751.20 Continuous Concrete Slab Bridges

From Engineering Policy Guide

Jump to: navigation, search

Contents

751.20.1 General

This article illustrates the general design procedure for Continuous Concrete Slab Bridge using AASHTO LRFD Bridge Design Specifications.


751.20.1.1 Material Properties

Concrete:
Unit weight of reinforced concrete, \, \boldsymbol{\gamma}_c = 0.150 kcf
Continuous Cast-In-Place Solid/Voided Concrete Slab
  Class B-2 \, f'_c = 4.0 ksi
  \, n=8
Precast Prestressed Multicell Voided Concrete Girders
  Class A-1 \, f'_c = 6.0 ksi
  \, f'_{ci} = 4.5 ksi
  <math\, >n=8</math>
Intermediate bent columns, end bents (below construction joint at bottom of slab)

in continuous concrete slab bridges

  Class B-1 \, f'_c = 4.0 ksi
  \, n=8
  Class B, Open bent, footing \, f'_c = 3.0 ksi
  \, n=10
Modulus of elasticity,

E_c = 33,000\ K_1 \ (w_c^{1.5}) \sqrt{f^'_c}

  Where:  
  f'c in ksi
  wc = unit weight of nonreinforced concrete = 0.145 kcf
  K1 = correction factor for source of aggregate
    ---- = 1.0 unless determined by physical testing
Modulus of rupture,

\,f_r = 0.24 \,\sqrt{f^'_c}     LRFD 5.4.2.6

  Where:  
  f'c in ksi
 
Reinforcing steel:
  Minimum yield strength, \, f_y = 60.0 ksi
  Steel modulus of elasticity \, E_s = 29000 ksi
Unit weight of future wearing surface, \, \boldsymbol{\gamma}_{fws} = 140 lb./ft^3

751.20.2 Design

751.20.2.1 Limit States and Load Factors

In general, each component shall satisfy the following equation:

\, Q = \textstyle \sum \eta_i  \gamma_i  Q_i  \le  \phi  R_n = R_r


Where:

\, Q= Total factored force effect
\, Q_i= Force effect
\, \eta_i= Load modifier
\, \gamma_i= Load factor
\, \phi= Resistance factor
\, R_n= Nominal resistance
\, R_r= Factored resistance


Limit States

The following limit states shall be considered for slab and edge beam design:

STRENGTH - I
SERVICE - I
FATIGUE
EXTREME EVENT - II


Resistance factors

For STRENGTH limit state,

Flexure and tension of reinforced concrete, φ = 0.90
Shear and torsion, \, \phi = 0.90

For all other limit states, \, \phi = 1.00


Load Modifiers

751.20.2.2 Loads

Permanent (Dead) Loads

Permanent loads include the following:

Slab weight
Future Wearing Surface
A 3” thick future wearing surface (35psf) shall be considered on the roadway.
Safety Barrier Curb (SBC)
For slab overhang design, assume the weight of the SBC acts at the centroid of the SBC.
(*) 12" For slab overhang design (LRFD (3.6.1.3.1), 2'-0" for other design
Application of Live Load to Slab


Gravity Live Loads

Gravity live loads include vehicular, dynamic load allowance, and pedestrian loads.


Vehicular
The design vehicular live load HL-93 shall be used. It consists of either the design truck or a combination of design truck and design lane load.
For slab design, where the primary strips are longitudinal, the force effects shall be determined on the following basis:
  • The longitudinal strips shall be designed for all loads specified in AASHTO Article 3.6.1.3.3 includuing lane load.
  • For the purpose of slab design, the lane load consists of a load equal to 0.640 klf uniformly distributed over 10 feet in the transverse direction.

For precast prestressed multicell girders, live load shall be distributed according to AASHTO LRFD Tables 4.6.2.2.2b-1, 4.6.2.2.2d-1, 4.6.2.2.3a-1 and 4.6.2.2.3b-1 for both moment and shear.


Dynamic Load Allowance

The dynamic load allowance replaces the effect of impact used in AASHTO Standard Specifications. It accounts for wheel load impact from moving vehicles. For slabs, the static effect of the vehicle live load shall be increased by the percentage specified in Table below.

Dynamic Load Allowance, \, IM
Slab Component\, IM
Deck Joints – All Limit States75%
All Other Limit States33%


The factor to be applied to the static load shall be taken as:


\, (1 + IM)


The dynamic load allowance is not to be applied to pedestrian or design lane loads.


Multiple Presence Factor, \, m:

The multiple presence factor accounts for the probability for multiple trucks passing over a multilane bridge simultaneously.

\, m = 1.20 for 1 Loaded Lane
 1.00 for 2 Loaded Lanes
 0.85 for 3 Loaded Lanes
 0.65 for more than 3 Loaded Lanes


Pedestrian

Pedestrian live load on sidewalks greater than 2 ft wide shall be:

\, PL = 0.075 ksf

This does not include bridges designed exclusively for pedestrians or bicycles.


For Additional Design Information, see LRFD 5.14.4.2

751.20.3 Details

751.20.3.1 Solid Slabs

SLAB LONGITUDINAL SECTIONS - SOLID SLABS

END SPANS
Image:751.20 cont conc slab longitudinal sections solid slab intermediate span.gif
INTERMEDIATE SPANS


Note:
All longitudinal dimensions shown are horizontal.

751.20.3.2 C.I.P. Voided Slabs

SLAB LONGITUDINAL SECTIONS - CAST-IN-PLACE VOIDED SLAB


Image:751.20 cont conc slab longitudinal sections cip voided slab end span.gif
END SPANS


Image:751.20 cont conc slab longitudinal sections cip voided slab intermediate span.gif
INTERMEDIATE SPANS
(*) 3'-0" or greater than or equal to 5% of span length.
(**) By Design (6" increments measured normal to the centerline of bent) (The minimum is
equal to the column diameter + 2'6")


Note:
All longitudinal dimensions shown are horizontal (Bridges on grades and vertical curves, included).
For sections A-A and B-B, see below.


SLAB CROSS SECTION


Image:751.20 cont conc slab cip voided slab cross section.gif
HALF SECTION A-A
CENTER OF SPAN
HALF SECTION B-B
NEAR INTERMEDIATE BENT

 


Sonovoids are produced in half
sizes 2" to 18". D = 4" to 36"
 
T=19" (Min. preferred. Consult
Structural Project Manager prior
to the use of a thinner slab.)
Image:751.20 cont conc slab cip voided slab part section & detail c.gif

Notes:
(*)   Increase the Dimension "T" by 1/2" for #14 bars placed in the top or bottom of the slab.

Increase the Dimension "T" by 1" for #14 bars placed in the top and bottom of the slab.
("T" and "D" are based on 3" clearance which includes the integral wearing surface to the top of the longitudinal bar.)

(**)  For Roadway with slab drains, use 10" minimum. For Roadways that require additional reinforcement for resisting moment of the edge

beam 20" minimum. Check for adequate space for accomodation for development of Safety Barrier reinforcement.
Personal tools