Answer & Solution
Answer: Option
C
Solution:
Let's break down this question about designing steel structures in a simple way.
First, what are racking forces?
Imagine you have a simple rectangular frame, like a picture frame or a doorframe. If you push hard on one of its top corners, the frame will try to get squashed and turn into a slanted shape, like a diamond or parallelogram. This sideways pushing force that tries to distort the shape is what we call a racking force.
In real buildings, these forces usually come from things like strong wind pushing on a wall or an earthquake shaking the structure sideways.
Now let's look at which parts of a steel structure's design consider these forces:
i) Lateral braces
Think of "braces" as diagonal bars or members that are specifically added to a rectangular frame. These diagonal pieces turn the squares or rectangles into stable triangles. Triangles are very strong and cannot easily be pushed sideways and distorted (racked).
So, lateral braces are specifically designed to resist these racking forces. Their main job is to provide stability against horizontal forces and prevent the entire structure from collapsing or distorting sideways.
ii) Chord members
Chord members are the main horizontal (and sometimes vertical) parts of a frame or truss, like the top and bottom beams. While lateral braces *primarily prevent* the racking, the entire structure is a connected system. When racking forces act on a building, even if braces are present, the forces are still distributed throughout the entire frame.
This means the chord members will also experience additional forces (like extra pushing, pulling, or bending) because of the racking effect on the overall structure. For example, the ends of the chords might experience more stress where they connect to columns.
Therefore, when engineers design these main chord members, they must also consider these additional forces caused by racking to make sure the chords are strong enough and don't fail under all possible load conditions.
In summary:
- Lateral braces are directly designed to resist racking forces and maintain the frame's shape.
- Chord members are also affected by racking forces, and their design must account for the stresses that racking induces in them as part of the overall structural response.
Since both elements are important and their design must consider the effects of racking forces, the correct answer is Option C: Both (i) and (ii).
