# Determine a suitable size beam (gantry girder)

1. Nov 13, 2013

### hershl

1. The problem statement, all variables and given/known data
In cross-section, the beam you are working on is a gantry girder.The maximum bending stress in the girder must not exceed 25MN/m2 and the modulus of elasticity can be assumed to be 210GN/m2.
Determine a suitable size for the beam, selected from the standard sections tables.
Also determine the maximum stress of the chosen beam and its radius of curvature.

2. Relevant equations

E=σ=M
R y I

3. The attempt at a solution
I have worked out the max bending moment as 36KN and drawn the SFD and BMD

Using the formula
σmax=M*ymax
I
This is where i get stuck as the notes i have are for universal beams and not gantry girders

I'm not after the answer but if any one can shine some light on how to go about solving this i'd be much appreciative.

The course is HNC electronic and electrical eng. With a mechanical principles module

2. Nov 13, 2013

### SteamKing

Staff Emeritus
Can you post the cross-section? I'm afraid 'gantry girder' is a rather vague description.

3. Nov 13, 2013

### hershl

Sorry
The table comes from a British standards book. showing universal, structural tees and gantry girders.
There is no cross section stated in the question(i think this is where i'm getting stuck as i have examples using the cross section).

Here is that table as well as my working of the SFD and BMD.

Thanks

#### Attached Files:

File size:
49.4 KB
Views:
206
• ###### photo (1).jpg
File size:
54.2 KB
Views:
188
4. Nov 13, 2013

### SteamKing

Staff Emeritus
I would work out the section modulus of the girder required to meet the max. allowable bending stress and then use the tables labeled 'gantry girder' to find a candidate beam.

5. Nov 13, 2013

### hershl

Is that worked out using the same "max bending moment times 1/2 of depth of beam over 2nd moment of area"
As you would with the universal beam?

6. Nov 13, 2013

### SteamKing

Staff Emeritus
The gantry beam sections are not symmetric w.r.t. the neutral axis of the beam. This is clearly shown in the cross section sketches of the attached tables. The section modulus value Z from the beam tables should be the minimum value for a given beam size, and they can be checked by using the appropriate I and y values (Z = I / y). The bending stress is calculated as σ = M / Z, using appropriate units of course.