moment and shear diagram 2

[WOZnTR]

See attached layout. In the 3 middle columns portion, what part of the beams would have the maximum moment and shear? The secondary beams attached to the girders can be said to be point loads?

 

[BAretired]

For Beams 2B-10, maximum negative moment and shear would occur at Column C-1. Maximum positive moment would occur at supported beam locations whose reaction could be considered a concentrated load.

Beams 2B-6 can be analyzed directly because they are supported by columns whose axial deformation is negligible. The magnitude of the reactions of Beams 2B-5 and 2B-7 are dependent on the relative deflection of Beams 2B-3, 2B-9 and 2B-10 which complicates the analysis.

BA

 

[SLTA]

Before I give any answers - is this a homework question?

 

[WOZnTR]

slta.. I often see this in buildings and would just like to understand the load transfer thing.

BA, in two columns with beam in between, moment is zero at the two supports. But with 3 columns with beams in between them. The middle column no longer have zero moment. What is the term for it? Which of the columns is pinned and fixed (or other terms) or what do you call the center column with respect to the two at the sides (pinned, fixed, supports, what else)?

 

[WOZnTR]

BAretired, you stated:

For Beams 2B-10, maximum negative moment and shear would occur at Column C-1. Maximum positive moment would occur at supported beam locations whose reaction could be considered a concentrated load.

http://people.stfx.ca/eoguejio/235/4_SF&BM_Diagrams/Solutions/example4_1.jpg

According to the above beam example I found at net with concentrated load at midspan of beam, the shear force is maximum from support or C-1 to the concentrated load at beam midspan. But you stated it is only maximum at C-1. What is the case?

 

[BAretired]

BA, in two columns with beam in between, moment is zero at the two supports. But with 3 columns with beams in between them. The middle column no longer have zero moment. What is the term for it? Which of the columns is pinned and fixed (or other terms) or what do you call the center column with respect to the two at the sides (pinned, fixed, supports, what else)?

If a concrete beam is supported by two columns, the moment is not zero unless the connection is hinged. In concrete structures, beams are rigidly connected to columns so that there will be some moment at each end of a single span beam.

Beam 2B-10 is continuous across two spans. It will develop small moments at the exterior columns C-2 and C-4 and a large moment at Column C-1. None of the columns are pinned. None of the columns are fixed. Columns and beams are assumed to be connected rigidly so that they each have the same rotation at the point of connection. The structure is indeterminate and can be solved by many different methods including slope-deflection, moment distribution or by stiffness methods using a computer.

According to the above beam example I found at net with concentrated load at midspan of beam, the shear force is maximum from support or C-1 to the concentrated load at beam midspan. But you stated it is only maximum at C-1. What is the case?

If concentrated load at midspan is the only load acting, then shear is constant from load point to Column C-1. In the real structure, the weight of beam adds a uniform load across the entire length of beam, so the maximum shear occurs at the face of Column C-1. However, shear at the concentrated load will differ by only a small amount from that at C-1.

BA

 

[WOZnTR]

Thanks BA. You stated:

Beam 2B-10 is continuous across two spans. It will develop small moments at the exterior columns C-2 and C-4 and a large moment at Column C-1.

If the Beam 2B-10 is not analyzed as continuous beam but separate from left to center column C-1 and from center column C-1 to right. Then the exterior columns C-2 and C-4 and the center column C-1 would be analyzed as a beam on two ends with maximum moment at midspan. Doesn't the result of this analysis produce the same results as the continuous beam across two spans (like maybe adding the column C-1 results from the two independent analysis can produce the same bigger moment as the continuous)?

So it seems that moments are lesser if the center column C-1 would be separated into 2 independent columns then it is no longer continuous. Then moments are lesser? Do people design such to lessen the moments?

 

[BAretired]

No, I don't believe so.

BA

 

[WOZnTR]

Ok. Because it costs more to build double columns just so the beams won't be continuous. Moments may be lesser but more cost.

Anyway. Going back to the original beam 2B-10 being continuous across two spans. You wrote that "... shear at the concentrated load will differ by only a small amount from that at C-1.". How about shear at the exterior columns compared to the concentrated load and at C-1? Much lesser or not far as C-1 (noting you wrote "It will develop small moments at the exterior columns C-2 and C-4 and a large moment at Column C-1")?

 

[hokie66]

BA has been very patient with you, but there is no substitute for a good engineering education, and mentoring by a senior engineer once you are in the workforce. Good luck.

 

[rapt]

WOZnTR,

They are both being too nice.

You obviously have no understanding of statics, structural analysis or building design. The questions you are asking are covered in the first year of an engineering course. If you have completed a degree course and are still asking these questions, I would suggest that you change the direction of your career to something outside of engineering.

If you have not completed an Engineering Degree, you should not be asking questions on this site.