Steel Design Test 3

Concept:

For economical spacing of truss,

The cost of truss should be equal to twice the cost of purlins plus the cost of roof covering.

t = 2p + r

where t= cost of truss, p= cost of purlin, r = cost of the roof.

However, the above expression is used to check the spacing of the roof truss. It can not be used to design the spacing as spacing does not occur in the equation. As a guide, the spacing of roof truss can be kept 1/4 of the span up to 15m and 1/5 of span for 15-30 m span of roof trusses.

Concept:

Economical depth of a plate girder is given by,

 \(d = {\left( {\frac{{Mk}}{{{f_y}}}} \right)^{1/3}}\)

\(k = \frac{d}{{{t_w}}} = \frac{{depth}}{{thickness\;of\;web}}\)

Where, M = Moment and f_y = yield stress of steel.

Calculation:

Given,

Load = 80 kN/m

Span (ℓ) = 25 m

k = 200, F_y = 250 MPa

\(B.{M_{max}} = \frac{{w{\ell ^2}}}{8} = \frac{{80 \times {{\left( {25} \right)}^2}}}{8} = 6250\;kNm\)

\(d = {\left( {\frac{{6250 \times {{10}^6} \times 200}}{{250}}} \right)^{1/3}} = 1710\;mm\)

\({t_w} = \frac{d}{k} = \frac{{1710}}{{200}} = 8.55\;mm\)

The effective length of groove weld:

As per IS: 800-2007, Clause 10.5.4.2 

The effective length of butt weld or groove weld shall be taken as the length of the continuous full-size butt weld, but not less than four times the size of the weld or minimum of 40 mm.

Maximum clear spacing between the effective length of the weld:

• For the welds in compression zone = Minimum of 12 × Thickness of the weld or 200 mm
• For the welds in tension zone = Minimum of 16 × Thickness of the weld or 200 mm

Ends of the longitudinal fillet welds shall not be less than the width of the member.

Concept:

As per IS 800:2007, Table 6, the permissible deflection in Gantry girder is tabulated below:

Calculation

Case 1: When 100 KN electrically operated crane (EOT) is provided on the gantry girder.

From the above table, the maximum permissible deflection, δ_max­ = L/750

Where, L is the effective span = 8 m given

∴ δ_max = 8000/750 = 10.67 mm

Case 2: When 50 kN hand-operated is provided on the same gantry girder

From the above table, the maximum permissible deflection, δ_max­ =  L/500.

This deflection must be equal to 10.67 mm as  per the condition  given in the question

∴ 10.67 = L/500

⇒ Effective span, L = 5.335 m