help with bending stiffness

[rudigger]

hello, im trying to workout the bending stiffness of a steel L shaped bracket.

E = 206,000 N/mm2

I = 1460.546 cm4

can snyone help with how to multiply these two??

thanks

 

[cntw1953]

rud:

Provide a sketch with loads indicated. Here are some fellows seem work from England that may help, once they understand the problem better (graphically).

 

[rudigger]

ok, here's a sketch

 

[cntw1953]

rud:

Nothing on the attached sketch. Looks like you didn't upload correctly. Try again.

 

[rudigger]

sketch

 

[Ussuri]

Moment capacity in BS 5950 is determined Mc=py x S, for the low shear case (cl 4.2.5.2). There are other limits also.

From what you have said above. You have a flat plate 100mm wide and 8mm deep. It is loaded with a 21kN point load at a distance 100mm along so as to load your plate in the minor axis (I assume you have a cantilever based on the way you calculated the moment). Is this correct?

You need to calculate the ULS load, i.e. factored, and compare it against the capacity.

According to your posts above, the structural engineer determined the moment capacity as 0.66kNm. You have calculated the applied moment as 1.05 kNm. This means it doesn't work. Either you have determined the moment incorrectly, or the your colleague determined the capacity incorrectly.

 

[cntw1953]

I gave up. HELP, someone!

 

[Ussuri]

still nuthin

 

[rudigger]

sketch is uploaded this time

thanks ussuri, yes the bracket failed but in practice the thickness was increased to make it work. im actually just trying to determine how the moment capacity of 0.66 kN.m is calculated, for future reference

i made a slight error, as you can see from the sketch the moment applied is 21 x 0.046 = 0.97 kN.m

 

[Ussuri]

In simple terms:

Moment Capacity Mc = py x S = py x b x d^2 x (1/4)

You need to know the yield strength of the material and the width, b.

 

[rb1957]

IMHO, this problem has nothing to do with the capacity of the angle. we're not bending the angle, only a flange.

with respect to ussuri, (1/4) should be (1/6) no ? ... bending stress = 6M/(b*t^2)

 

[Ussuri]

rb1957,

BS5950 typically states the moment capacity is the yield strength multiplied by the plastic section modulus (S, in the UK, Z in the US). This reduces depending on the section classification (b/T ratios) so that it may be the elastic section modulus (Z in the UK, S in the US) that should be used, so you may indeed be correct. In the case of particularly slender sections is will be less than Z (or S).

 

[BAretired]

In Canada, we use Z for plastic modulus and S for section modulus. So Z = bd²/4 and S = bd²/6.

The resisting moment, Mr = [φ]Fy*Z

Resistance Factor,[φ] = 0.90

Z = 100*8²/4 = 1600 mm³

For a yield of 350 MPa, Mr = 0.9*350*1600 = 504,000N-mm or 0.504kN-m (Note that this is the factored capacity)

M = 21*50/1000 = 1.05 kN-m. > 0.504 [NO Good]

Maximum factored load Pf = 0.504/0.05 = 10 kN

Maximum allowable load (assuming live load) = 10/1.5 = 6.7 kN.


BA

 

[dhengr]

rudigger:

I commend you for wanting to learn, and for taking on a problem like this for your own edification, even though it wasn’t your problem to solve. This shows some real initiative on your part. At the moment you may only be a vBE (“very basic engineer”, Mike never could get his degrees straight), but you could very quickly graduate to a lbvBE (less bashful very basic engineer) if you would get with that other structural engineer you mentioned. Hopefully he is in the same office as you are. Ask him to be your mentor, you will find that most engineers are glad to help a beginner learn the ropes, as is evidenced by the many responses you got on this thread, as long as you show the initiative.

As someone else said ‘you need a few text books, not this forum’ to do what you are trying to do.
You need a basic strength of materials book and a steel design text, and your own time to study them. Then develop that relationship with a smart engineer who will spend some time with you. He can see the problem with you and send you off in the right direction, or tell you where to look for the answer or what to study. And he can correct you, immediately, when you are going off track, more efficiently than we can on this forum, i.e. it took us 20 posts to get a picture of the problem.

If you can find that mentor in your own company, and really show that initiative, that can lead to promotion. If you go outside for that mentor, hard work could lead to a new job. Don’t be bashful, none of us objects to an honest question, and we all admire honest initiative.
Good Luck

 

[Qshake]

I hope that we've not assisted yet another student with a homework problem.....it's getting to be the end of term these days ya know.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[rudigger]

thanks all for the help, and dhengr for the encouraging advice. i am infact, Qshake, a part-time civils student but this wasnt homework. i have plenty of that though if anyone knows their water infrastructure and retaining walls ;0)