Week 10 Challenge

Aim

To Calculate the Snow Load accumulated on a Bridge of span 10m and width 18m as shown below and apply the calculated loads on the StaadPro model.

Data Provided:

Span of bridge = 10m

Width of bridge = 18m

The snow is of settled type

The deck of bridge is flat with β <30 degree.

The depth of accumulation of snow is = 1250 mm = 1.25 m

Introduction

Snow load is calculated as per IS 875 Part-IV and the loads are applied in accordance with IRC 6. Minimum snow load that is considered for deisgn of bridges is 900kg/m^3 as per clause 224 of IRC 6.

Snowy regions in India are, Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Haryana, West Bengal, Arunachal Pradesh, Assam, Manipur, Meghalaya.

Calculation of Snow Load as per IS 875 Part-IV

• Design snow load S = μ S0

where,

μ= shape coefficient

S0 = Snow load on ground

In most regions, snowfall is accompanied or followed by wind. The wind will redistribute the snow. In multi level roofs, the accumulated drift load may reach a value that is a multiple of the ground load.

Manual Calculation:

Assuming the density of settled snow = 3kN/m^3

Therefore,

Snow load, S0 = 3 (density of settled snow) x 1.25 (depth of snow accumulation) x 10(span of the bridge)

SnowLoad, S0 = 37.5 kN/m

For β < 30 deg, μ = 0.8

Therefore,

S = μS0

S = 0.8 x 37.5 = 30kN/m

Procedure in Staad.Pro

Modelling:

Structure is modelled in Staad.Pro as per the given data.

• Deck slab of width 18 m and span 10 m is modelled in Staad.Pro using Geometry option.
• 4 Girders with spacing 4.0m and two Cross girders at the start and end of the span. Mesh of size 0.5 x0.5 is drawn and plates are drawn for deck slab.
• Simple Supports are assigned at base of longitudinal girder resting on Bearing by using the option, General > supports > create > pinned > add > assign to selected nodes.

With this, modelling is complete as shown below

Specifications

Sectional properties of various structural elements are defined and assigned.

• Property > define > rectangle > 1.00 x 0.5 m (assumed) , Material : Concrete > add > assign to selected members of Longitudinal Girders

• Property > define > rectangle > 0.75 x 0.5 m (assumed) , Material : Concrete > add > assign to selected members for Cross Girders.

• Property > Thickness > 0.9 m (given) , Material : Concrete > add > assign to selected plates for deck slab.

For rendered view of structure we select 3D rendering, the result is as shwon below

Loading

Snow load calculated above as per the given data shall now be assigned in the model as elaborated below

• Load & Definition > Load case details > add > Snow load > type : Live load > add
• Snow load > add > member load > -30kN/m > add > assign to selected beams > assign

Minimum snow load considered for deisgn of bridges is 900kg/m3 as per clause 224 IRC 6 which shall be applied on the plate elements for deck slab.

• Snow load > add > plate load > -9kN/m2 > add > assign to selected plates

Analysis

The structure shall now be analzed to obtain the bending moment and shear force results

• Analysis > perform analysis > Ok
• Analyse > Run Analysis > ok

Now we go to post processing mode

Results

For bending moment of longitudinal girder, we select all the longitudinal girders

• Go to Report > section forces > Moment Mz > sort Low to high (sagging moment) > ok

BM max = -7.389 Mton-m for beam no. 747 for snow load

• Go to Report > section forces > Moment Mz > sort high to low (hogging moment) > ok

BM max = 0.015 Mton-m for beam no. 579 for snow load

For shear force of Longitudinal girder, we select all the longitudinal girders

• Go to Report > section forces > Shear Y > ok

SF max = 8.797 Mton for beam no. 13 for snow load

For Bending moment of cross girder, we select all the cross girders

• Go to Report > section forces > Moment Mz > sort Low to high (sagging moment) > ok

BM max = -1.146 Mton-m for beam no. 48 for snow load

• Go to Report > section forces > Moment Mz > sort high to low (hogging moment) > ok

BM max = 6.575 Mton-m for beam no. 12 for snow load

For shear force of cross girder, we select all the cross girders

• Go to Report > section forces > Shear Y > ok

SF max = 9.774 Mton for beam no. 12 for snow load

For Bending stress of deck slab, we select all the plates elements and go to plate results and select for Mx and My

Similarly Shear stress SQx and SQy are obtained as shown below