Construction Cable Statics Problem Video Lecture - Civil Engineering (CE)

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your journey to pass the PD all right
welcome to civil engineering Academy
today we're selling prop from
construction and says a hoisting device
use uses the cable system shown at the
load of P equals five tons so this load
P is five tons tons the load and pounds
at point B is most nearly what
so we're asked to solve this unknown
load here okay so this is a statics
problem and the civil engineering
reference manual does go over statics
problems so that's a good resource for
you but all I'm going to do is start at
Point C and work my way back to point B
I start at Point C because I know we
know what load P is and I know that if I
do the sum of the forces in the X in the
Y direction I'll get two equations with
two unknowns if I started at point B I I
don't know anything so and be able to
solve there so that's what I'm going to
do I'm also with the information given
I'm noticing that they're giving slopes
so I'm going to solve for the hypotenuse
I punch of each of these so one squared
plus one squared take the square root of
that that gives me what that's one point
four one four three squared plus one
squared square root of that does that
give me they give me three point one six
and one squared plus 2 squared squared
that gives me two point two four that's
going to help me when I take some of the
forces in the X and y direction I'm
going to use the information they give
me instead of solving for the angle here
and using sines and cosines to get some
of the forces so if I were to draw out
Point C let's just do that right here we
got here CD C D
and then B goes off something like that
so and then we got this force P which
equals five tons okay so I just do this
diagram here for us so let's go ahead
and take some of the forces in the
x-direction and that's going to give me
force of CD times one it's the X
component over the hypotenuse
- point 2 4 or - force of BC times three
over three point one six and that equals
0 so 3 is the X component times two
point one over three point one six okay
so some of the forces in the Y direction
and you're going to get minus five push
the load which is given plus force of BC
times one over three point one six plus
four so CD times two over two point two
four equals 0 so now you have two
equations we have two unknowns force of
CD and force of BC I want to find out
what force of BC is so I'm going to
solve for this first equation what force
of CD is so force of CD is going to
equal bring force of BC to their side so
that's four so BC times three over one
three point one six and then you're
going to be multiplying all that by the
2.24 bring that this to the other side
okay so that gives you force of CD and I
get two
one two seven now I need to do is plug
this in here and we'll solve for BC so
that's minus five plus four so BC times
one over three point one six plus two
point one two seven four sub BC times
0.8 ninety-four I just solve for that
okay that good set that equal to zero
and what do you get you get a force of
BC equal to BC equal to two point two
five tonnes okay so now we know force BC
we can work our way back and I'll draw
this over here so here's point B or C a
shoots up like this and you've got this
guy what we're solving for so now we're
going to do the same thing over here so
we'll just take some of the forces in
the x-direction which is three over
three point one six again that's the X
component divided by the hypotenuse all
times by the two point two five because
we know what force of BC is we just
solve for that so that's times 2.25
minus one over one point 4 1 4 so 1 over
the botanist x force of a B and solve
for force of a B and you get three point
zero two tons just take the sum of
forces in the Y direction and we'll get
our your answer I'll just call this
negative B this end this unknown so
negative B plus three point out of two
times one over one point four one four
- two point two five times one over
three point one six that gives me one
point four two tonnes so converting this
there's two thousand pounds in a ton and
you're going to get twenty eight forty
pounds whoops
can't spell here pounds that is answer C
so this was a pretty good statics
problem um it can get confusing I guess
not knowing where to start
but once you get going you can wind
yourself back to get the answer so I
hope this helped you out head to civil
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