Analysis and Design of Voided Slab Bridge

International Journal of Trend in Research and Development, Volume 7(3), ISSN: 2394-9333

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IJTRD | May – Jun 2020 Available [email protected] 207

Analysis and Design of Voided Slab Bridge

1Brajesh Kumar,

2Dr. Pankaj Singh,

3Ravindra Gautam,

1M.Tech. Scholar

2HOD,

3Associate Professor,

1,2S R K University, Bhopal, India,

3KIET Group of Institutions Ghaziabad, India

Abstract: Circular voids are often incorporated into concrete

bridge decks to reduce their self-weight without greatly

reducing their flexural stiffness. Incorporating voids within the

deck slab offers many advantages over a conventional solid

concrete slab like lower total cost of construction, reduced

material use and increased structural efficiency. However the

voids within the structure complicate the analysis of the

structure. In this thesis a manual analysis for both longitudinal

and transverse direction of voided slab bridge is done as per

the industrial standards. For transverse analysis the bridge is

idealized using STAAD pro software. The detailing of the

complicated structure is also included in order to understand

how the reinforcement is placed in the structure.

Keywords — Voids, Longitudinal Analysis, Transverse

Analysis, STAAD pro

I. INTRODUCTION

Bridge construction plays a vital role in development of cities

and thus, has achieved great significance these days. It

facilitates a free and undisturbed movement for traffic. It also

plays an important role in development of trade and industries,

enhancing progress of the nation. A prestressed concrete slab

constitutes a great portion of bridges all around the world.

Prestressed concrete was introduced into bridge construction

since very early period of time. In past decades wide varieties

of new techniques have been developed. Along with new

developments in technologies spans became longer and

aesthetics and appearance of bridge became more important.

Prestressed concrete bridges include a wide variety of different

forms like cast-in-situ or precast concrete; continuous or cable-

stayed; simply supported; box-girders, slabs or beams. Among

wide variety of prestressed concrete bridges, a study on solid

and voided slabs has been carried out. A comparison regarding

forces and moment in both cases for different spans and

sections is done; thus, analyzing both cases for a better section

to be used.

Prestress Concrete Bridges

High-strength concrete and high-tensile steel, besides being

economical, make for slender sections, which are aesthetically

superior. Prestressed concrete bridges can be designed as class

I type structures without any tensile stresses under service

loads, thus resulting in a crack-free structure. In comparison

with steel bridges, prestressed concrete bridges require very

little maintenance. Prestressed concrete is ideally suited for

composite bridge construction in which precast prestressed

girders support the cast in situ slab deck.

This type of construction is very popular since it involves

minimum disruption of traffic. Post-tensioned prestressed

concrete finds extensive applications in long-span continuous

girder bridges of variable cross-section. Not only does it make

for sleek. structures, but it also effects considerable saving in

the overall cost of construction. In recent years, partially

prestressed concrete (type-3 structure) has been preferred for

bridge construction, because it offers considerable economy in

the use of costly high-tensile steel in the girder. Slab type

superstructure These types of structures require more steel and

concrete compared to that of girder bridges of same span. The

overall cost of construction of these bridges is lower and is

easier to construct. The limit of span of slab bridges depends

on magnitude of load and relative cost of frame work,

materials and labour. In slab type superstructure bridges, it is

the slabs that are major load carrying elements. The loads are

directly transferred to substructure through slabs. Bridges are

referred as slab bridges if ratio between W/D ≥ 5, where W

=total width of slab and D= Depth of slab ( if W/D < 5, then

the bridge is referred as beam bridge). Slab bridges can be

classified as per their construction: Solid slab bridges, voided

slab bridges and ribbed bridges. The study here consists of

solid slabs and voided slabs. A comparative analysis is done

for both type of slabs.

Solid slab bridges

Solid slab decks comprises of a solid section, without beams

or voids. This type of deck is commonly used in the

construction of short span bridges and culverts. As the slabs

are solid, the cross section from any point is a homogeneous

structure. The construction of solid slab bridge decks is straight

forward and easier. Also the formwork required is very simple

and easy. As the structure is solid and the cross section

homogeneous, the layout of reinforcement becomes very easy.

There is no congestion of reinforcements created and thus,

placing concrete becomeThe one and major backdrop of solid

slabs is a very large volume of concrete. This effects cost and

self-weight both. Due to high concrete volume, these slabs

have a greater self-weight. This can be avoided to certain

extent by providing suitable variations in thickness of the slab.

But this shall be checked properly before commencing.

Another method to reduce self-weight is by providing voids in

the slabs. This is explained in voided slabs.

Voided slab Bridges

Voided slabs are characterized by presence of voids within the

slab. As it was discussed in solid slab section voids in the slab

helps reducing the self weight of structure. Thus, the major

function of voided slabs is to reduce the concrete volume and

thereby decrease the self weight of slab. When the concrete is

being cast, the void formers are subjected to large buoyancy

forces. These buoyancy forces are resisted by straps tied to

bearers below the formwork. If proper care is not taken during

this process, it can create major difficulties; sometimes to the

extent of demolishing the slab. Hence, it should be properly

designed and the formers to be used must be sufficiently rigid,

thoroughly sealed and tied before commencement. The voids

are usually cylindrical and are constructed using a hollow thin

walled steel sections placed in the slab. If designed properly, it

can reduce the self weight of slab up to 35% as compared to

solid slab for same section and span. The voided slabs can be

modelled and designed by the method same as that used for

solid slab in case of void diameters less than 60% of the depth

of slab. In case of diameters larger than 60% of slab depth the


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behaviour of the same becomes a cellular behaviour. Apart

from reducing the slab weight and concrete volume of the slab,

the voided slabs helps providing advantages also like; it

provides large open floor areas. Also, large spans are possible

to construct without beams by providing voided slabs. The

biaxial forces created helps in reduction of deflection. Again

by reducing material consumption, it is thus economic

compared solid slab or beams.es easier.

Figure 1.1: Typical voided slab

Concrete is strong in compression; but is weak in tension.

Concrete experiences prestressing because of the force that is

transferred between the prestressed tendon and concrete. The

tendons when pulled or stressed are under tension. This tension

in tendons is balanced by compression in concrete. Thus, an

external compressive force applied to concrete counteracts the

tensile stresses generated under shear forces and bending

moments caused due applied loading. The tendons can be

externally placed or can be internal tendons. Also, it can be

bonded tendons or unbounded tendons. Again, they could be

pre-tensioned or post tensioned. However, its effect and

principles remains the same. The concrete if prestressed fully;

that is full prestressed concrete, ensures longitudinal stress in

compression always and a partially prestressed concrete allows

a part of tension to occur under some specific loading

conditions.

Materials for Void Formers

Various types of void formers have been used, spirally wound

sheet metal being an early form. The voids became full of

water during construction which resulted in the overstressing

of the deck. The use of expanded polystyrene overcomes the

problem of water logging since the material consists of series

of small closed cells whose porosity is less when compared to

the total volume. It also has an advantage of being readily cut

using hot wire inbfactory or using hand saw in site.

Figure 1.6: Spirally wound metal sheet

Figure 1.7: Expanded Polystyrene

Objectives

(i) To perform a manual analysis for different loading

combinations as per IRC loading criteria.

(ii) To obtain the critical load positions that causes severe

distress in the structure.

(iii) To get an overall idea of how the complicated bridge

structures are designed in the industry.

II. METHODOLOGY

Analysis and design by STAAD pro.

Preliminary Data For Analysis

The voided slab bridge considered here is a RCC cast in

situ simply supported bridge of span 30 m.

Carriageway width =12.1 m

Overall width = 13 m

Width of crash barrier = 0.45 m

Thickness of wearing coat=75 mm

Cantilever portion of deck=1.5 m


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Thickness of cantilever deck slab=0.35 m

Depth of voided slab =2.1 m

No of voids in the deck=4

Length of solid section at both ends= 3 m

Grade of concrete=M35

Grade of steel =Fe 500

Figure 3.1: Cross section details of voided slab

Figure 3.2: Cross section details of end section

III. ANALYSIS OF VOIDED SLAB BRIDGE

The analysis is done separately for both longitudinal and

transverse direction.

Longitudinal Analysis

In longitudinal analysis, the bending moment and shear force

due to dead load and live load are found in the longitudinal

direction. The effect due to impact, temperature stresses are

also included and also a reduction is done in the longitudinal

effect as all the lanes will not be subjected to characteristic

loading.

Transverse Analysis

Different load positions for each type of loading are checked

and the maximum response has to be found out. For this the

wheel load of the vehicle is moved transversely such that it

occupies different positions on the deck and the maximum

response can be calculated for each of the loading case. Since it

will be tedious to manually calculate the response at every 0.1

m, computer analysis program such as STAAD can be used.

To analyze the effect due to live loads, first unit load is run to

obtain the influence ordinates. Then at each position of loading

and combination, effective width and equivalent concentrated

load per meter run is calculated. Effective width for each

location of load is calculated and thus each wheel load is

converted to wheel load per unit width of the slab. For

transverse analysis of the mid-section, the deck is idealized as

13 beam elements connected with nodes at the ends.

Fig.6 Idealized bridge deck for transverse analysis

Transverse analysis is carried out for the two cases separately

i. When the wheel is in the simply supported portion

ii. When the wheel is in cantilever portion

Transverse Analysis of Simply Supported Portion

In order to find the locations of maximum bending moment

along the transverse direction, the deck is idealized into beam

elements. The deck is modeled using STAAD software and a

unit load is run and the influence ordinates are obtained. For

example for beam 4 the influence ordinate at every 0.1 m is

obtained from STAAD as shown in the figure below.


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Now at each location effective width is calculated for four

combinations i.e., 1 laneA, 2 lane A, 3lane A and 1 laneA+1

lane 70R wheeled and the maximum value isselected.

According to IRC 21:2000 Pg 52, effective width for a solid

slab spanning in onedirection for a single concentrated load

isgiven by

beff= α a[1-a/lo]+bt,

Where

lo is the effective span

a is the centre of gravity of the concentrated loadfrom

thenearer support.

bt is the breadth of tyre+ twice the thickness ofwearing coat.

α is a coefficient depending upon the value b/lo ,where b is the

width of the slab.

Fig. 5.1 Beam end forces for beam 4 from STAAD

Fig. 5.2 Influence line diagram for moment beam 4node 4

The effective width at the position of maximum bending

moment is calculated. Then load per effective width is

found and it is checked whether the loads will overlap and if

so the load is modified

Fig. 5.3 Wheel `position for maximum positive bending

moment for beam 4

Now this concentrated load is applied on the model and is

analyzed to get the moments at each section. This procedure

is repeated for all the other beams and the moments are

obtained at each section.

Thus, for various positions of loading the maximum

moment from the different combination are summarized as

shown below.


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Transverse Analysis of Cantilever Portion

The cantilever portion of the voided slab bridge consists of the

crash barrier and wearing coat. The dead load on the cantilever

portion includes the weight from crash barrier, wearing coat

and the self-weight of the slab. For live load cantilever portion

has to be checked

Individually for 70R loading and class A loading. Minimum

clearance required in the case of 70Rloading is greater than the

length of the cantilever. Hence no live load will be acting in

70R loading. Thus live load due to class A loading is

considered. The impact load is also added to get the final live

load bending moment.

Longitudinal Moments

The reinforcement details for the longitudinal moments at mid

span, l/3 and at a distance “d” from the face of support are

summarized as shown below.

Table 6.1: Area of Steel in Longitudinal Direction


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Minimum steel in longitudinal direction at solid portion as per

clause 305.19 of RC: 21-2000, = 0.15% of total cross sectional

area=1650 mm2So provide 16 mm Dia. bar at 120 mm c/c.

Transverse Moments

As per clause 5.2.2.of IRC: SP: 64-2005,

Area of steel reinforcement

where

he= c/c distance of compression and tension flange,

d=diameter of voids

Minimum transverse reinforcement in the bottom,as per clause

8 of IRC: SP: 64-2005, In top and bottom of bottom flange is

750mm2 /m or 0.5% of minimum flange area.

Minimum transverse reinforcement in the top, as per clause 8

of IRC: SP: 64-2005, In top and bottom of top flange is

500mm2 /m or 0.35% of minimum flange area. The

reinforcement details in the transverse direction are

summarized below. Cross reinforcement of 12 dia bars at 150

mm c/c are provided in order to keep the voids intact.

Table 6.2: Area of Steel in Transverse Direction

DETAILING

Figure 6.1: Plan of the bridge

Figure 6.2: Detailing of Section A-A of the bridge


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Figure 6.3: Detailing of Section 1-1

Figure 6.4: Detailing of Section 2-2

CONCLUSION

The conclusions arrived after the analysis and design of the

voided slab bridge can be summarized as follows:

(i) The design of the bridge superstructure is a work

that requires great expertise and also knowledge to

foresee unexpected situations that may come

during the construction stage.

(ii) The longitudinal moments are the governing

moments and if the main bending has to suffice it

should not fail in transverse direction.

(iii) Transverse analysis makes the whole structure as a

single unit thereby taking care of the stresses due to

individual loadings.

(iv) Detailed analysis for the various IRC loadings have

helped in understanding the critical load position

and combinations that govern the entire design.

(v) If the void ratio is less than 40 %, the voided slab

can be analyzed using the samemethods that of

solid slab in longitudinal direction.

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Analysis and Design of Voided Slab Bridge

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