# Moment in beam (caused by influence line)

• fonseh
In summary: The author asked for find the influence line for the moment at C . If the unit load is not considered in AC or CB , then where should the unit load be considered for the moment?
fonseh

## Homework Statement

http://cereference.com/book/structural-analysis/influence-lines-beams
In the example , i don't understand the moment in the part of beyond AC . I don't understand why we don't have to consider the momenty due to unit virtual load in the part beyond AC , just like what we did for section AC

## The Attempt at a Solution

Why shouldn't the moment equation beyond AC look same as the region within AC ? , which is Mc = -ax/L + x [/B]

#### Attachments

• 538.PNG
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What the author did is evaluate the Moment when the virtual load is on segment CB. To do so, you can analyse segment AC or CB, and note in segment AC, it won't have the unit load.

If you perform the same analysis on segment BC , you will have to consider the unit load, take into consideration the different member lengths for moment calculations - but ultimately, you will get the same answer.

sakonpure6 said:
What the author did is evaluate the Moment when the virtual load is on segment CB. To do so, you can analyse segment AC or CB, and note in segment AC, it won't have the unit load.
Do you mean the author is evaluating the moment when the virtual load is in segment CB , so in the segment AC , it wouldnt have to include unit load ?

It seems contrary to what the author did , in photo 538 i uploaded earlier , I found that when it's within segment AC , the author still consider the unit load , while in CB ( segment beyond AC ) , the unity load is not considered ...

sakonpure6 said:
What the author did is evaluate the Moment when the virtual load is on segment CB. To do so, you can analyse segment AC or CB, and note in segment AC, it won't have the unit load.

If you perform the same analysis on segment BC , you will have to consider the unit load, take into consideration the different member lengths for moment calculations - but ultimately, you will get the same answer.
How do we know we need to apply the unit load within AC or CB ?
btw , the author asked for find the influence line for the moment at C ...

## 1. What is a moment in a beam?

A moment in a beam refers to the force that causes the beam to rotate around a fixed point, also known as the beam's support. It is measured in units of force multiplied by distance, such as newton-meters (Nm) or pound-feet (lb-ft).

## 2. How is the moment in a beam calculated?

The moment in a beam is calculated by multiplying the force acting on the beam by the distance from the support to the point where the force is applied. This calculation is based on the principle of moments, which states that the sum of clockwise moments must be equal to the sum of counterclockwise moments in a system in equilibrium.

## 3. What is an influence line for a moment in a beam?

An influence line for a moment in a beam is a graphical representation of the relationship between the position of a single moving load and the resulting moment at any point along the beam. It is a useful tool for determining the maximum moment that a beam may experience due to different loading scenarios.

## 4. How is an influence line for a moment in a beam constructed?

To construct an influence line for a moment in a beam, a unit load is placed at different points along the beam and the resulting moments are calculated. These moments are then plotted on a graph, with the distance along the beam on the horizontal axis and the moment on the vertical axis. The resulting curve is the influence line for that particular moment.

## 5. Why are influence lines important in structural analysis?

Influence lines are important in structural analysis because they provide a quick and visual method for determining the maximum moment that a beam may experience. They are also useful for optimizing beam design and determining the locations of critical loads. Influence lines are especially helpful in the design of bridges and other structures with moving loads.

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