one for the civil engineers

[Dave Stones]

no need to worry if no one has the foggiest idea what i am on about, this has been posted in a couple of areas ;)

anyway, see the attached diagram:

it is a statically determinate pin-jointed truss, i have to "show" this. i know the simple equation relating joints, members and reactions. i am just checking, that the force acting downwards at E does not count as an external reaction, and that the only reactions are those that are acting horizontally and up-vertically at A and B?

I have been doing too much work today and have developed a mental block...

oh and no jokes about pin joints, members and trusses please ;) the lecturer goes on abuot losing erection stability enough without random strangers doing it as well...:erm:

[kronas]

the lecturer goes on abuot losing erection stability enough without random strangers doing it as well...:erm:
[/font]

ooer wheres ramrod ;) :D :p:

[SOSAGES]

anyone done a stiffy joke yet?

[Dave Stones]

all to be expected ;) you should have sat through a year's worth of lectures with this guy. imagine a really unfunny uni lecturer, making jokes about erection problems.....

[Ramrod]

ooer wheres ramrod ;) :D :p:he's staying out of this one :erm: :D

[Maggy]

he's staying out of this one :erm: :D

Why?Are you an uncivil engineer then? ;)

[martinell]

so is a reaction the point at where a force is acting on the structure?

[Dave Stones]

so is a reaction the point at where a force is acting on the structure?
naw, a reaction is the "reaction" that the structure gives to the force. simple example:

you sit down on a chair and exert a force of 900 newtons on it. the chair then has an upward reaction to your downward force of 900 newtons. hence you don't fall through the chair :)

reactions can occur in seemingly random places somtimes, it doesn't have to be anywhere near where the force is acting.

[martinell]

then doesn't every joint in the structure have a reaction?
d carries the weight of e while pulling on c. b is holding c in position through the brace and a is countering the over balance of e and all the weight is borne by poor little b at the bottom
..and no i havent got a clue really but e has not got an external reaction

[Dave Stones]

then doesn't every joint in the structure have a reaction?
d carries the weight of e while pulling on c. b is holding c in position through the brace and a is countering the over balance of e and all the weight is borne by poor little b at the bottom
..and no i havent got a clue really but e has not got an external reaction yes, every joing in the structure does have a reaction. however, those are *internal* reactions in the force-system. at the moment with this diagram, i am only concentrating on the *external* forces... its evil i swear :)

oh, and pin-joints (the circles) are completely frictionless too :)

i think i worked it out now anyway, i found a lost temple of notes and it made slightly more sense :)

[BBKing]

Gosh - it's seven years since I graduated, but this brings it all back.

Structures was never my strong point though - fluid flow equations I'm your man.

Statically determinate is when the number of equations you could write down was equal or more than the number of unknowns, so you can solve the equations to get all the unknowns with no bother. Statically indeterminate structures, on the other hand, are a bugger.

[BBKing]

Either I've gone completely ga-ga on red wine, or this is actually quite easy - there's only a single force on the structure and it's vertically downwards, hence there's no horizontal reaction at either support - Ha and Hb both = 0 (imagine what would happen if A and B were rollers instead of fixed - the structure wouldn't move under that single 20kN force).

Hence we only have two unknowns Va and Vb (vertical reaction at A and B).

Obviously Va+Vb = 20 otherwise the structure would accelerate into the ground or into orbit.

From the point of view of A, in order for the structure not to spin round and round, the downward moment at E has to be balanced by an upward moment at B, caused by Vb

Thus 2m x 20kN = 1m x Vb

Vb thus = 40kN upwards

Since Va+Vb=20, Va = -20 or 20kN downwards.

Thus the support at A is being pulled upwards, B is being pushed down twice as hard.

[Dave Stones]

Either I've gone completely ga-ga on red wine, or this is actually quite easy - there's only a single force on the structure and it's vertically downwards, hence there's no horizontal reaction at either support - Ha and Hb both = 0 (imagine what would happen if A and B were rollers instead of fixed - the structure wouldn't move under that single 20kN force).

Hence we only have two unknowns Va and Vb (vertical reaction at A and B).

Obviously Va+Vb = 20 otherwise the structure would accelerate into the ground or into orbit.

From the point of view of A, in order for the structure not to spin round and round, the downward moment at E has to be balanced by an upward moment at B, caused by Vb

Thus 2m x 20kN = 1m x Vb

Vb thus = 40kN upwards

Since Va+Vb=20, Va = -20 or 20kN downwards.

Thus the support at A is being pulled upwards, B is being pushed down twice as hard.
woohoo that's what i got :D believe it or not...

oh, and i am resitting fluid mechanics as well, those questions to come soon ;) fourier's law is a complete bast*rd....

[BBKing]

They've probably stopped you doing it now, but I used to programme my calculator with fluid flow equations - made things a lot easier.

Wonder if they'll take my degree away? :erm:

[Mr_love_monkey]

Wonder if they'll take my degree away? :erm:

You shouldn't say things like that... the degree police could be watching... they're real, I tell you! :)

[basa]

IMO if all the joints are frictionless there must be a tension force in brace B-C. This must in turn introduce a force in link C-D.

Imagine the structure with no brace B-C - it would collapse ! But if it were on frictionless wheels (at A & B) it wouldn't move - so no net forces there.

How this affects the forces in the structure I dunno (it is at least 30 years since I completed my HND in Structural Engineering !!)