Help! Thaddeus Lecture Question -- Shear Diagrams

Structural Layout Vignette and Multiple Choice

Help! Thaddeus Lecture Question -- Shear Diagrams

Postby mpc123 » Tue Jan 16, 2018 1:09 am

Question for someone who has been through the Thaddeus lectures!

I am trying to grasp shear and moment diagram concepts and am understanding things for the most part. My questions have to do with the conceptual diagrams he explains in workbook page 23.

In question 1 on the left (see below), he states greatest maximum shear would be in diagram B (continuous beam), not diagram A (2 simply supported beams). Can someone explain why the center supports/loads have more shear than the outer edges? This would also be the case in the lower continuous beam diagrams with 4-5 supports, but I'm not quite understanding why.
sheardiagrams1.jpg


Second question is for the diagrams on the right side of the page, showing concentrated/uniform loads on either simply supported or fixed supports. Again, same question about maximum shear. How can one assume without numbers that B (fixed ends, concentrated load) has greatest shear? How is this different than reading the maximum moment, which is you can look at as max deflection?
sheardiagrams2.jpg


Please let me know if anyone can help explain this in simple terms -- I may be overthinking this, but it would be good to know conceptually for the exam. Thank you!!
You do not have the required permissions to view the files attached to this post.
mpc123
 
Posts: 74
Joined: Sat Aug 15, 2015 11:39 pm

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby alexwilkosz24 » Tue Jan 16, 2018 9:21 am

look up Dilip Khatri's videos on You Tube. Very Helpful.
alexwilkosz24
 
Posts: 104
Joined: Wed Nov 13, 2013 1:05 pm

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby cabara01 » Wed Jan 17, 2018 12:21 am

How many times have you watched his videos? I suggest you watch them while taking notes and then watch (at minimum one more time) without taking notes. On the second round I was able to understand it better.

I’m pretty sure he went over it in the video.
cabara01
 
Posts: 73
Joined: Thu Apr 06, 2017 12:35 am

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby nickedemus » Wed Jan 17, 2018 9:25 am

mpc123 wrote:In question 1 on the left (see below), he states greatest maximum shear would be in diagram B (continuous beam), not diagram A (2 simply supported beams). Can someone explain why the center supports/loads have more shear than the outer edges? This would also be the case in the lower continuous beam diagrams with 4-5 supports, but I'm not quite understanding why.
sheardiagrams1.jpg


It's really very simple.

First, the shear diagram, which looks like two triangles touching each other between supports, shows the gradation of shear force through the beam. It uses the beam as a y-axis. Think, "Beam = 0." The highest point of each triangle is at the supports. That means the greatest shear is at the supports. Note the height of the vertical lines at these points.

Next, notice how two shear diagrams terminate at the center support: One that runs between the left support and the middle support, and one that runs between the middle support and the right support. Note the height of the combined shear diagram verticals at that point. Do you see how the height there is twice as high as the the height of the verticals at the ends?

That is saying that this center support bears the shear stress of the load between the left support and the middle support PLUS the shear stress of the load between the middle support and the right support.

Does that help?
nickedemus
 
Posts: 192
Joined: Wed Nov 19, 2014 4:43 am

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby mpc123 » Wed Jan 17, 2018 9:59 am

Thanks all for the responses. These are helpful. Someone on the Facebook group described looking at amount the shear stress as being similar to tributary area/load of the force/support. This was helpful as I kept getting caught up trying to understand numbers that could be associated. This is all assuming the beams are in equilibrium and forces are equal right? theoretically the center support could be a lot more than its adjacent which would affect the height of the shear stress line right?
mpc123
 
Posts: 74
Joined: Sat Aug 15, 2015 11:39 pm

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby nickedemus » Wed Jan 17, 2018 11:00 am

mpc123 wrote:theoretically the center support could be a lot more than its adjacent which would affect the height of the shear stress line right?


I don't understand your statement. A lot more what than its adjacent?
nickedemus
 
Posts: 192
Joined: Wed Nov 19, 2014 4:43 am

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby mpc123 » Wed Jan 17, 2018 10:08 pm

Sorry - the adjacent supports. For example, in the beam with five supports all the center shear lines are the same height assuming they have the same force and is in equilibrium. If the center one happens to have more force upon it, the shear stress would be more than the one to its right or left -- but we that would only be known if numbers were tied to this problem. Is that correct?
mpc123
 
Posts: 74
Joined: Sat Aug 15, 2015 11:39 pm

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby nickedemus » Thu Jan 18, 2018 3:16 am

I guess you are referring to the reaction of the support. But think about a uniform load: by definition, the force is distributed equally across the entire length of the beam. If there is no point load (downward arrow), and the supports are equidistant, you can assume that all of the supports have the same reaction. You really don't need numbers, just concepts.
nickedemus
 
Posts: 192
Joined: Wed Nov 19, 2014 4:43 am

Re: Help! Thaddeus Lecture Question -- Shear Diagrams

Postby mpc123 » Sun Jan 28, 2018 5:18 pm

makes sense. thank you nickedemus!
mpc123
 
Posts: 74
Joined: Sat Aug 15, 2015 11:39 pm


Return to SS - STRUCTURAL SYSTEMS

Who is online

Users browsing this forum: No registered users and 44 guests

cron