78 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 ...
Harper - Reinstate Monica's user avatar
44 votes
Accepted

Why are engine blocks so robust apart from containing high pressure?

If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few ...
NMech's user avatar
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32 votes
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Why is this bridge thickest above the support pillars instead of the mid-span where the bending stress is highest?

If I model this as a simply supported beam having load at mid span [...] I suspect that this is where your analysis went awry. First off, you should always model bridges with distributed loads, not a ...
Wasabi's user avatar
  • 13.1k
26 votes
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Why are truss bridges the way they are?

That looks like a Pratt truss. These trusses have diagonals which go from the outer-top nodes to the inner-bottom nodes (i.e. they connect to the top chord on the node furthest from the center of the ...
Wasabi's user avatar
  • 13.1k
26 votes

Why are some public benches made with arm rests that waste so much space?

I am not sure about this particular design. But in public places, the benches are designed to provide temporary comfort but discourage using them as a vagrant or homeless hang out, hence designed ...
kamran's user avatar
  • 22.2k
24 votes
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Why would you launch a large ship by dropping it sideways?

A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either ...
Chris Johns's user avatar
  • 15.2k
22 votes
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Why would a bearing plate that further concentrates load onto small surface be used?

Because bridges and other structures are not static objects. They must be allowed to flex under varying loads and also accommodate changes in length from thermal expansion. The hinge pin allows ...
Dave Tweed's user avatar
  • 6,874
22 votes
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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 ...
Wasabi's user avatar
  • 13.1k
19 votes

Is it structurally sound to cut an I beam to a T on one end?

While not an ideal situation, it is common enough that this type of cut/reduction of the beam as it comes to its support actually has a name. This is more often referred to as a coped or dapped steel ...
Forward Ed's user avatar
  • 1,201
18 votes
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What is the purpose of building foundations?

Down, Not Out Building foundations don't always have to spread the load out to a larger area. Sometimes the load only needs to be transferred to a stronger (harder) layer. This layer may be deep in ...
hazzey's user avatar
  • 10.7k
16 votes

Is it structurally sound to cut an I beam to a T on one end?

This is a textbook example of what not to do. We don't get into stress concentration at the cut off of the corner of the beam, or the fact that the two very different stiffnesses of the beams are a ...
kamran's user avatar
  • 22.2k
15 votes
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Why don't we just build a giant wind turbine?

Not everything scales linearly. In particular, the cross-sectional area of supports required scales faster than height of a structure, all else held constant. This explains why ants have tiny thin ...
Olin Lathrop's user avatar
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15 votes
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Why does column buckling occur when the load is parallel to the column?

Euler buckling occurs because the world isn't perfect. So that theory assumes that there is an initial infinitesimal deviation along the column (assuming the column is in fact not perfectly vertical*)....
Wasabi's user avatar
  • 13.1k
15 votes

Why are engine blocks so robust apart from containing high pressure?

You need to consider that the complete engine block has to withstand the reciprocating forces generated by each of the pistons and con rods moving as well as the rotational forces from the rotating ...
Solar Mike's user avatar
  • 15.6k
15 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 ...
Ilmari Karonen's user avatar
12 votes
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How can large bridges be made resistant to earthquakes?

You can never make something earthquake proof, but there are many things that can be done to resist earthquakes. There are long-span bridges built in earthquake zones. For example, the Akashi Kaikyo ...
atom44's user avatar
  • 2,631
12 votes
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Why is the point of failure load lower than the peak load?

That is due to necking. When a bar is under tension, two opposing mechanisms take place: The bar loses cross-sectional area since it attempts to retain its volume while being stretched (see Poisson'...
Wasabi's user avatar
  • 13.1k
12 votes
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Is it useful to stuff my glasses with newspaper to protect them?

In itself putting paper inside a glass will make very little difference. Glass is a brittle material and tends to fail by shock and point loading. Its static tensile strength is actually pretty good. ...
Chris Johns's user avatar
  • 15.2k
12 votes

What is the maximum angle of a shelf angled brace?

The angle is arbitrary. One can decide which angle is more practical. However, it comes at a cost: the more the angle from the vertical the more pulling and pushing force on the anchors or escrows. ...
kamran's user avatar
  • 22.2k
12 votes

Why are engine blocks so robust apart from containing high pressure?

The vibration, loading, and fatigue aspects have already been addressed, but a wide range of operating temperatures is another factor. A typical consumer engine can be deployed in anything from say -...
brichins's user avatar
  • 221
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 ...
jayben's user avatar
  • 451
11 votes

Why are concrete bridges more prevalent than steel bridges in the United States?

I live in Brazil, so the relevance of my answer to your US experience is somewhat questionable. This is exacerbated by the fact that Brazil is almost 100% concrete, with very few steel or wooden ...
Wasabi's user avatar
  • 13.1k
11 votes
Accepted

How do the columns in the 'magic mountains' sculpture stay up?

This marketing video shows moments of their construction. At 0:40 you can see them all upright. They aren't painted, but you can tell it's the same installation. At 1:37 and 1:46 you can see guys ...
Wasabi's user avatar
  • 13.1k
11 votes
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What is the advantage of statically indeterminate structures?

For example, take a look at the following static systems. Assume they have the same length and the same (constant) cross-section. Thus an equal allowed bending moment $M_u$. The first system is ...
Andrew's user avatar
  • 1,166
11 votes

Why are some public benches made with arm rests that waste so much space?

Benches are not usually engineered as such. They are being made by furniture designers, (landscape/interior) architects etc. Now generally these professions put a lot of work into making their items ...
joojaa's user avatar
  • 3,597
11 votes
Accepted

What happens from B to C in this stress strain diagram of mild steel?

One point of note is that the yield region is not as cleanly defined as BCD is in the image (although most books have it that way). In reality the yield region looks like The following image is one ...
NMech's user avatar
  • 24.3k
10 votes
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What is the difference between the Polar Moment of Inertia, $ I_P $ and the torsional constant, $ J_T $ of a cross section?

The torsion constant $J_T$ relates the angle of twist to applied torque via the equation: $$ \phi = \frac{TL}{J_T G} $$ where $T$ is the applied torque, $L$ is the length of the member, $G$ is ...
atom44's user avatar
  • 2,631
10 votes
Accepted

Why does a continuous beam have less deflection than a pair of simply supported beams?

For a simple visual demonstration, take one of the spans in your example. If it is fully hinged, then each span can be represented as a simply-supported beam. If the beam is continuous, then each ...
Wasabi's user avatar
  • 13.1k
10 votes

Why would a bearing plate that further concentrates load onto small surface be used?

The reason is pretty simple. Steel is significantly stronger than concrete. Nowadays we have high-performance concretes with $f_c > 100~\text{MPa}$ (and ultra-high-performance, which is ...
Wasabi's user avatar
  • 13.1k
10 votes
Accepted

How does the strength of a square tube depend on the side size?

Flexural capacity is based on the stress at the extreme fiber (the point farthest vertically from the neutral axis). $$\sigma = \frac{My}{I}$$ Or, rearranging $$M = \sigma \frac{I}{y} = \sigma S$$ ...
CableStay's user avatar
  • 2,760

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