3. 3
FEATURES
• Design of Gantry Girder is a classic example
of laterally unsupported beam.
• It is subjected to in addition to vertical loads
horizontal loads along and perpendicular to its
axis.
• Loads are dynamic which produces vibration.
• Compression flange requires critical attention.
13. Steps for Design
Assume that the lateral load is resisted entirely
by the top flange of the beam plus any
reinforcing plates, channels etc. and the
vertical load is resisted by the combined beam.
1. Find the maximum wheel load: This load is
maximum when the trolley is closest to the
gantry girder. Increase it for the impact
2. Calculate the maximum bending moment in
the gantry girder due to vertical loads.
3. To simplify the calculations, add the
maximum bending moment due to dead load
to the maximum wheel load moment
13
14. Steps for Design (cont.)
4. The maximum shear force is calculated. When the
gantry is not laterally supported, the following may
be used to select a trail section.
Zp = Mu / fy Zp (trial) = k Zp (k = 1.40-1.50)
Economic depth ≈ 1/12th of the span.
Width of flange ≈ 1/40 to 1/30th of the span
5. The plastic section modulus of the assumed
combined section
Mp = Zp fy
where Zp is called the plastic modulus
14
15. Steps for Design (cont.)
6. Check for moment capacity of the whole
section (as lateral support is provided at the
compression flange)
Mcz = βb Zp fy ≤ 1.2 Ze fy / γm0 <Mu
7. Check top flange for bending in both the axes
using the interaction equation
(My / Mndy)+ (M2/Mndz) ≤ 1.0
8. If the top (compression) flange is not
supported, Check for buckling resistance in the
same way as in step 6 but replacing fy with the
design bending compressive stress fbd.
15
16. Steps for Design (cont.)
9. Check web of the girder at points
of concentrated load for local
buckling or local crushing, and
provide load carrying/ bearing
stiffeners, if necessary.
10. Check for deflection under
working loads
16
