Sriram Aaleti and Sri Sritharan 1 DESIGN OF UHPC WAFFLE DECK FOR ACCELERATED BRIDGE 1 CONSTRUCTION 2 3 Sriram Aaleti 1 and Sri Sritharan 2 4 5 1 Assistant Professor, Department of Civil, Construction and Environmental Engineering, University of Alabama, 2037C, SERC Building, Tuscaloosa, AL, 35487; Phone: 205-348-5110; Fax: 205-348-0783; [email protected](Corresponding Author) 2 Wilson Engineering Professor, Department of Civil, Construction, and Environmental Engineering, Iowa State University, 376 Town Engineering Building, Ames, IA, 50011-3232; Phone: 515-294-5238; Fax: 515 -294 -7424; [email protected]6 7 8 Submission Date: August 1 st , 2013 9 10 Words in Text: 4793 11 Number of Figures: 7 12 Number of Tables: 2 13 Total Word Count: 7043 14 15 TRB 2014 Annual Meeting Paper revised from original submittal.
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Sriram Aaleti and Sri Sritharan 1
DESIGN OF UHPC WAFFLE DECK FOR ACCELERATED BRIDGE 1
CONSTRUCTION 2
3
Sriram Aaleti1 and Sri Sritharan
2 4
5 1
Assistant Professor, Department of Civil, Construction and Environmental Engineering,
University of Alabama, 2037C, SERC Building, Tuscaloosa, AL, 35487; Phone: 205-348-5110;
The tension behavior of UHPC can be represented with an elastic-perfectly plastic 26
curve. The design tension strength of the UHPC can be taken as 1.2 ksi. 27
The elastic modulus of UHPC can be approximated to √ . In the absence 28
of exact concrete strength, a modulus value of 7,500 ksi can be used for design purposes. 29
The unit weight of the UHPC is 157 lb/ft3. 30
The minimum concrete cover for unprotected mild steel reinforcement in UHPC shall be 31
0.75 inches because of excellent durability properties of UHPC. 32
33
UHPC WAFFLE DESK SYSTEM 34
Analogous to the typical full-depth precast deck systems currently used in practice and 35
developed in previous research (12), the waffle deck system consists of a series of UHPC waffle 36
deck panels that are full-depth in thickness (as required by the structural design) and connected 37
to the supporting girders with robust connections. A UHPC waffle deck panel consists of a thin 38
slab, cast integrally with concrete ribs spanning in the transverse and longitudinal directions. 39
This system is similar to the two-way joist system used by the building industry. The schematic 40
of the waffle deck system is shown in FIGURE 1. The transverse ribs along the deck panel acts 41
as T-beams, distributing wheel load effects to the adjacent bridge girders. The longitudinal ribs 42
help in distributing the wheel load to the adjacent panels through the panel-to-panel connections. 43
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 5
The reinforcement needed to resist the wheel loads is provided in the ribs along both directions. 1
The spacing of the ribs in both directions is determined based on the girder-to-girder spacing, 2
panel dimensions, and minimum detailing requirements for panel-to-panel connections. 3
4
5
6
FIGURE 1 Schematic of UHPC waffle deck system 7
8
The UHPC waffle deck system for a given thickness has the same or higher capacity and 9
is 30 to 40 percent lighter than a comparable solid precast full-depth panel made of normal 10
strength concrete due to the improved structural properties of UHPC. The decreased weight of 11
the UHPC panel has significant benefits including, increase in span length for a given girder size, 12
increase in girder-to-girder spacing, improvement in bridge ratings when used for deck 13
replacement projects, and reduction in seismic, substructure, and foundation loads when 14
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 6
compared to solid precast deck panel systems. The presence of the steel fibers in UHPC and very 1
minimal shrinkage of UHPC after steam curing of the precast elements also decreases the 2
reinforcement requirements when compared to traditional precast deck panels. 3
DESIGN OF WAFFLE DECK PANELS 4
The design of the waffle deck panels consist of two main steps: geometrical design and 5
structural design. In geometrical design, critical dimensions of the waffle deck panel are arrived 6
based primarily on the bridge functional requirements. The structural design phase consists of the 7
design of the primary deck reinforcement (both transverse and longitudinal) to resist the 8
AASHTO design loads. 9
Geometrical Design: 10
In this section, several recommendations to arrive at the dimensions of the UHPC waffle 11
deck panel are provided. 12
Thickness of the waffle deck panel: Considering the minimum thickness requirements of 13
Article 9.7.5 of AASHTO 2010 and structural capacity requirements, an 8-inch thick panel 14
was found to be structurally sufficient for most cases. 15
Length and width (dimensions perpendicular and parallel to direction of the traffic): 16 length and width of the panel depends on the handling requirements of the panels at the 17
precast plant and at the job site along with the transportation constraints. If the roadway 18
width is more than 24 ft, it is recommended to use waffle panels with lengths equal to half of 19
the roadway width. The width of the panel will depend on the bridge geometry and thus left 20
to a designer’s judgment. However, 8ft to 12 ft wide precast panels are appropriate for 21
practical use. 22
Thickness of slab: The thickness of the slab connecting the ribs on top in the UHPC waffle 23
deck panel is dictated by the punching shear capacity of the plate between the ribs, cover 24
requirements of top transverse and longitudinal reinforcement, and any anticipated surface 25
wearing over time. Based on an experimental test completed at Iowa State University (ISU) 26
(9) and the limited data available on punching shear capacity of UHPC (8, 13), flat plate 27
thickness of 2.5 inches is recommended. 28
Dimensions of the longitudinal and transverse ribs: Based on the side cover requirements 29
for the reinforcement, as well as the previous studies completed by the FHWA (14) and ISU 30
(9), the width of transverse and longitudinal ribs was chosen to be 3 inches at the bottom with 31
a gradual increase to 4 inches at the top of the ribs at the rib-to-plate interface (see FIGURE 32
2b). 33
Spacing of the longitudinal and transverse ribs: The spacing between the transverse and 34
longitudinal ribs will depend on the girder-to-girder spacing, width of the panel, and 35
minimum number of dowels required for establishing sufficient panel-to-panel connections 36
as the dowels are located within the longitudinal ribs. Based on the limited available data on 37
punching shear behavior and the results from detailed 3D finite element analyses of waffle 38
deck panels with different rib spacing in ABAQUS (15), the maximum allowable rib spacing 39
of 36 inches is suggested to control the extent of flexural panel cracking under service loads 40
and limit the extent of local damage to the flat slab at ultimate loads. 41
Support rib spacing: The support longitudinal ribs, which are located at the girder lines (see 42
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 7
FIGURE 1b), provide an enclosure for the girder-to-panel connection, and is referred to as 1
the shear pocket connection, making the support rib spacing dependent on the top flange 2
width of the girder. It is recommended that the support rib spacing is limited to a value less 3
than the beam top flange width, with a minimum value of 12 inches (see FIGURE 2c). 4
Shear Pockets: Shear pockets facilitate the connection to achieve full composite action 5
between the precast waffle panels and supporting systems (concrete girders, steel girders, 6
stringers, etc.). If the shear studs/hooks are positioned uniformly along the girder length 7
(typical in concrete girder), the shear pocket spacing should be equal to the transverse rib 8
spacing. However, if a group configuration is used for the shear studs, the shear pockets can 9
be placed at spacing of 2 to 4 ft apart, in agreement with the maximum shear stud group 10
spacing allowed by the AASHTO guidelines (16) and the recent studies on shear stud group 11
spacing (17). 12
13
14
FIGURE 2 Recommended geometric dimensions for waffle deck panels 15
Flexural Design: 16
The experimental testing of waffle deck system at Iowa State University demonstrated 17
that the wheel load is distributed to the supporting girders in a similar fashion to the traditional 18
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 8
cast-in-place deck system (9). The extent of distribution of the wheel load among the transverse 1
ribs of the waffle panel is dependent on the rib spacing and girder spacing. Therefore, the waffle 2
deck panel system can be designed conservatively using the strip method as described by the 3
current AASHTO LRFD specifications (16). The transverse strip, whose width is estimated 4
according to the Article 4.6.2.1.3 in the AASHTO specifications (16), is analyzed as a 5
continuous beam supported by bridge girders, which are assumed to be considered as non-6
settling rigid supports. The transverse strip width depends on the location of the critical section 7
along the length of the panel for in the +ve moment (+M), -ve moment (-M) or overhang regions. 8
The transverse strip width for the waffle deck system can be arrived using AASHTO 9
specifications and is given by Eq.(1);. The entire transverse strip is designed to resist the dead 10
load and live load effects with appropriate load factors at different limit states. 11
ts
26 + 6.6 S (ft) for +ve moment
transverse strip width W (in.) 48 + 3.0 S (ft) for -ve moment
45 +10.0 X (ft) overhang
(1) 12
13
where S = girder-to-girder spacing in feet and X = distance of the critical location from 14
the centerline of exterior girder (in feet). 15
Design Loads 16
The design loads include the dead load due to self-weight of waffle panel and wearing 17
surface (if used), live load (design truck load) and collision loads. Design moments are 18
determined at three different regions along the panel cross-section including, section at the center 19
of span between the girders, sections over interior girders, and the overhang section. As detailed 20
in the AASHTO guidelines, the interior spans between girders are investigated for positive 21
bending at the strength-I limit state. Sections over interior girders are examined for negative 22
bending at the strength-I limit, while the overhang region is investigated for different 23
combinations of dead, live, and collision loads for the strength-I and extreme event II limit states. 24
The deck system should be also designed to satisfy the serviceability requirements as required by 25
the AASHTO 5.7.3.4 article. The previously established geometric details along with the 26
following design parameters are used while arriving at the loads on the deck panels. 27
The longitudinal and transverse rib spacing vary between 18 to 36 inches. 28
A girder centerline spacing of 4 to 10 ft, which was established based on an extensive review 29
of frequently used standard details used by several State DOTs including Alabama, Florida, 30
Georgia, Illinois, Indiana, Iowa, Kentucky, Nebraska, New Jersey, New York, Ohio, 31
Oklahoma, Virginia, and Wisconsin. 32
Dead Load 33
Dead load on the waffle panel includes the self-weight of the panel (DC) and the weight 34
of any future wearing surface or overlays (wws) (if used by the DOT). The self-weight of the 35
waffle deck panel will depend on the rib spacing and is given by Eq.(2). 36
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 9
(in psf.) 112
uhpctr w wwaffle slab
lr tr
S b hw h
S S
(2) 1
where hslab = thickness of top slab in inches (= 2.5 in.), Str = transverse rib spacing in 2
inches, Slr = longitudinal rib spacing in inches, hw = rib height in inches (= hdeck – hslab) (= 8 in. - 3
2.5 in. = 5.5 in.), and uhpc = unit density of UHPC (= 157 pcf ). 4
The design dead load is given by following equation. 5
ts tsw =1.25 W 1.5 Wdead
u waffle wsw w (3) 6
Live Load 7
The precast deck panel is designed for HL-93 truck loading. More details of the HL-93 8
truck loading can be found in Section 3.6 of the AASHTO LRFD Bridge Design Specifications 9
(16). 10
Design moment 11
The moment demand for deck panel between the girders (+ve moment region) and at the 12
interior girder locations (-ve moment region) is estimated using the strength-I limit state. The 13
positive and negative moment demand due to the dead load can be estimated using Eq.(4). 14
2
whew
M = M = , re S = girder spacing10
.dead
DL DL uu u
S (4) 15
The positive and negative design moments due to the live load ( M and MLL LL
u u
) can be 16
arrived at using the AASHTO LRFD Bridge Design Specifications Table A4-1. The maximum 17
+ve and –ve moment demand varies from 8.29 kip-ft/ft to 13.17 kip-ft/ft and 3.81 kip-ft/ft to 18
13.41 kip-ft/ft respectively, with the girder spacing changing from 4 ft to 10 ft. Design moment 19
values for different girder spacing can be found in Aaleti et al. (11). 20
Flexural Capacity Calculation 21
Moment capacity of the waffle deck panel in the positive and negative bending directions 22
can be estimated using a transverse strip along the deck panel (see FIGURE 3a). As shown in 23
FIGURE 3b, the equivalent strip width contains a number of ribs depending on the girder span 24
and rib spacing in the waffle deck panel. The cross-section of the transverse strip can be further 25
divided into a combination of T-beams with a cross-section, as shown in FIGURE 3c. The flange 26
width for positive bending (bf+ve
) or negative bending (bf-ve
) can be estimated using the Eq.(5). 27
+ve -ve
+ve -vets tsf f+ve -ve
ts ts
tr tr
W Wb = and b
W W1+interger value of 1+interger value of
S S
(5) 28
where Wts+ve
and Wts-ve
= equivalent strip width for positive moment and negative moment 29
regions, respectively. 30
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 10
The moment capacity for the T-beam cross-section can be estimated using the strain 1
compatibility approach as shown schematically in FIGURE 4. 2
3
4
5
6
FIGURE 3 Cross-section of an equivalent strip for positive and negative bending 7
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 11
1
FIGURE 4 Strain and stress profiles for estimating the positive nominal moment capacity of a T-2 shaped UHPC beam 3
Deck Reinforcement 4
From the observations from the experimental testing of waffle deck panel and a detailed 5
3D finite element model, two configurations for deck reinforcement in transverse ribs using #6 6
and #7 bars are proposed. The nominal positive and negative bending moment capacities of 7
waffle deck panels for different girder spacing and transverse rib spacing configurations can be 8
estimated using the strain compatibility approach as illustrated in FIGURE 4. The cross-section 9
configurations are shown in FIGURE 5 and denoted by UWD6T6B and UWD6T7B. The 10
estimated nominal moment capacities of the two cross-sections different girder and transverse rib 11
spacing are presented in Table 1 and Table 2, which simplifies the design process. 12
13 FIGURE 5 The reinforcement details recommended for transverse ribs 14
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 12
TABLE 1 Nominal moment capacities of UWP6T6B in kip-ft/ft. 1
2
TABLE 2 Nominal moment capacities of UWP6T7B in kip-ft/ft. 3
4
TRB 2014 Annual Meeting Paper revised from original submittal.
Sriram Aaleti and Sri Sritharan 13
Overhang Design 1
The overhang region is designed for different combinations of dead, live, and collision 2
loads for the Strength I and Extreme Event II limit states as required by AASHTO guidelines 3
(16). An F-shape standard concrete railing is used for designing the overhang region for collision 4
loads. In addition, based on the suggestions from Iowa DOT designers, it is recommended to use 5
a solid cross-section for the overhang region instead of a waffle configuration. The solid section 6
for the overhang will not only help in addressing the variability in the types of railings and their 7
capacities as used by DOTs, but also provide adequate space to include the necessary details for 8
attaching the railing to the precast deck. The negative moment capacity of the solid overhang for 9
the UWP6B6T and UWP6T7B configurations was found to vary between 37.6 kip-ft/ft to 41.83 10
kip-ft/ft depending on the transverse rib spacing. 11
Connection Details 12
The short-term and long-term performance and durability of bridges constructed using 13
these deck panels will be influenced by the quality of the connections among the panels (i.e., 14
panel-to-panel connections in both longitudinal and transverse directions) and with panels to 15
girders. Panel-to-panel connections are subjected to bending moments and vertical shear forces 16
under vehicular loading. In recent decades, a wide variety of deck level connection designs have 17
been deployed in bridge projects involving full-depth precast panels with substantial variance in 18
observed performance under traffic loads. Several of these connection details are provided in the 19
design guide (11). The connections that perform well typically consist of match-cast shear keys 20
with epoxy adhesive or grouted female-to-female joints with discrete reinforcement, combined 21
with field-cast concrete or grouted together with quality construction. A few connection details 22
that are appropriate for the waffle deck panel-to-panel connection are shown in FIGURE 6. By 23
realizing the superior durability and bond characteristics of UHPC, all the connection regions are 24
designed with field casting of UHPC. The connection details presented in Figures 6 b, c and d 25
were developed for solid deck panels. However, these connections can be adopted for waffle 26
deck panels by making the cells adjacent to the connections to be solid. 27
28
TRB 2014 Annual Meeting Paper revised from original submittal.