate/bar lateral connections were spaced at 4-6 ft centers. At the time of the study, no

Grouted Shear Key \

- - - - - - =O ------"'(:: 4" Long Steel Plate

Steel Bar

Figure 4. Connection Detail Used by Nebraska DOT.

difficulties were reported after approximately five years of service on a low-volume road with

unknown proportion of truck traffic.

El Shahawy (1990] describes a double tee design developed by Florida DOT for state and

interstate class highways with spans up to 80 ft. The lateral connection detail is seen in Figure 5

and consists of a continuous grouted shear key ("V-joint") and heavy transverse post-tensioning.

A half-scale bridge model was constructed and tested to determine the performance of this type

lateral connection. The model had 3 .25 in thick flanges, and non-shrink portland cement grout

with minimum strength of 6,500 psi was used to form the shear key. Post-tensioning was applied

to produce a transverse normal stress across the joint of 150 psi in the central region and 300 psi

in the end regions. Various forces were applied to the bridge to cause load transfer across the

joint. Deflection measurements taken during tests were used to argue the absence of slip across

the joint at this level of prestress. Crack width (joint opening) was also monitored during testing,

and was found to be small (less than 0.005 in) at loads equivalent to an HS-20 truck.

6½"

1 5/s"

*

3 - 1 /2 f Post-Tension Strands @ 4'- 6" o/c Spacing

Non-Shrink Grout

Figure 5. Connection Detail Developed by Florida DOT.

El Shahway and Issa [ 1992] describe load tests on a full-scale bridge similar in detail to

that described above. The structure spanned 60 ft, had an overall width of 30 ft, and was

constructed from six 34 in deep tees with 6.5 in thick flange and no wearing surface or composite

deck. The V-joint in Figure 5 joined the edges of adjacent flanges and a transverse post-tensioning

level of200 psi was achieved by placing three half-inch diameter grade 270K strands in 1.25 in by

3.25 in galvanized metal ducts. The bridge was loaded with two five-axle trucks, each weighing

204,000 lbs and deflection measurements taken at ends, quarter points, and mid-span of each

beam stem. After comparing measured deflections with theoretical values, the authors conclude

"the results strongly suggest practically perfect moment and shear transfer between the double tee

beams." In addition, they recommend a minimum of 150 psi transverse post-tensioning for

satisfactory performance of their joint. Arockiasamy et al [ 1991] reported the results of cyclic

loading of the Florida V-joint i~ a 1:3 .5 scale model. While cracking in the longitudinal joints was

reported, it appears as if it is related to the magnitude of loads and not the result of degrading

performance under the application of two million cycles of load.

5

Martin et al [ 1983] reported the use of the welded plate detail shown in Figure 6 by a state

DOT, but does not identify which state. The spacing of the horizontal steel plates was not given,

but likely is in the range of 4--6 ft as cited earlier in their report. A non-shrink grout was used to

fill the key. The same report presents several schemes for forming a lateral connection between

precast deck panels. Figure 7 shows one of those connection details which might have application

to joining of flanges in double tees. It also cites the use of epoxy grout for shear keys by railroads

on box beam bridges. These railroads report good results with this material. They form shear keys

by prefilling the keyway with aggregate and then pouring the liquid epoxy directly into the key,

making the installation much faster . They report using aggregate ratios up to 70 percent.

3/8"

Fill with Approved Non-Shrink Grout. Cover for Curing

Flat Bar 3/4" x -4" Long Steel Plate

~--Steel Stud

L 318 11 ± th.

Figure 6. Welded Plate Detail by Martin.

6

Mild Steel Reinforcement

Tape

C.I.P. Closure Pour

Lap/Welded Bars

Partial Depth Continuous

Partial Depth Keys

Figure 7. Connection Details for Joining of Flanges in Double Tees.

7

Stanton and Mattock [ 1986] reviewed methodologies for connecting multi-stemmed

precast members. They point out that the American Association of State Highway and

Transportation Officials (AASHTO) presently [1986] provides no guidelines for the design of

joints between multi-stemmed members and in practice, grout key size and shapes and connector

requirements are determined by using rule-of-thumb methods and historical performance, rather

than rational analysis. They collected details which had been proposed or used by transportation

agencies and which are shown in Figures 8 through 11 . All are a combination of grouted shear

key and steel connectors, which were described as being spaced at 4-8 ft centers. Stanton and

Mattock, as well as others, suggest that the role of the steel connectors is to prevent the joint

from separating under the action of loads and temperature change, while the grouted key transfers

vertical shear across the joint. A survey of county transportation officials in the state of

Washington where these details had been used suggested "a combination of a grout key and

welded connectors function very well." No indication of the volume of truck traffic each structure

carried was given, although being county roads, it is unlikely that it is comparable to that on

interstate highways.

A significant effort was made by Stanton and Mattock to characterize the strength of the

studs used to anchor typical welded connectors (see Figures 4 and 9). They reviewed available

design procedures for estimating the shear and tension resistence of these elements. In addition,

they conducted a series of six load tests on the connection details shown in Figures 12 through 14

to access the effects of the following variables on the response of the connection: (i) location of

hardware within the thickness of the slab, (ii) weight of the connector hardware, ·and·(iii) the size

and shape of the grouted shear keys. Each test specimen contained a single welded connection

between two 5 ft long by 6 in thick concrete panels. In some tests there was a grouted shear key

and in others no key was poured. All specimens were loaded by a pair of concentrated forces

acting on 6x6xl in steel plates both situated on one side of the panel joint. The total shear force

acting across the joint was recorded at failure. Those results are shown in Table 1.

From the results in Table 1 it is obvious the total shear transmitted across a joint is greatly

enhanced by the shear key (compare lA, 2A, and 3A with the remaining specimens which had a

grouted key). The welded connections alone sustained between 4,700 and 6,700 lbs of shear

before failure.

8

Keyway Detail

Grout 1/2" --.1 ~

--"------1

5 II

1,2 .. ~L 1 "j=

2 II

2½" 1"__.. __

Backer Rod

Welded Connections at 48 in centers

1/4" 4" _i_.__

1 ½ II

T

L2"x1½"x¼ X 6 11

2"x¼"x4"

1/4" 1½"

#4 Bars welded to connector at opposite edge of flange

Figure 8. Methodologies for Connecting Multi-Stemmed Precast Members (48 in crs) .

9

Keyway Detail

Grout

Backer Rod

Welded Connections at Up to 96 in Centers

L 2½" x 2" x 3 /s" X 4"

2" x ¾" x 3 ¼"

6 ed Studs


Keyway Detail

6" ¼"-.. 1¼'T

Grout

¼"t 2" 1 "_., ,._ t

Backer Rod

Welded Connections at Up to 60 in crs . typ.

¼" 2"

3½1 t-------- 2 11 x ¼ 11 x 7 11

2½" f t,-2-"---.1...----,1 ½ 11 f Studs

7" #5 X 30 -- Anchor Bolts ------

Figure 10. Methodologies for Connecting Multi-Stemmed Precast Members (60 in crs).

11

Keyway Detail

6½"

Grout ,f 1½"

1¼" 3 II

¾"

Backer Rod

·Welded Connections at 55 in crs. typ.

2" X 5/1611

X 8"

2"

L 4" X 4" X 5f15" typ.5, II X 8" _i ,1~ 6 __ ~--"--r-, r--ri;;;t:-------

r I

4" t: -{rr: :-: : :-: : ~ ~::::::: m , , ¾" f x 6 I I

: : Headed Studs -c~: :-: : :-: : ~~ ~~::::::::) 2" t


12

1½"t ½"

1 " 1 II

2"

¼" 4"

____________ i1"\. __ _

V ,., ________ u_

L 2" X 1½" X ¼ 11

Length 6

2" X 1/4" X 4"

2"

¼" 1 ½" #5 Bar Slab

_ 7 i-----~ Reinforcement L __

---------7 ["""_J ---------7 ["""_J

----- ---

L_""'J ,---------L_""'J ,----------

(a) Connector details, specimens 1 A, 1 B, & 1 C

½" ½" ½"

fftM 1" ►. ~ 1" Mttti

Structural -i-1½"

-r,i½" Grout

-i-1½" Foam Backer =11½" Rod

-.JI+ 3/a" _.I ~3/a"

Type A Type B

(b) Keyway details

Figure 12. Connector Details of Specimens lA, lB, and lC.

13

6 II

L 2" x 1½" x ¼" Length 6

¼" 1½"

~-

_i_ 2"

#5 Bar Slab Reinforcement

(a) Connector details, specimen 2A

Figure 13. Connector Details of Specimen 2A.

. 14 ..

t 211

t 1 II

L 4 II X 4 II X 5/16 11

Length 8

1 II

3/s"

,--,- #5 Bar Slab I 1 1 1 Reinforcement _i ______ ! L:.-=.::::::::........L....

211 =-~~~~~~~~]L_ µ::!;!;.~~,-r, ~~~~~-~-t:eaded Studs I ------1 -~ 1------T ------,,-~ ~-7,------

(b) Connector details, specimens 3A and 38

Figure 14. Connector Details of Specimens 3A and 3B.

15

Table 1. Total Shear Strength.

Specimen No. Concrete Strength Grout Strength Shear in Connection

(psi) (psi) at Failure (kips)

1A 5,470 * 4.78

18 5,895 3,280 11.60

1C 5,775 3,615 17.35

2A 5,680 * 4.95

3A 5,600 * 6.70

38 4,400 4,175 20.38

TxDOT STANDARD CONNECTION

Figure 15 shows a connection used by TxDOT for double tee bridges in the past.

Although a modified version of this connection has been adopted which replaces the vertical

plates with angles, the plated connection was examined because its stiffness properties are more

reliably computed. An analytical study was performed to gather information on the performance

of this connection detail and its effect on the beams in a bridge.

Five bridge configurations ranging in roadway width from 24-44 ft were studied. Table 2

lists the beam arrangements for each of these designs. For each bridge listed in the table, 22, 28,

and 36 in deep tees were considered, and for each depth tee, a short, medium, and long span (see

Table 3) was analyzed. For each of the combinations of span and tee size, both nominal 5 and 10

ft connection spacings were examined. Details of the connection locations are contained in Table

4. Section properties for the beams were provided by TxDOT and only the 6 in flange without

composite concrete deck slab was examined.

The program :MBBA used in a previous study (Jones [1999]) and described in Jones and

Boaz [1986] was utilized for the analyses. This structural model idealizes each discrete connection

between adjacent beam flanges as a series of four linear springs which develop the four

components of force s~own in Figure 16. The forces are proportional to the relative separation of

the joint in the X, Y, and Z directions and differential rotation about the X-axis. Prediction of the

four force components provides a means of estimating the stresses induced

16

--...J

5 X 3/e" X 0'-6" le. with ¾ftf Hole

Field Trim as Necessary __

~---6"

( 3"

-'---L.--+--t.._-:..,-;..,._ ___ - _-T""~ rl-;-~-,

~ Joint& 3f4R fHole

PLAN

½ft f x 8" Studs (Bend 1 ¼" In Last 3")

Edge of Beam

6 X ½" X 0'-8" le.

- ~Joint

SECTION

Fill with Non-Shrink Mortar

3 Position D1 Bars as Shown

Figure 15. Connection Used by TxDOT for Double Tee Bridges.

Table 2. Beam Arrangements.

Roadway Width Double Tee (ft) Arrangement

24 6-7-7-6

28 6-6-6-6-6

30 6-6-8-6-6

38 6-6-8-8-6-6

44 6-6-8-6-8-6-6

Table 3. Analysis of Spans.

Double Tee Short Span Medium Span Long Span Depth (ft) (ft) (ft)

(in)

I

22

I

22

I

28

I

36

I 28 30 42 54

36 40 52 64

Table 4. Details of Connection Locations.

5 FT SPACING 10 FT SPACING

Distance of First and Distance of First and Span (ft) Number of Last Connection From Number of Last Connection From

Connections Bridge Ends Connections Bridge Ends (ft) (ft)

22 4 3.5 2 6

28 5 4 3 4

30 5 5 3 5

36 7 3 4 3

40 7 5 4 5

42 8 3.5 4 6

52 10 3.5 5 6

54 10 4.5 5 7

64 12 4.5 6 7

18

X

z

Figure 16. Four Components of Force Acting on a Connection.

19

N 0

Configuration

6-7-7-6 24 ft roadway

6-6-6-6-6 28 ft roadway

6-6-8-6-6 30 ft roadway

6-6-8-8-6-6 38 ft roadway

6-6-8-6-8-6-6 44 ft roadway

Span (ft)

22

28

36

22

28

36

22

28

36

22

28

36

22

28

36

Table 5. Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (22 in Tee).

6 ft Tee 7 ft Tee 8 ft Tee

Interior Beams Exterior Beams Interior Beams Exterior Beams Interior Beams Exterior Beams

5ft 10 ft 5 ft 10 ft 5ft 10 ft 5ft 10 ft 5ft 10 ft 5ft 10 ft Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac-

ing ing ing ing ing ing ing ing ing ing ing ing

0.526 0.526 0.645 0.644

0.514 0.511 0.608 0.589

0.521 0.517 0.577 0.586

0.650 0.677 0.510 0.511

0.617 0.675 0.495 0.497

0.559 0.569 0.484 0.486

0.555 0.557 0.510 0.510 0.752 0.776

0.514 0.505 0.495 0.497 0.695 0.740

0.487 0.492 0.487 0.487 0.624 0.639

0.522 0.539 0.510 0.510 0.612 0.649

0.446 0.444 0.495 0.497 0.520 0.574

0.392 0.407 0.571 0.474 0.417 0.452

0.532 0.544 0.510 0.511 0.596 0.633

0.451 0.449 0.495 0.497 0.503 0.558

0.393 0.407 0.471 0.474 0.417 0.436

Configuration Span (ft)

6-7-7-6 30 24 ft roadway

42

54

6-6-6-6-6 30 28 ft roadway 42

54

6-6-8-6-6 30 30 ft roadway

42

54

6-6-8-8-6-6 30 38 ft roadway

42

54

6-6-8-6-8-6-6 30 44 ft roadway 42

54




5 ft 10 ft 5ft 10 ft 5 ft 10 ft 5 ft 10 ft 5 ft 10 ft 5ft 10 ft

Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac-ing ing ing ing ing ing ing ing ing ing ing ing

0.525 0.523 0.634 0.615

0.521 0.518 0.588 0.611

0.527 0.521 0.560 0.562

0.637 0.690 0.508 0.509

0.574 0.559 0.487 0.488

0.520 0.554 0.483 0.481

0.535 0.525 0.508 0.508 0.728 0.768

0.498 0.510 0.489 0.489 0.642 0.643

0.471 0.477 0.486 0.484 0.577 0.610

0.481 0.479 0.508 0.509 0.558 0.606

0.400 0.453 0.483 0.502 0.445 0.559

0.364 0.370 0.456 0.461 0.370 0.409

0.489 0.489 0.508 0.508 0.540 0.588

0.402 0.429 0.484 0.486 0.429 0.435

0.354 0.368 0.456 0.461 0.358 0.395

N N

Configuration

6-7-7-6 24 ft roadway

6-6-6-6-6 28 ft roadway

6-6-8-6-6 30 ft roadway

6-6-8-8-6-6 38 ft roadway

6-6-8-6-8-6-6 44 ft roadway

Span (ft)

40

52

64

40

52

64

40

52

64

40

52

64

40

52

64




5ft 10 ft 5ft 10 ft 5 ft 10 ft 5 ft 10 ft 5ft 10 ft 5 ft 10 ft Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac- Spac-

ing ing ing ing ing ing ing ing ing ing ing ing

0.528 0.530 0.648 0.659

0.523 0.516 0.595 0.600

0.530 0.524 0.569 0.591

0.599 0.605 0.511 0.512

0.493 0.610 0.574 0.495

0.488 0.532 0.529 0.486

0.546 0.558 0.510 0.511 0.699 0.712

:o.505 0.513 0.493 0.495 0.646 0.682

0.480 0.497 0.491 0.489 0.591 0.606

0.480 0.509 0.510 0.511 0.517 0.538

0.401 0.420 0.492 0.495 0.441 0.484

0.368 0.394 0.471 0.476 0.382 0.401

0.487 0.510 0.511 0.512 0.499 0.518

0.404 0.423 0.493 0.495 0.424 0.466

0.363 0.393 0.472 0.477 0.368 0.385

in a connection by vehicular traffic. For a given loading (e.g., HS-20 truck), the model gives stress

resultants at the beam's center of gravity, providing a means of predicting forces needed in the

design of the double tee reinforcing. Modifications to the basic MBBA program allow the

computation of live load lateral distribution factors for each beam in the structure, by dividing the

bridge into traffic lanes and moving an HS-20 truck about within each of the lanes to obtain the

extreme beam moments. For this study, additional modifications were made to the program to

track the largest force components occurring in the connections.

LIVE LOAD DISTRIBUTION FACTOR

Tables 5 through 7 show the live load distribution factors computed for the sample

structures described above. Each table corresponds to a different depth tee, and within each table,

the distribution factors for interior and exterior beams are listed for both 5 ft and 10 ft spacing of

connections.

Several trends are suggested by these results. First, the distribution factor for a particular

structure is not very sensitive to the connection spacing. In comparing two structures which differ

only in that one has a nominal 5 ft spacing and the other 10 ft, a 2-5 percent difference in

distribution factor is typical, although some isolated cases are higher. Experimentation with

selected bridges in which the stiffnesses of the connections were increased by threefold (while

keeping the spacing the same) produced only about a 4 percent reduction in distribution factor. It

appears from these results that adding more of the current connections along adjoining flanges or

making the connection larger and stiffer would yield a very marginal reduction in the lateral

distribution factor. Furthermore, adjusting discrete connection details alone doesn't appear to be a

viable means of gaining beam economy through reduction in maximum design moment.

CONNECTION FORCES

The four components of force acting on a connection were described earlier and shown in

Figure 16. The same analyses described above were used to estimate the maximum force

components developed in the connection when used with 5 ft and 10 ft nominal spacing. Among

the five different bridge widths, three span lengths, and three depths of tee (45 different cases in

all), the maximum force components found are listed in Table 8. Generally speaking, the

23

connection nearest mid-span tended to develop the largest forces. In all cases, the longitudinal

component Fx was small relative to the other components and is therefore omitted from the table.

As one would expect, the larger connection spacing and resulting fewer connections available for

load transfer leads to larger force components. The normal stress produced by the force

component FY in both cases is under 5 ksi, and the shear stress produced by component Fz is 6-8

ksi. The predicted normal stress produced by the transverse moment Mc is enormous, on the

order of 85 ksi in the case of 5 ft spacing and 100 ksi for the larger spacing. Such stresses

obviously could not be generated in a connection of grade A36 steel with a nominal yield strength

of 36 ksi, and certainly raises the possibility of low cycle fatigue induced fractures in such

connections.

NEW CONNECTION RECOMMENDATIONS-TEES WITHOUT

COMPOSITE DECK

In reviewing the connection details used by others it is clear that most incorporate some

sort of continuous grouted shear key. Several writers describe the discrete connections as simply

devices whose function is to keep the beams from separating, presumably to ensure the integrity

of the key. The same comments apply to transverse post-tensioning of the deck in lieu of discrete

connections. The limited test data cited in Table 1 certainly suggest that in such connections, the

shear key transfers most of the load with the connectors playing a secondary role. However,

regardless of efficiency, other considerations may make any type of grouted key unattractive to

TxDOT.

The discrete connections for joining flanges developed by others and described earlier in

this report don't appear to offer any particular advantages over the basic design now used by

TxDOT, although the predicted bending stress level in the TxDOT connection mentioned above is

of concern. This not withstanding, the ability of this connection to transfer load appears

equivalent to those used by others. The relative economics among them is not clear at this point,

and guidance ofTxDOT designers is needed in assessing it. Those connections utilizing a

continuous shear key and/or post-tensioning are attractive from a potential performance

standpoint, but again, their cost-effectiveness is unclear.

24

The addition of an asphalt wearing surface to the double tee bridge leads to longitudinal

cracking of the asphalt over the joint between adjacent beam flanges. This problem has been

documented by Cotham (1997] for double tee bridges in Texas. Differential rotation about a

longitudinal axis of adjacent beams (i.e., a torsional rotation) produces transverse moment in the

asphalt layer over the joint between beam flanges and the resulting tensile stresses cannot be

resisted by the asphalt material. The analyses described earlier on TXDOT bridges make it

apparent that no discrete connection like those in Figures 1-4, 6, and 15 can restrict this rotation

and prevent longitudinal cracking of the asphalt layer. If this type of connection is to be

continued, then it is recommended that a longitudinal joint in the asphalt be placed over the

juncture between flanges, perhaps sealed from below with a waterproof membrane over the joint

to control staining of the underside of beams where they are visible.

NEW CONNECTION RECOMMENDATIONS-TEES WITH

COMPOSITE DECK

When a composite concrete deck is added to the double tee bridge, the nature of force

transfer between adjacent beams is substantially altered. The slab becomes the primary force

transfer agent and the discrete connections play a secondary role. Based on previous experience

with box beam bridges (Jones (1999]), we know that significant transverse bending moment will

develop in the slab which typically leads to objectionable longitudinal cracking in the deck over

the joint between adjacent flanges. Inspection of seven composite slab bridges in Texas by

Cotham (1997] supports this conjecture. Because the overhanging flange of the douf>le tee is not

as stiff as the outside walls of the box beam, the author has reservations that the MBBA model

used in the box beam study can be applied without modifications when a concrete deck overlays

the tees. We are still working to resolve this issue by constructing detailed finite element models

of selected bridges and comparing the predicted response with the results from the MBBA

software.

While all the information needed is not yet in hand, we strongly suspect that the situation

will tum out to be comparable to the condition which exists in box beam bridges. If this is in fact

the case, a greater slab thickness and more reinforcing are going to be needed in an effort to

25

/

control the longitudinal cracking. Additionally, we found in the box beam study that longitudinal

shrinkage cracking typically occurs in the slab between beams, making the growth of longitudinal

cracks due to transverse moment more pronounced. In short, it seems preventing the formation of

longitudinal cracks is considerably more challenging than controlling them. The author therefore

proposes, as a point of departure for further discussion, creating a longitudinal joint like that

shown in Figure 17. The goal of the detail is to eliminate transverse moment in the slab at this

location, while allowing for adequate transfer of force between adjacent beams with a discrete

connection or continuous shear key used when no composite deck is placed on the double tees. A

sheet metal waterstop is provided to prevent moisture from staining the underside of the flange

member. Transverse reinforcing in the deck slab is not continuous across the cold joint, thus

removing the ability to develop transverse moment.

Sheet Metal Strip Bonded to Tee Flange for Moisture Barrier

-------o-

Cold Joint

►

Terminate Transverse Steel at Joint

, -------.__a- _____ _ . C. I. P. Deck Slab

-- Double Tee Flange

Same Connection Used without Composite Deck

Figure 17. Suggested Joint Detail with Composite Deck Slab.

26

BIBLIOGRAPHY

American Association of State Highway and Transportation Officials, Standard Specifications for Highway Bridges, 14th Edition, 1986.

Arockiasamy, M., et al. , Fatigue Strength of Joints in a Precast Prestressed Concrete Double Tee Bridge, PCI Journal, January-February, 1991.

Cotham, J., Double Tee Field Survey, Internal Report, Design Division, TxDOT, 1997.

El Shahawy, M., Feasibility Study of Transversely Prestressed Double Tee Bridges, PCI Journal, September-October, 1990.

El Shahawy, M., and Issa, M., Load Testing of Transversely Prestressed Double Tee Bridges, PCI Journal, March-April, 1992.

Jones, H. L., Multi-Box Beam Bridges with Composite Deck, TTI Final Report 0-1709, April 1999.

Jones, H.L., and Boaz, I.B., Skewed Discretely Connected Multi-Beam Bridges, Journal of · Structural Engineering, American Society of Civil Engineers, February 1986.

Martin, L. D., et al., Connections/or Modular Precast Concrete Bridge Decks, Federal Highway Administration, Report FHW A/RD-12/106, 1983.

PCI Committee on Connection Design, Standard Precast Connections, PCI Journal, July-August, 1998.

Stanton, J. F., and Mattock, A.H., Load Distribution and Connection Design/or Precast Stemmed Multibeam Bridge Superstructures, NCHRP Report 287, Transportation Research Board, November 1986.

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