ii i ill ii i ii i i ii i i ii ii i 111111111111111
TRANSCRIPT
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
Figure 9. Methodologies for Connecting Multi-Stemmed Precast Members (96 in crs) .
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
Figure 11. Methodologies for Connecting Multi-Stemmed Precast Members (55 in crs) .
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
Table 6. Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (28 in Tee).
6 ft Tee 7 ft Tee 8 ft Tee
Interior Beams Exterior Beams Interior Beams Exterior Beams Interior Beams Exterior Beams
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
Table 7. Live Load Distribution Factor (fraction of a truck) Standard TxDOT Lateral Connections (36 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 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.
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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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