# Inputting known values and being able to select them depending on calculated value

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Finlay Brierton am 8 Jan. 2020
Beantwortet: Finlay Brierton am 10 Jan. 2020
Hi,
Sorry I worded this question badly beforehand.
As you can see from my screenshot and code, I have calculated the Mr (max bending moment) for a steel portal frame of dimensions (w,L,h).
Having calculated this moment I would like the programme to suggest a steel section with a suitable max bending moment resistance. A made a small example list at bottom of my coding, but I will be putting in all the sections and their corresponding bending moment resistance so I can get suggested sections for when I change the frame variables.
How can I get the program to suggest suitable section sizes for the calculated Mr?
Thanks % program code equivalent to the graphical design method %
% as far as rafter selection %
w = 10; % w is the udl on the fram
L = 30; % L is the span of the frame
h = 6.5; % h is the height to eaves of the frame
theta = 12; % theta is the roof pitch of the frame
N = L/2 + h/tand(theta); % N is the distance from stanchion to base of reactant line where it meets the ground level
syms m
eqn = m^2/(2*w) + m*(5*N/3 - L/3) + w*L^2/8; % the derived equation from the graphical method to find gradient
M = solve(eqn,m); % solving the equation
m1 = max(double(M)); % choosing the smaller negative negative gradient (so smaller number)
Y = -(w/2)*(-m1/w).^2 + w*L^2/8; % the equation for the parabola at max bending moment (at x-bar)
R = -(m1.^2/w) - m1.*N; % the equation for the reactant at max bending moment (at x-bar)
Mr = Y - R; % (KNm) the differce in y values at x-bar gives the max bending moment in the rafter
steel sections: 406x178x67, 406x178x74, 406x178x85 % dimensions of specific steel sections (taken from steel design book)
section moment: 370, 413, 459 % Bending moment resistance for the sections labelled above (taken from steel design book)
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### Akzeptierte Antwort

Sylvain Lacaze am 8 Jan. 2020
Hi Finlay,
You could turn pproject into a function:
Say:
function Mr = pproject(w, L, h)
end
Then compute the Mr for all your cross sections:
w = [..., ..., ...];
L = [..., ..., ...];
h = [..., ..., ...];
Mr = zeros( 1, numel( w ) );
for i = 1:numel( w )
Mr(i) = pproject( w(i), L(i), h(i) );
end
And rank best on closest to target Mr:
[~, rank] = sort( abs( Mr - MrTarget ) );
disp( 'Best cross section' )
disp( rank )
You could also consider vectorizing pproject:
HTH,
Sylvain
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Sylvain Lacaze am 9 Jan. 2020
Hi Finlay,
OK, I think I see.
So let's say you have Mr, your computed max bending moment.
Then, let's say you have a table like this:
Name = ["crossSection1"; "crossSection2"; "crossSection3"];
Resistance = [370; 413; 459];
crossSectionInfo = table( Name, Resistance );
First, sort by Resistance, to make sure you go from weakest to strongest:
crossSectionInfo = sortrows( crossSectionInfo, 'Resistance' );
Then find the find cross section with a Resistance higher than the max bending moment Mr:
bestCrossSectionIndex = find( crossSectionInfo.Resistance > Mr, 1, 'first' );
disp( crossSectionInfo(bestCrossSectionIndex,:) )
HTH,
Sylvain

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### Weitere Antworten (1)

Finlay Brierton am 10 Jan. 2020
Sylvain,
This is exactly what I was looking for, thank you so much.
Finlay
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