huangyhg

bending moment
hi ... sorry this is so simple for you lot but i never did well at mechanics and it was sooo long ago.
1) i know if i plant a 5 meter high post in concrete and apply a 1 kn horizontal force at the pole tip, the pole moment is 5 knm. that's easy enough, but what if i weld a right-angled fitting to the top and apply the load 1 meter down like so ...
_____a
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| |<< 1 kn
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------------ fixed in concrete
assume the right-angled fitting is rigid. please tell me how to calc the post bending moment.
2) and the second question i have (again using the 5 m high post fixed in conc) ... instead of a horizontal load (please disregard the 1 kn horz load shown in drawing), i apply a 1 kn vertical load at 1 meter out from the post at point 'a' in drawing. how is the post moment calculated? is it just 1 knm?
thanks!
bill

sabrepoint,
1)bending moment at base = 1.0kn x (5.0m - 1.0m) = 4.0knm
2)you are spot on, bm(base) = 1.0kn x 1.0m = 1.0knm
regards,
neilmo
i go for neilmo's solution.
it is quite easy to figure out for the cantilever. point a is experiencing a load of the 1 kn horizontal load plus a moment of 1knm in "clockwise direction". thus for the fixed end to reach equilibrium the original fixed end moment of (anticlockwise) 5knm has to offset the 1knm clockwise moment there.
if one sketch out the bm diagram then the short 1m
bbird,
nice explanation, better than mine.
neilmo
neilmo,
thanks for the complement. yours more concise. i wasn't going to bother at all but i thought the young engineer could do with a tip here.
nice thing about engineering is we can always agree on facts and physical laws.
bbird
they're both great ... thanks!
actually i had the correct answers as it turned out, but couldn't really explain. values used were hypothetical ... amongst other things i occasionally survey power lines. the first scenario roughly simulates the transverse wind applied to a wire on a suspension insulator; the second the dead load on the structure when considering overturning.
thanks again.

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