Box Culvert Structural Design Report Template

1.

STRUCTURAL DESIGN OF THE BOX CULVERTS

The hydraulic design process yielded two box culverts; a single cell type on chainage 1+095 on 4th Circular road and double cell type on 2+940 on 5 th Circular Road. 1.1

STANDARDS

AND

CODES

OF

PRACTICE

The culverts were designed according to the following documents:      1.2

Ministry of Works and Transport (MoWT) Road Design Manual Vol. 4, Bridge Design Manual, January 2010 in conjunction with, BS 5400 - Steel, Concrete and Composite Bridges, BD31 - Design of Buried Concrete Box & Portal Frame Structures, and, BS8110 – Structural Use of Concrete. DESIGN LOADINGS

The loads that were applied in the design of the box culverts were: the dead load of the top slabs, the dead load of the vertical side walls applied onto the bottom slabs of the culverts, vertical earth loads applied onto the top slabs of the culverts, vertical and horizontal loads imparted by the internal water, and vertical loads applied to the top slabs due to vehicular traffic. Each of these loads will be discussed in detail in the following paragraphs. 1.2.1

Top Slab Dead Load

The Top Slab dead load was calculated from the dimensions of the member assuming a unit weight of reinforced concrete of 24kN/m 3. The equation for calculating this load is: DLTS = (gc) x TTS Where; DLTS - Top Slab dead load in kN/m gc - unit weight of reinforced concrete (normally assume 24kN/m 3) TTS - Thickness of the top slab in meters The Top slab dead load was applied as a uniform load across the top members. 1.2.2

Vertical Wall Dead Load

The dead load of the vertical walls acting on the bottom slab was applied as vertical concentrated load acting at the joints between the vertical walls and the bottom slab.

Page | 1 Preliminary design report May 2015 Consultancy Services for Engineering Design of Roads at South B Estate (4.3km) at Kampala Industrial and Business Park - Namanve

The self weights of two side walls acting as concentrated loads were assumed to produce uniform soil reaction on the bottom slab. The loads, due to the external vertical walls, were calculated using the equation: DLSW = (gc) x H x Tw + 0.5 x DLTS Where; DLSW - load of exterior vertical wall acting on the base slab in kN/m gc - unit weight of reinforced concrete (normally assume 24kN/m 3) TSW - Sidewall thickness in meters, H - Height of the culvert opening in meters (The Rise), DLTS - Top Slab dead load in kN/m For the two-cell culvert, the load on the base slab due to the interior wall was calculated as follows: DLIW = (gc) x H x TIW + 0.33 x DLTS Where; DLIW - load of interior vertical wall acting on the base slab in kN/m gc - unit weight of reinforced concrete (normally assume 24kN/m 3) TIW – Interior wall thickness in meters, H - Height of the culvert opening in meters (The Rise), The weight of the base slab was assumed to have no influence on the internal forces in the structure.


1.2.3

Soil-Pavement Overburden

The vertical load applied to the Top Slab by the overlying soil and pavement was taken to be the unit weight of the material multiplied by the depth of the material. This load was calculated from the equation below and was applied as a uniform load across the top slab, as shown in Figure 2.

DLSP = (gP) x TP + (gb) x h1+ (gs) x h2 Where; DLSP - uniform load due to the pavement and soil in kN/m gP - unit weight of pavement surface (assumed 12kN/m 3) Tp - thickness of the pavement surface material in meters gb - unit weight of the granular pavement materials (base/subbase) in kN/m 3 h1 - thickness of granular or soil material between the bottom of the pavement surface and the top of the subgrade in meters. gs - unit weight of soil (backfill or subgrade) in kN/m 3 h2 - thickness of soil between bottom of granular layer and top of culvert in meters. In general for this evaluation, the layers of granular material (base and subbase) were considered as one layer providing with the assumption that the density of both layers was approximately the same. The same was true for layers of soil backfill over the culvert.


1.2.4

Lateral Earth Pressure

There were two types of lateral earth pressures on the vertical side walls of the box culvert that were considered for design; the one due to increase in depth was computed according to the Coulomb’s theory with the assumption of a triangular distribution and a uniform lateral pressure due to the effect of live load surcharge. By combining these two pressures, a trapezoidal pressure distribution on side walls due to embankment loading was obtained. The distribution of soil pressure on the side wall is shown in Figure below:

p1 = k * q s Where; p1 - uniform load component of the lateral soil load in kN/m k - Coefficient of lateral earth pressure qs – Surcharge load And p2 = k * (gs) * h Where; p2 - triangular load component of the lateral soil load in kN/m k - Coefficient of lateral earth pressure gs - Unit weight of soil (backfill or subgrade) in kN/m 3 h - Distance from top to bottom of culvert


1.2.5

Water Pressure inside the Culvert

This type of loading was assumed to occur when the culvert is full with water. The pressure distribution on side walls was assumed to be triangular with a maximum pressure intensity given by the equation below; p = (gw) x h at the base Where; gw = density of water and h is the depth of flow

1.2.6

Vehicular Traffic Loads

There were two types of lateral earth pressures on the vertical side walls of the box culvert that were considered for design; the one due to increase in depth was computed according to the Coulomb’s theory with the assumption of a triangular distribution and a uniform lateral pressure due to the effect of live load surcharge. By combining these two pressures, a trapezoidal pressure distribution on side walls due to embankment loading was obtained. Load Factors (RDM, Ref: 5.1.2)

The structural design procedure used in the design of the concrete box culverts was based on the Limit State Design procedure which requires that the load, shear, or moment be obtained by applying load factors to the service values. The factors, yfL to be applied to all parts of the dead load, irrespective of whether these parts have an adverse or relieving effect, weree taken for all load combinations as follows:

BS 5400 table 1


For the Ultimate Limit State 1.05 1.15

For the Serviceability Limit State 1.0 1.0

Steel Concret e Design Superimposed Dead Load (RDM, Ref: 5.2.2) The factor, yfL, to be applied to all parts of the superimposed dead load, irrespective of whether these parts have adverse or relieving effect, shall be taken for all load combinations as follows: For the Ultimate Limit State 1.75

For the Serviceability Limit State 1.20

Wind Load (RDM, C5.3.1.1) For small and/or low structures, wind usually does not govern. Temperature (RDM, Ref: 5.4.2) For all bridges, extremes of shade air temperatures for the location of the bridge shall be obtained from available maps of isotherms and a 50-year return period may be adopted. The design range of movement shall be taken as 1.3 times the appropriate nominal value for the ultimate limit state and 1.0 times the nominal value for the serviceability limit state. Earth Pressure on Retaining Structures (RDM, Ref: 5.8.1) Where filling materials are retained by abutments or other parts of the structure, the loads calculated by soil mechanics principles from the properties of the filling materials shall be regarded as nominal loads. For all design load combinations, yfL, shall be taken as follows: For the Ultimate Limit For the Serviceability Limit State State 1.5 1.00 Highway Bridge Live Loads (RDM, Ref: 6.2.1) Nominal Uniformly Distributed Load (UDL) (RDM, Ref: 6.2.1) The UDL shall be taken as 30 kN per linear meter of notional lane for loaded lengths up to 30 m, and for loaded lengths in excess of 30 m, it shall be derived from the equation. W =

151(1/L) 0.475 but not less than 9.

Where L is the loaded length (in m) and W is the load per metre of the lane (in kN)


Nominal Knife Edge Load (KEL) (RDM, Ref: 6.2.2) The KEL per notional lane shall be taken as 120 kN Design HA Loading (RDM, Ref: 6.2.7) For design, HA load considered alone, yfL, shall be taken as follows:

Combination 1 Combination 2 & 3

For the Ultimate Limit State 1.50 1.25

For the Serviceability Limit State 1.20 1.00

Design HB Loading (RDM, Ref: 6.3.4) For design HB load, yfL, shall be taken as follows: For the Ultimate Limit For the Serviceability Limit State State For Combination 1 1.30 1.10 For Combination 2 1.10 1.00 &3 The design will be checked for 37 units of HB loading. Longitudinal Load (RDM, Ref: 6.6) For the longitudinal and primary live load, yfL, shall be taken as follows:

For HA load For HB load

For the Ultimate Limit For the Serviceability Limit State State 1.25 1.10 1.10 1.00

The longitudinal load resulting from traction or braking of vehicles shall be taken as more severe of nominal load for type HA or HB, applied at the road surface and parallel to it in one notional lane only. The nominal load for HA shall be 8 kN/m of loaded length plus 200kN, subject to a maximum of 700kN, applied to an area one notional lane wide x the loaded length. The nominal load for HB shall be 25% of the total nominal HB load adopted, applied as equally distributed between the eight wheels of two axles of the vehicle, 1.8 m apart. The nominal load shall be taken as 250 kN. Type HA loading, applied in accordance with (RDM, Ref: 6.4.1), shall be considered to act with the accidental skidding load. For the skidding and primary live load, yfL, shall be taken as follows: For the Ultimate Limit State 1.25

For the Serviceability Limit State 1.00

Page | 7

Preliminary design report May 2015 Consultancy Services for Engineering Design of Roads at South B Estate (4.3km) at Kampala Industrial and Business Park - Namanve

1.3

MATERIALS SPECIFICATIONS

All component materials will be in conformance with the durability requirement of the project sites/localities. Material specifications shall meet the requirements of MOWT General Specifications Part 6 and the Special Specifications. 1.3.1

Concrete Grades

Concrete grades for the design of the bridge elements will be as shown in Table 12. Table 1: Concrete grades

Table 6.2: Grades of Concretes Superstructure (deck slab and beams) Foundation, piers, walls and abutments Blinding and Mass Concrete Source: BS 5400 1.3.2

Grades C30/20 C30/20 C15/20

Minimum Concrete Cover

Minimum concrete covers to reinforcement for the bridge elements, based on exposure. Conditions and classes according to BS 8500 as presented in Table 13 will be adopted. Table 2: Minimum Concrete Covers to Reinforcements

Element Foundation Base or Footing Abutments and Wing Walls Beams and Slabs

Minimum Cover to Reinforcement (mm) > (75mm or equivalent diameter of bundled bars + 5mm) > (50mm or equivalent diameter of bundled bars + 5mm) > (40mm or bar diameter +5mm or equivalent diameter of bundled bars + 5mm)

Source: BS 5400 1.3.3

Reinforcement

Reinforcement will be hot rolled steel bars to BS 449 with the following properties: High Yield - Type 2 Deformed Bar Yield Strength: Fy = 460 N/mm2 Modulus of Elasticity: E = 200 kN/mm2


1.3.4

Concrete Protection

Concrete surfaces below finished ground level will be protected by either bituminous coating or other equivalent chemical waterproofing materials. 1.4

STRUCTURAL DESIGN

1.4.1

Superstructure Design

Superstructure will be designed in accordance with BS 5400. Superstructure analysis will be carried out using SAP 2000 and customised Excel Sheets by the Consultant. AutoCAD software will be used for detailing. The deck will be analyzed taking account of the composite action between the beams (concrete or steel) and the in-situ reinforced concrete slab. In order to optimize the bridge construction costs, span standardization will be adopted. The beams for the superstructure will be standardized and shall be reinforced concrete. In selecting the type of beam, the following advantages will be considered; 

Availability of materials (cement , reinforcing steel) locally;



Cost;



Ease of construction or installation; and



Extent of maintenance required; etc.

1.4.2

Substructure Design

Abutment wall, wing walls, and foundation will be designed as a continuous system that prevents movement of the walls. Reinforced Concrete Abutments will be designed to BS 5400 and checked using an Excel Spreadsheet that has been pre-programmed by the Consultant, based on conventional reinforced concrete theory. Soil’s angle of internal friction and the unit weight of the backfill will be taken from the Materials Report. Stability checks will be carried out for overturning, sliding and bearing pressures under active and passive earth pressures. Factors of safety against overturning and sliding which will be used are as follows: 

Service Loads ≥ 2.0; and



Seismic Loads ≥ 1.5.



Box Culvert Structural Design Report Template

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