- #1

dichotomy

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i've to design a 3 or 5 bar statically determinate frame, out of Al, so that it'll support a certain load. now I thought it would be a veritable piece of cake to figure out the forces in the individual members, but so far I've tried everything under the sun, and my answers from resolving at joints (see diagram) for B1 will just not agree.

i'm trying to minimise the bar forces by using excel to calculate the best design from a number of cases (specifying the angles and lengths using a coordinate system), and frankly I've no idea if this is a genuinely hard problem or if I'm just being an idiot.

i end up with:

B1 = R2/(sin(a)+tan(c)cos(a))

by resolving at joint C, and:

B1 = R1/(tan(b)cos(a)-sin(a))

by resolving at joint A. (ie resolving vertically and horizontally, and substituting in, just like every other simultaneous equation.) I've checked my sign convention, checked issues with positive and negative angle values, and still nothing!

these give diff. answers in excel, and if you boil them down to pure trig they also are not equal. so what gives? this is a basic issue ffs.

i'm trying to minimise the bar forces by using excel to calculate the best design from a number of cases (specifying the angles and lengths using a coordinate system), and frankly I've no idea if this is a genuinely hard problem or if I'm just being an idiot.

i end up with:

B1 = R2/(sin(a)+tan(c)cos(a))

by resolving at joint C, and:

B1 = R1/(tan(b)cos(a)-sin(a))

by resolving at joint A. (ie resolving vertically and horizontally, and substituting in, just like every other simultaneous equation.) I've checked my sign convention, checked issues with positive and negative angle values, and still nothing!

these give diff. answers in excel, and if you boil them down to pure trig they also are not equal. so what gives? this is a basic issue ffs.

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