Introduction:
Inside dimensions of the box culvert (SPAN x RISE) The fill height (H) above the culvert are as per the below
t bl A t i l ti f th l t i h i Fi M t i l d d i t i i T bl
table. A typical section of the culvert is shown in Figure. Material and design parameters are given in Table.
24 kN/m3
Compressive Strength, f’c Span L
Rise R
Top Slab Thickness, Tt
Bottom Slab Thickness Tb 320 mm
4500 mm
Modulus Elasticity of Steel Reinforcement, E
s 200000 MpaModulus Elasticity of Concrete, E
cHaunch Thickness, Th
The approximate strip method is used for the design with the 1m wide design strip oriented parallel to the direction of traffic.
A 2-Dimensional (2D) plane frame model is used to analyze the box culvert. Beam elements in the 2D model are assumed to be centered in the concrete members. The model is assumed to be externally supported by a pinned support on one end and a roller support on the other end. In addition, the model is always assumed to be in equilibrium so external reactions to loads applied to the structure were assumed to act equal and opposite. A “w”
dimension of 1 m is added to the calculations to convert the units to kN/m for consistency with national conventions.
A. Dead Load
The total self-weight of the culvert top slab is:
47.04
kN (for 1 m wide) The self-weight of one culvert side wall is:
8.74
kN (for 1 m wide) Self weight of Haunch
0.03 kN (for 1 m wide)
The top slab weight, wall weights, and all four haunch weights are applied to the bottom slab as an upward
reaction from the soil assuming an equivalent uniform pressure. The bottom slab weight is not applied in the model because its load is assumed to be directly resisted by the soil.
Dcbottom = 53.44 kN (for 1 m wide) B. Earth Pressure Loads
1.29
The interaction factor for embankment conditions is dependent on the height of fill (H) and the outside width of the culvert (Bc):
The weight of fill on top of the culvert produces vertical earth pressure (EV). The fill height is measured from the top surface of the top slab to the top of the pavement or fill. The unit weight of the fill is 19.2 kN/m3
The design vertical earth pressure at the top of the culvert is:
104.82
kN/m
The lateral earth pressure (EH) on the culvert is found using the equivalent fluid method. For at-rest conditions, a maximum stage full soil unit weight and a minimum stage half soil unit weight will be are
At the top of the culvert, the lateral earth pressure is:
81.00
kN/m (Top)
40.50
kN/m (Top)
At the bottom of the culvert, the lateral earth pressure is:
used.
= 109.80 kN/m (Bottom)
= 54.90 kN/m (Bottom)
C. Live Load Surcharge
Use an active coefficient of lateral earth pressure ka = 0.2827
The height is:
H top of culvert = 4500 mm
The height for the live load surcharge calculation at the top of the culvert is the distance from the top surface of the top slab to the top of the pavement or fill.
p
The equivalent fill height, heq is dependent on the depth of fill and can be found using AASHTO Table 3.11.6.4-1.
By interpolation, the equivalent height for a fill depth of 4500 mm is:
heq = 2.52ft heq = 0.77 m
The corresponding lateral live load surcharge on the top of the culvert is given as:
3.91 kN/m
6100 mm
Again using interpolation and AASHTO Table 3.11.6.4.1, the equivalent height is:
heq = 2ft heq = 0.61 m
The height for the live load surcharge calculation at the bottom of the culvert is the distance from the bottom surface of the bottom slab to the top of the pavement or fill.
eq
The lateral live load surcharge located at the bottom of the culvert is given as:
3.10 kN/m D. Water Load
At the inside of the culvert, the lateral water pressure is:
WAtop = 0 kN/m2
Designers need to consider load cases where the culvert is full of water as well as cases where the culvert is empty. A simple hydrostatic distribution is used for the water load:
9.81 kN/m2
E. Live Load
Using a 2D frame model there is an opposite upward reaction from the soil caused by the water inside the culvert:
Wabottom reaction = WA bottom * Span = 8.82 kN/m
(Span + Ts)
The design live loads include the HL-93 truck and tandem loads. Since the span of the box culvert is less than 15 ft, no lane load is applied.
Dynamic Load Allowance
-27.885 = 0
The dynamic load allowance may not be taken less than zero.
The dynamic load allowance (IM) for culverts and other buried structures is reduced based on the depth of fill over the culvert. For strength and service limit states:
Live Load Distribution
Live loads are assumed to distribute laterally with depth. The specifications permit designers to increase the footprint of the load with increasing depth of fill. The load is assumed to spread laterally 1.15 times H horizontally in each direction for every foot of fill above the culvert. The intensity of live loads at any depth is assumed to be uniform over the entire footprint.
The assumed tire contact area for each wheel has a width of 20 inches and a length of 10 inches.
Using the distances between wheel lines and axles, the live load intensities at the top of the box culvert can be found. For truck and tandem loadings, the influence area or footprint of the live load is found first. Then the sum of the weights of the wheels is used to determine the intensity of the live load.
To determine the live load, use multiple presence factors (MPF). A single loaded lane with a MPF of 1.20 is used for strength and service limit states.
where Pw = 130kN
Axlespacing = 1.8m
Wtire = 0.51m g
A single 3 Axle with 600 kN Truck configuration produces a live load intensity of:
tire
W = 7.49 m
Ltire = 0.25m L= 7.23 m Therefore
WLL+IM = 5.77 kN/m
A tandem truck axle configuration produces a live load intensity of:
The live load intensities of the single and tandem axle configurations are compared. Since the tandem axle configuration produces a live load intensity slightly larger than that of the single axle
configuration, the tandem axle configuration is used for design in both the strength and service limit states.
1 Maximum Vertical Load and Maximum Horizontal Load
1.25 DC + (1.30)(1.05) EV +1.75 (LL+IM)+ (1.35)(1.05)EHmax + 1.75LS 2 Maximum Vertical Lod and Minimum Horizontal Load
1.25 DC + (1.30)(1.05) EV +1.75 (LL+IM)+1.00WA+(0.9/1.05)EHmin 3 Minimum Vertical load and Maximum Horizontal Load
0.90 DC + (0.9/1.05)EV + (1.35)(1.05) EHmax +1.75LS
Service Limit State
1 Maximum Vertical Load and Maximum Horizontal Load 1.00 DC + 1.00 EV + 1.00 (LL+IM)+1.00EHmax + 1.00LS 2 Maximum Vertical Lod and Minimum Horizontal Load
1.00 DC + 1.00 EV + 1.00 (LL+IM)+1.00WA+1.00EHmin
3 Minimum Vertical load and Maximum Horizontal Load 1.00 DC + 1.00EV + 1.00EHmax +1.00LS
A structural analysis is performed using a standard commercial matrix-analysis program. The bottom slab of the box culvert is assumed rigid compared to the subgrade. Reactions to vertical loads applied to the culvert (earth, water, live load) are assumed to be carried by uniform, triangular or trapezoidal distributed reactions applied to the bottom slab. Box culverts supported on stiff or rigid subgrades (rock) would require further investigation. The haunches are included in the analysis by increasing the thickness of members near each corner
Reinforcement Design
where,
F = 1
fy = 420 Mpa
fc' = 35 MPa
thickness of members near each corner.
fc' = 35 MPa
420∗ √ 176400 ^2 11.844 /5.922
Side Wall
Max @ Slab End@OS Max @ Mid Slab@IS
From staad Model
Outside Mu = 86.88 kNm Main bar dia = 14 mm
d = thickness - Cover - dm/2
d= 233 mm
As = 913.02 mm2 Minimum sidewall flexural reinforcement
Asmin = 0.002 x b x Ts = 560.00 mm2 therefore As = 913 02 mm2
therefore As 913.02 mm2
As Provided = 14 mm dia X 150 c/c
for 1 m length 14 mm dia X 6.67 Pcs 1,026.25 mm2
Inside Mu = 53.33 kNm Main bar dia = 14 mm
d = thickness - Cover - dm/2
d= 233 mm
As = 554.26 mm2 Minimum sidewall flexural reinforcement
Asmin = 0 002 x b x Ts = 560 00 mm2 Asmin = 0.002 x b x Ts = 560.00 mm2 therefore As = 560.00 mm2
As Provided = 14 mm dia X 150 c/c
for 1 m length 14 mm dia X 6.67 Pcs 1,026.25 mm2
Top Slab
Max @ Slab End @OS
Max @ Mid Slab @IS
From staad Model
Inside Mu = 98.83 kNm Main bar dia = 16 mm
d = thickness - Cover - dm/2
d= 232 mm
As = 1,047.62 mm2 Minimum sidewall flexural reinforcement
Asmin = 0.002 x b x Tt = 560.00 mm2 therefore As = 1,047.62 mm2
As Provided = 16 mm dia X 150 c/c
for 1 m length 16 mm dia X 6.67 Pcs 1,340.41 mm2
Outside Mu = 47.09 kNm Main bar dia = 12 mm
d = thickness - Cover - dm/2
d= 234 mm
As = 486 27 mm2
As = 486.27 mm2 Minimum sidewall flexural reinforcement
Asmin = 0.002 x b x Tt = 560.00 mm2 therefore As = 560.00 mm2
As Provided = 12 mm dia X 150 c/c
for 1 m length 12 mm dia X 6.67 Pcs 753.98 mm2
Bottom Slab
Max @ Slab End@OS Max @ Mid Slab @IS
From staad Model
Inside Mu = 73.58 kNm Main bar dia = 16 mm
d = thickness - Cover - dm/2
d= 272 mm
As = 655.21 mm2 Minimum sidewall flexural reinforcement
Asmin = 0.002 x b x Tb = 640.00 mm2 therefore As = 655.21 mm2
As Provided = 16 mm dia X 150 c/c
for 1 m length 16 mm dia X 6.67 Pcs 1,340.41 mm2
Outside Mu = 82 kNm Main bar dia = 12 mm
d = thickness - Cover - dm/2
d= 274 mm
As = 726 11 mm2
As = 726.11 mm2 Minimum sidewall flexural reinforcement
Asmin = 0.002 x b x Tb = 640.00 mm2 therefore As = 726.11 mm2
As Provided = 12 mm dia X 150 c/c
for 1 m length 12 mm dia X 6.67 Pcs 753.98 mm2
Checking of Shear
Top Slab
Maximum Shear (Vu) = 225.17 kN Shear Capacity (Vc) =
241.38
kN > Vu
Bottom Slab
0.17∗√ ^′
Bottom Slab
Maximum Shear (Vu) = 207.29 kN Shear Capacity (Vc) =
0.17∗√ ^′
281.61
kN > Vu
Side Wall
Maximum Shear (Vu) = 121.96 kN Shear Capacity (Vc) =
241.38
kN > Vu Final Size and Reinforcement
Box culvert Inside Dimensions = 2500 x 1000 mm Top Slab Thickness = 280 mm Bottom Slab Thickness = 320 mm
id ll hi k 280
0.17∗√ ^′
Side Wall Thickness = 280 mm Reinforcement
Dia (mm) C/C Dia (mm) C/C
16 150 12 150
16 150 12 150
14 150 14 150
Top Slab Bottom Slab
Side Wall
Location Inside Outside