How Do You Resolve Forces in a Statically Determinate Truss?

[Sharifullah]

Homework Statement

Help required please. Method to be used is resolution of forces.

thanks,
sharifullah.

Homework Equations

The Attempt at a Solution

 

[Sharifullah]

help required

 

[Sourabh N]

where is the figure?

 

[z-component]

The figure was approved by a moderator. You must show an attempt at a solution before anyone can help.

 

[Sharifullah]

truss problem help

as it can be seen from the figure attached that the truss is symetrically loaded, there fore,i was able to solve for the reactions forces indicated by the arrows pointing upwards.
From the figure,
The method i used is that
Sovling for vertical forcves:

Reaction force1+Reaction force2=7w

" As the truss is symmetrically loaded therefore,"
Reactionforce1=Reactionforce2
"SO,"
Reactionforce1+ Reactionforce1=7w
2(Reactionforce1)=7w
Reactionforce1=3.5w
Similiarly,Reactionforce2=3.5 ("As Reaction1=Reaction2")

Once i found the reaction forces i started with the rigth side of the truss and solved for internal forces in the member using resolution of forces at joints method. Looking into the first joint two angles (30, 60 degree)are given, so i solved for the internal members but at the next joint the angle between the joint and the member is not given and there is no enough info given to derive an angle from. So i was wondering if some one could help with it.

 

[Sharifullah]

in the next figure where the reaction as been moved from the end to certain distance i cannot figure out the way to solve for reaction and as well as internal forces. I have tried much and then i jumped into asking others so i would kindly be aspecting a help this time. The figure was attached a should be available to all by now.

 

[Astronuc]

For the problem in the second figure, in addition to the sum of the reaction forces, one must also look at the sum of the moments.

Pick one of the reaction points as a pivot and look at the sum of the moments about that pivot.

 

[PhanthomJay]

Although not explictly clear from the sketch, the angles between the members are assumed to be either 30, 60, or 90 degrees. There are a few equilateral triangles in there.

 

[Sharifullah]

For the problem in the second figure, in addition to the sum of the reaction forces, one must also look at the sum of the moments.

Pick one of the reaction points as a pivot and look at the sum of the moments about that pivot.

THANKS,FOR THE HINT,ASRTRONUC BUT TO FIND THE MOMENT ABOUT A PIVOT THERE HAS TO BE A DISTANCE GIVEN FROM THE PIVOT TO THE POINT OF APPLICATION OF FORCES BUT THERE IS NO SUCH THINGS GIVEN THERE? A SOLVED THE FIGURE1 BY ASSUMING THE ANGLES FROM THE TRIANGLES AS THERE ARE EQUILATERAL triangles BUT DON'T FIND ANY USEFUL APPLICATION OF THE TRUSS HEIGTH GIVEN IN THE SKETCH. ANY IDEA OF HOW TO PROCEED WITH THE PROBLEM FIGURE 2? wHEN I SOLVED THE 2ND PROBLEM I GOT SOME ANSWERS FOR FEW MEMBERS AND THE REST ARE LOTS OF EQUATION WHICH I COULD NOT RELATE Anyhow TO EACH OTHER TO FIND THE FORCES REMAINING.

 

[Sharifullah]

I was able to find the internal forces in the member, thanks to the help provided by this active forum. Another question which i have is regarding the static dertermiancy of the system. As it has been asked in the first part of the figure 1(First figure) to find static determiancy of the struture.

I assumed all the jpints within the struture to be rigid and support to be roller supports( having only 1 reation which is vertical, as in the figure). The formula which is being used is
(m+r-f)-2j
m=number of member (19 members)
r=number of rigid joints including supports(11)
f=number of roller supports.(2)
j=number of joints including supports(13)

SO THAT STRUTURE IS STATICALLY INDETERMINANT WITH INDERMINANCY OF 2 DEGREE.am I GOING ON RIGTH TRACK)

THANKS FOR HELP!

 

[PhanthomJay]

Well I never could master that equation for determining determinancy, but if you were able to solve the reactions, and internal member forces by the method of joints, using the ordinary static equilibrium equations, without having to calculate deflections, then the truss is determinate. Also, there are no rigid supports (they are pinned or sliding), and all members are assumed connected by pinned joints.

 

[Sharifullah]

thanks phanthomjay, u mean if i could solve the whole unknowns within the structure using statics equation(Sum Fx=0 Fy=0 moment=0) then the structure is statically determinant otherwise indeterminant. In this case, as there is no indication whether the truss is pinned member joints can't it be rigid?The supports are sliding as the reaction is single so it is an indication or at least i could justify my assumption in the assignment.

 

[PhanthomJay]

thanks phanthomjay, u mean if i could solve the whole unknowns within the structure using statics equation(Sum Fx=0 Fy=0 moment=0) then the structure is statically determinant otherwise indeterminant. In this case, as there is no indication whether the truss is pinned member joints can't it be rigid?The supports are sliding as the reaction is single so it is an indication or at least i could justify my assumption in the assignment.

In reality, members are seldom truly connected by pins; often there are several bolts on each member at the joints, or gusset plates, or welds, that tend to make the connections more rigid than pinned. However, tests and computer modeling show that the error is generally very small when assuming the members are connected by pins. Whenever you see a truss problem, you can assume pinned connections at the menmber joints, if not otherwise stated. BTW, I found a formula for determining determinancy: if m= 2j-3, the truss is determinant. In your case, m=19 and j =11, so the truss is determinate. That doesn't work all the time, though, especially if there are zero force members.