72 votes

Why are two wires used in railway overhead equipment?

Overhead line worker here. It's about speed. To answer your question, the purpose of the catenary wire is to get the contact wire to lay flat, so it isn't catenary itself. In your second photo, there ...
user avatar
21 votes
Accepted

Why do wooden gate designs recommend a brace under compression instead of tension?

The reason is simple: the load applied on the gate is pretty trivial, and the brace (be it a 2x4 or a wire) is mostly added for rigidity. Indeed, you could put that 2x4 in tension and it would work ...
user avatar
  • 12.7k
13 votes

Why do wooden gate designs recommend a brace under compression instead of tension?

The frame and the brace in the first gate shown in your question seem to be joined together with simple butt joints, probably with nails to hold them together. Such joints are strong under compression ...
user avatar
12 votes

Why are two wires used in railway overhead equipment?

It isn't to do with current-carrying capacity; it is to maintain good contact between the current-carrying wire and the pantograph pickup on the locomotive, as it allows the contact wire to be ...
user avatar
  • 341
7 votes

Why do wooden gate designs recommend a brace under compression instead of tension?

Wood expands when exposed to moist conditions. For the first case, the elongation of the diagonal will be offset by the compressive stress, resulting in a smaller geometry change. For the second case, ...
user avatar
  • 7,293
6 votes

Why are two wires used in railway overhead equipment?

Why two wires: A wire hanged at two points sags (it forms a "catenary curve"). The pantograph can only handle a certain sag before losing contact and arcing. The acceptable sag depends on ...
user avatar
  • 231
3 votes

Why do wooden gate designs recommend a brace under compression instead of tension?

Because the wood brace works in compression and the wire brace works in tension. Just make a model and see how that wire will behave in compression. Also you would need different joints for the wood ...
user avatar
  • 13.6k
3 votes
Accepted

Transforming the Area of a Hollow Square Section filled with concrete into the equivalent area of just steel

You can simply calculate "equivalent" cross-sectional stiffness values. Note that all non-zero terms of the stiffness matrix depend on one of these values: $EA$ or $EI$. These are the cross-...
user avatar
  • 12.7k
2 votes

Work done by a force in a deformable body

Work is a strange but useful mathematical concept. It is very handy because it is conservative in inertial frames. It also has an intuitive meaning most of the time, but not always. This is what gets ...
user avatar
  • 3,381
2 votes

Can/should you support a sagging ceiling from above?

You don't seem to have an urgent problem, as the ceiling may have stayed there for a long period of time. However, according to the modern building code for attic space without storage, the 50 mm ...
user avatar
  • 7,293
2 votes
Accepted

Natural Frequency of a Shear Frame

The basic unit of the natural frequency is $Hz$, $cycle/s$, or just $1/s$. Let's check the results by using "$m$" and "$mm$" as the base unit - 1. Use $m$ as the base unit: $\...
user avatar
  • 7,293
2 votes
Accepted

Justification of static equilibrium for a cut structure/frame

For your first question regarding whether the shear in BC equals the shear in DE: yes, they are the same. An easy way of seeing this is that some fraction of $P$ is going to be absorbed by BC as shear,...
user avatar
  • 12.7k
1 vote

Strain energy, Conservative force and Potential energy

What will be the conservative force $F$ in this case, which I will be using in this $\int_1 ^2F.ds = - (U_2 - U_1) $ equation to find the potential energy? I know you know that the force is $F=-kx$ (...
user avatar
1 vote

Why do wooden gate designs recommend a brace under compression instead of tension?

I think @r13 nailed it. FWIW... My preference for the cable version is based on the following, FWIW: It adds less weight to the gate. I like the "mechanical assist" look. And most ...
user avatar
1 vote
Accepted

Symmetry to determine the support reactions of a statically indeterminate frame

The members and loads of the frame are symmetric about joint "B", but the supports are not, which is the source that causes non-conforming deflections when loaded. The sketches below depict ...
user avatar
  • 7,293
1 vote

Can/should you support a sagging ceiling from above?

You can use 2x10s at 48" O.C. from above and use available fasteners by Simpson or other brands to hang the 2x4d from down below. it is the common way.
user avatar
  • 20.9k
1 vote

Work done by a force in a deformable body

Individual work done on the body by a single force equals $F*s$ as shown on the sketch to the right. The work done by multiple forces equals the vector sum of the forces ($R$) and the absolute/net ...
user avatar
  • 7,293
1 vote

Work done by a force in a deformable body

The work of a force is by definition vector multiplication of the force by displacement. $$W =F*x$$ Let's use two examples to clarify. In a pendulum, the work done on the part of the descending of ...
user avatar
  • 20.9k
1 vote
Accepted

Direction confusion for shear and moment when sectioning a beam

This is the first step to simplify the analytical model by applying the equivalent load concept as depicted below: Another way to examine the direction of forces at where the cantilever is cut off.
user avatar
  • 7,293

Only top scored, non community-wiki answers of a minimum length are eligible