Structural steel "T" cantilever beam design.
Structural steel "T" cantilever beam design.
(OP)
I am attempting to design a structural steel "T" cantilever beam for a balcony. This is proving very difficult. I have managed to model it to find a deflection but moment capacity is proving impossible. Can anyone help?
Thanks.
Thanks.






RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
Finding the relevant section properties i.e. Z eff are proving beyond me.
See architect Sketch.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
It is the only code I am familiar with.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
If your T section is oriented such that the flange is in tension then I don't beleive Lateral Torsional Buckling is a problem, you'll only have to check that the stem (web) doesn't locally buckle and then make sure it doesn't exceed it's yeild or plastic strength as SEIT stated.
Also for cantilevers, the deflection requirements are less stringent per foot of length compared to a simple supported beam. In AISC your aloud to double the length of a cantilever to caculate the allowable deflection. This is considering that a cantilever assumes the curvature of a beam that has a length 4 times greater, but the deflection limit at the tip of the cantilever should be 1/2 the deflection at midspan of this equivalent simple beam. Which gives you the factor of 2. This is what I've always used to convince myself of this provision. Is this wrong?
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
The stem/web local buckling check is the check which is troubling me. The effective section properties (i.e. Z eff) is what I cannot put my finger in.
It is not as simple as using Zx to evaluate moment capacity. According to BS5950 anyway. It says for Class 4 slender members Z eff defines moment capacity.
Appreciate everyones input. I am young engineer with little experience.
RE: Structural steel "T" cantilever beam design.
Grab your Mechanics of Materials book...both are simple to compute.
How does your code define Z eff?
RE: Structural steel "T" cantilever beam design.
Sorry, had to do it.
RE: Structural steel "T" cantilever beam design.
However the code does not give a clear indication on how to evaluate this number.
That is where I am stuck.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
For instance, I know that "fb" is the "allowable bending stress" in AISC ASD, but depending on the situation "fb" might = 0.6FY, 0.66Fy etc. Follow me here?
RE: Structural steel "T" cantilever beam design.
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Yes i follow you. But it is working out the "reduction penalty" for Z in a slender member that is not clear.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
But................Clause 3.6.2 Is titled.........Doubly symmetric cross-sections. In this case, the section only has only one axis of symmetry.
Clause 3.6.3 is for Singly symmetric and unsymmetric cross-sections but this clause is not telling me Zeff = or Aeff = etc.
Thanks for your time.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
Read closely 3.6.2.4.
Section modulus is simply I/C.
or Zx = Ix/Cx.
In your case Ix will be based on effective widths "b eff" and therefore will change or reduce Ix for the full section. I am assuming that Cx (distance to the cnetroid) will also be based on the REDUCED Ix.
Also, you will wind up with two different Z values as there will be two values for "C" as for a Tee shape the centroid will not be at mid depth of the section.
RE: Structural steel "T" cantilever beam design.
Unfortunately I have got to go.
Thanks to everyone again.
I will have a look at this thread again tomorrow.
Cheers.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
Will read up again tomorrow.
RE: Structural steel "T" cantilever beam design.
The supporting beam will rotate torsionally so that the deflection will be greater for interior members than exterior. This will be partially offset by the 80 x 60 HSS at the tips of the cantilevers.
Your connection to the wall is questionable. Are you expecting to transfer all of the moment at each end through the 120 x 80 HSS by means of a couple of through bolts? Not a good idea!
BA
RE: Structural steel "T" cantilever beam design.
I would also look closely at your wall connection. The last time I did one of these using Hilti anchors the recommended shear and tensile capacity for an M12 was around 3.0kN. See if its possible to do some pull out tests if your brickwork properties are unknown.
RE: Structural steel "T" cantilever beam design.
Can I suggest you read the SCI advisory desk note AD311 T sections in bending - stem in compression.
It provides two methods for designing the T using modifications to BS 5950.
You know how to calculate Z from I? Zeff is done the same way but instead of using the I value for the full section, you use the I value for the effective section as detailed in Figure 8.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
Instead, focus on the issue of how to safely secure the steel structure to the wall.
BA
RE: Structural steel "T" cantilever beam design.
I am trying to give the client and architect what he/she wants.
I have managed to do some analysis of moment and deflection.
Thanks all.
RE: Structural steel "T" cantilever beam design.
I suggest that if you are forced to do this, you should look at using a welded tee instead of a tee split from a wide flange section. That way, you can use a thicker web, more resistant to buckling.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
RE: Structural steel "T" cantilever beam design.
I have only uploaded the exe file as I think most PCs will have the supporting files necessary.
RE: Structural steel "T" cantilever beam design.
Your program does not consider unbraced length of the bottom of the web. Or am I missing something?
BA
RE: Structural steel "T" cantilever beam design.
No you're not missing anything. It's one of the things still to add, just ran out of time, I'm only considering local capacities at present (still to digest the appendices of the BS). I hope to get this done by the end of the week, and I'll post again.
RE: Structural steel "T" cantilever beam design.
I am amazed there is not a similar example published somewhere.
RE: Structural steel "T" cantilever beam design.
One method of tackling the problem is to treat the bottom of the web as a compression strut, calculate its L/r and use a bending stress equivalent to the permissible axial stress. To make that work, you would want to use hokie's suggestion of a built-up member with a fairly thick web.
BA