Canadian Code S16-14-TEE section bending question

[850R]

Hello all,

I have a question about a Tee section strong axis bending.
Have 2 load cases, both are simply supported beam and concentrated load in the middle,
One case the load is downward in the middle (stem in tension)
the other case load is upward and in the middle (stem in compression)

I am looking at section 13.6.(e)
My section is WT12x27.5 which is classified as flange 1 web 4
From the code I understand that I need to replace Mp with My for the second case where I have the stem in compression and use the the original formula Mr as it is in the code for the first case but I am not exactly sure.
Can you guys please confirm this or if I am wrong can you please explain where I go wrong?

Thanks in advance.

 

[jayrod12]

If it is class 4 web then I would be considering the whole section to be class 4 and it would need to be designed accordingly.

Which would mean using reduced section properties for both load cases (or reduced yield stresses, which is generally easier to accomplish)

 

[850R]

Thanks for your response,

Have two more questions now based on your answer.
First question: under 13.5(c) it says Mr should be determined in accordance with CSA S136 for class 4 sections, is this where I should calculate the Mr for my case?
Second question: If I had Class 1.2 or 3 Tee section, is my approach correct in the my first message?

 

[jayrod12]

It would be correct however if you had a class 1 or 2 section (both web and flange) you could use Mp for both upward and downward loading. Class 3 you would need to use My for both upward and downward loading.

For class 4 sections, it is easiest to use the formulas for class 3 sections from table 2 and the section properties of the section you want to use and back calculate a reduced fy to use in the regular formulas. It seems like more work to use S136

 

[850R]

Thanks again.

I just compared the laterally supported and unsupported class 1 Tee sections with clauses 13.5(a) and 13.6(e)
The laterally supported option gives more capacity using 13.5(a), I thought the unsupported clause would give less capacity?

Is that possible?

 

[850R]

Jayrod,

I got the class 1 2 and 3.
Do you have any example problems for a Class 4 section?

 

[jayrod12]

Nope. What I normally do is use the b and t (or h and w) values from your class 4 section and using the formulae for class 3 sections in table 2 back calculate for a reduced fy to use in the regular capacity formulae.

Option 2 would be to determine what b or h value works for your given t or w for the class 3 formulae, then calculate an effective section modulus Se using this newly determined b or h and use that in your capacity equations.

Option 2 always seems like too much work for me.

 

[850R]

So lets say I have a Tee section WT12.27.5 with
d=11.8in
bf=7.01in
tw=0.395in
tf=0.505
Fy= 248MPa

The flange is classified as Class 3 and stem is class 4

Since flange is Class 3, no need for a reduced Fy but for stem,
(d-tf)/tw=340/Fy

Fy= 11.295MPa

Is this how you do it? The reduced Fy is 11.3MPa?

 

[jayrod12]

That is correct in theory but I believe the formula has the square root of fy so your reduced fy should be around 120 MPA. But I'm out of the office this week so can't confirm

 

[850R]

Yes you are right, forgot the sq root

 

[850R]

Jayrod,

In the code Cl 13.5a and 13.6e says T-sections shall not yield under service loads, what does this exactly mean?
I am still not that clear if i should use Mp or My for class 1 Tee section for both cases stem in compression or tension.

 

[jayrod12]

It means that your calculated stress at full service loading must be less than fy. For the class 1 section you can use 350MPa. for the class 4 we've been discussing it must be less than your reduced fy. That is why more often than not people avoid class 3 and 4 sections at all costs.

For a class 1 tee section you can use Mp. This is where the service loading comes into play more often than not, because Mp allows the entire section to yield under ultimate loading, however for service loading your extreme fibres must stay below fy which will often end up governing the section size.

Why the fascination with Tee sections?

 

[850R]

Jayrod,

I really appreciate for your every response.
The company I am working for purchased a structural analysis software and I am checking it out for validation. That is why I am asking all these questions about the Tee section, once I complete it, will get on the angle section.

 

[jayrod12]

I see.

I also hope that you do not take my word as gospel (although I think if I were to be leading you down a wrong path that one of the other bright individuals on here would be correcting me).

Which software if you don't mind me asking?

 

[850R]

Staad.
Your responses make me understand the code better, I think the code could be written more clear.
One other thing that I couldn't find on the code is that if you have a class 2 web and class 3 flange the whole section will be classified as class 3 section? This is how it works in Eurocode but couldn't confirm it on the Canadian code.

 

[jayrod12]

That is correct. Always default to the worst case scenario.

 

[canwesteng]

FYI there is nothing for bending of angles in S16. I use AISC to determine member capacity if I think LTB may govern over yield moment.

 

[850R]

Thank you canwesteng, will keep in mind.

I have one last questions about bending of a Tee section.
If I have a class 1 or 2 tee section to check for bending and use Sy instead of Zy would that be wrong? If not wrong, would that be too conservative? (that last sentence in clause 13.6e "except that these sections shall not yield under service loads" just makes me uncomfortable using Zy for Class 1 and Class 2 tee sections.

 

[jayrod12]

IT would be conservative but not incorrect.

 

[850R]

Awesome, thank you so much...