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Roof Live Load

Forum|Forum|2 months ago
January 26, 2026
2 replies
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Good Afternoon, Community,
Would anybody be kind to help on how to program the Roof Live Loads tributary area as one function to take care of column or can I do this in Comb in Mathcad Prime. Please see attached. Thanks,.

Olu

1_Roof_Live_Loads.zip

Civil_Engineering

Best answer by Werner_E

Basically you have five different cases (actually nine, but five return the same value 12).

Your two programs just cover three of them.

What is the meaning of

It looks like a boolean comparison which could be true or false (in Prime that's 1 or 0), but I guess it has a different meaning.

I have no experience in your filed of work but according to the table the outcome should be a load in psf and not as you programmed it an area?

I also don't understand what you mean by "take care of column" and "do this in Comb"!?

There are many ways to skin a cat, so here is a way which could also easily deal with nine different outcomes and also with larger matrices.

I interpreted the center expression as "at least 12 or more". Not sure if this makes sense, though.

W

Werner_E

Answer

25-Diamond I

Basically you have five different cases (actually nine, but five return the same value 12).

Your two programs just cover three of them.

What is the meaning of

It looks like a boolean comparison which could be true or false (in Prime that's 1 or 0), but I guess it has a different meaning.

I have no experience in your filed of work but according to the table the outcome should be a load in psf and not as you programmed it an area?

I also don't understand what you mean by "take care of column" and "do this in Comb"!?

There are many ways to skin a cat, so here is a way which could also easily deal with nine different outcomes and also with larger matrices.

I interpreted the center expression as "at least 12 or more". Not sure if this makes sense, though.

2_WE_20260126_Roof_Live_Loads.zip

T

terryhendicott

21-Topaz II

Hi Werner and Olu

Corrected Werner's sheet to reference A_t not A_trib.

Gets a better surface.

Cheers

Terry

1_WE_20260126_Roof_Live_Loads.zip

W

Werner_E

25-Diamond I

Ah, yes, thanks! I sure should not have used A.trib in the program instead of A.t.

After that fix the value now never runs below 12 psf.

BTW, here is a slightly simpler way to get the correct matrix indices 'row' and "col'.

IMHO its clearer and easier to read if we just check for the upper limits. Because of the index ORIGIN we are still correctly using the first index where the condition is fulfilled.

EDIT: Added an attempt to graphically show the nine areas defined by the matrix.

Unfortunately Prime's 3D plots miss a lot of standard formatting options like some sort of transparency. And the contour plot does not allow for additional traces and does not offer marker lines 😞