How do you calculate the force in a compression flange?
How do you calculate the force in a compression flange?
(OP)
How do you calculate the force in a compression flange based on the moment?
Hi,
I am trying to calculate how much force is in the compression flange of a portal column based on its moment. I have to design a torsional restraint (capable of withstanding 2.5% of the force of the compression flange). The angle of the tie from the compression flange to the restraining member is very accute and as such I am a little worried about the steel angles which have been added. Was thinking it would go something like this:
Calculate the Z vale of the compression flange only.
Divide the moment by the Z value to get a stress.
Multiply the cross sectional area by the stress. That should give me a maximum applied force.
Find the resultant force based on the angle of the tie.
Check the angle against the maximum force in compression to see if it passes.
Any help would be really appreciated.
Thanks!!!
Hi,
I am trying to calculate how much force is in the compression flange of a portal column based on its moment. I have to design a torsional restraint (capable of withstanding 2.5% of the force of the compression flange). The angle of the tie from the compression flange to the restraining member is very accute and as such I am a little worried about the steel angles which have been added. Was thinking it would go something like this:
Calculate the Z vale of the compression flange only.
Divide the moment by the Z value to get a stress.
Multiply the cross sectional area by the stress. That should give me a maximum applied force.
Find the resultant force based on the angle of the tie.
Check the angle against the maximum force in compression to see if it passes.
Any help would be really appreciated.
Thanks!!!






RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
Thanks again for the reply!
RE: How do you calculate the force in a compression flange?
The stress in the flange wont vary greatly across the thickness.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
M=650kNm at brace
Area of compression flange= 4486mm^2
I=111444cm^4
Axial Force in Column=430kN
M/I=F
F*Area of compression flange=CF
CF*tan Angle=Compression Force the angle has to resist
I don't quite understand why you would require the compressive force of the column.
One another note if I was to be conservative and multiply the cross section of the flange and multiply with by the tensile capacity of the steel (fy=355N/mm^2) I would end up with a force of 1682kN *2.5=42kN?
Thanks for your help as always.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
P/A + Mc/I (or M/S)
Take this vale and multiply by the area of the flange.
This = the force in the flange.
Design your brace withstand 2.5% of this force.
RE: How do you calculate the force in a compression flange?
the axial load is also compressing the flange (along with the rest of the section).
if you're designing the connection, ie what's the maximum load that could be in the flange, then using fty is conservative. if you're designing the flange, the allowable will be less than this, due to buckling.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
Multiply that with the flange width x flange thickness...
Dik
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
You left the area out of your first expression.
All,
So much ado about nothing. Why does everybody seem to object to simplification? Does anybody seriously see anything wrong with my approach? Of course, this has to be resolved into a bracing force as the OP knows.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
Thanks again to everyone who added to this thread.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban
RE: How do you calculate the force in a compression flange?
As rowing notes above, almost everything in structural engineering is approximated on some level.
RE: How do you calculate the force in a compression flange?
"If you understand why what you're doing works, you'll be much more effective at it!"
RE: How do you calculate the force in a compression flange?
I tried to oblige.
However, If I was designing a brace like this (and I have many times) I would use the yield stress of the steel in the calc.
You'll find after designing a few such braces that 2.5% equates to a very small, easy to accommodate, load.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
Lion, I fully appreciate that it would take you much less time if you just approximated but for the sake of 10 minutes I now have an accurate answer, a method for carrying out this type of analysis, a better grasp of what value of the applied load I would need to start worrying about, etc.
Being conservative is one thing but actually knowing how to carry out the process correctly is far more important for a graduate engineer don't we all agree?
On that note I just want to post a thank you to TJ for his help.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
If you have ever designed a moment connection, the force in the flange is M*12/(d-tf). This calculation is littered throughout the AISC manuals. So if it is not exact then why hasn't the AISC revised the calculation in all their design examples?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
I would not necessarily use the yield stress times the area of the flange because slenderness might have controlled the allowable compression in the column.
RE: How do you calculate the force in a compression flange?
using yield*area as an allowable can be unconservative if the flange has a lower crippling/buckling allowable.
if you're just getting started in the business, i think it's a good idea to calc the load the long way a couple of times to learn how conservative is the yield stress simplification and how much (or little) time it saves, and to be carfeul in applying it.
it might be a "clever" way to show the calc and then to use the higher yield stress load as a conservatism.
the long method, as pointed out above, is the combination of axial compression and bending stresses.
RE: How do you calculate the force in a compression flange?
Not much of what we do has to do with "actual values". For instance, the 2.5% used in your example is arbitrary, and could just as well be 3, 4, or 5. Some codes differ in the amount of bracing required.
RE: How do you calculate the force in a compression flange?
There is a correct method of carrying out a calculation. That's important for a graduate, not just to quickly scoot around it and get a rough answer. I like facts, clear and in black and white. Even if the people responsible for writing the codes are completely wrong I am sure the insurers etc would be willing to accept that based on the code and through proper analysis my design was safe and proper.
Until I get to a stage where I am comfortable in my analysis I will always do the correct calculation to the button and not just approximate.
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
RE: How do you calculate the force in a compression flange?
Graybeach, I'll upload the sketch tomorrow as I'm out the office now.
Once the sketch is uploaded and the variables and unknows set I would really appreciate the clear and definitive approach to the problem.
Once again all your help and advice no matter how little or expansive is always appreciated (that gos for the oldies too lol)
RE: How do you calculate the force in a compression flange?
However, once you have been out of university for a while, you will being to realise that the best engineer is not the one being the most accurate or carrying figures to 6 decimal places. The main things is always understanding all the simplifications you make, and ensuring a safe and logical load path for all applied loads.
I did my thesis under one of the authors of the Australian wind code. I called him up one day to ask him for advice on a condition not covered in the code. He said "handex, your guess is as good as mine. Just make a conservative approximation".
Once you get on site and see how a lot of things are done, you will realise that the following quote holds a lot of truth for engineering calculations (quoted from a user of these forums)..
"We tend to measure with a micrometer, mark it with a crayon, and cut it with an axe"