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[rahulranjan_31]

Dear SEFIans,
For a foot over bridge I want to calculate the wind force over the purlin, but as per table-6 (External pressure coefficients for pitched roofs of rectangular clad buildings) for h/w ratio i am confused if i should take the height from ground level or from top of girder level (i.e. walkway level).
Any suggestion would be highly appreciated and thank you in advance.

[Dr. N. Subramanian]

[vikram.jeet]

The height at which roof spring level is located from GL is height h to be taken . It is already made clear by Dr NS  Sir.

Confusion to you occured , I think , due to foot bridge deck and roof forming a sheltered formation.

But it seems a case of open roof  without wall cladding and exterior wind coefficient needs to be plus/minus 0.7 as per IS.

[vikram.jeet]

Minor slip in earlier post , inconvenience regretted

The internal wind pr coefficient needs to be plus/minus 0.70 , as roof is open , no cladding from sides except railing posts and roof posts in a foot over bridge.

[rahulranjan_31]

Thank you so much for your response sir. Actually that is the confusion which i am not clear about. Since bridge deck comes in between GL and roof truss level then why should we consider height from the GL for calculating external wind pressure coefficient.

[vikram.jeet]

The bridge deck is also subject to external and internal wind pressure based on height h of deck from ground surface .

The stability of overall bridge structure under wind case , will get uplift force  (by summing up) from (I) wind pressures on deck (ii) wind pressure on walkway A type sloping roof.