Rafter Loading

[Guest090822]

Help! A young coworker is studying for the PE exam and asked me for help with this problem. I did what I thought was correct in a few minutes only to find out my answer was not correct. Just curious what everyone else gets for an answer. See attachment.

 

[oldestguy]

Been a long time since college, but I always thought gravity was vertical direction.

 

[Guest090822]

I don’t like the problem at all. If it’s a dead load then the arrows on the sloped load should be pointed down. They also don’t give the support type (pinned or roller).

 

[WARose]

I came out with none of the above. (I.e. 420 lbs.)

 

[oldestguy]

I get 432

 

[NorthCivil]

Agree with OG and Rick. Also, why have 5 significant figures on a problem that should only have 3? maximum (arguably 2 or even 1 in the real world). They taught us about appropriate use of sig figs first semester, first year of engineering school.

all logic aside I get D. 400 lb. (on my second try)

 

[IFRs]

It's not 413 pounds?

 

[civeng80]

I get 428 pounds.

 

[BAretired]

LL = 16*30/2 = 240
DL = 20*16*13/12*1/2 = 173 (assuming 20 plf per foot of sloping roof)

DL + LL = 413

BA

 

[Guest090822]

The answer is 413 lbs according to the book the problem came from.

The solution exactly as BAretired shows above.

I arrived at 400, 413, and 428 taking different approaches.

 

[msquared48]

I got 409, 427 and 454



Mike McCann, PE, SE (WA)

 

[sticksandtriangles]

voting for 413

S&T

 

[IFRs]

I thought you'd have to ignore the designations WLL and WD and just go with the free body diagram, exactly as BARetired said, and I computed. I think that's the lesson they are trying to teach. Screw gravity, or the orientation the drawing happens to be in.

 

[hokie66]

The horizontal component of the sloping load has to go somewhere. Taking moments about the support, I get 428, so agree with civeng80.

 

[oldestguy]

This OG got side tracked with those arrows. 413 is mine now. Darn stuff like this kept me awake after bed time and I had to getup and fix it. Why is it your best thinking comes after bed time? Anyhow isn't student help frowned upon here and for good reason as proved here? Now back to bed. Another lesson learned. No Eng Tips after dinner, like coffee.

 

[hokie66]

So maybe this is a trick question. The arrows perpendicular to the rafter would lead one to think the force is applied in that direction, but dead load is gravitational. If that load is a wind load as indicated by the arrows, I stick with 428.

 

[civeng80]

I thought the loading perpendicular to the rafter was a Wind Load, but after reviewing the diagram it is Dead Load. It is a confusing question because Dead load acts vertically down all the time so to show it perpendicular to the rafter is plain wrong.
I worked out the reaction by taking moments about the right support.

 

[IFRs]

So, the question is: did they intend you to rotate the FBD so the DL component is vertical or simply compute the support vector component as shown. Is this a FBD exercise or a trick question? Are we reading DL to be Dead Load or could it be Design Load, Dumb Load, etc? My vote originally was to ignore the LL and DL designations since their definitions were not included with the problem. If LL and DL were not written there would be no question - the answer is 413. I think the real-life answer is: what is meant by LL and DL since it makes a difference to the answer. And petition the test giver to fix this idiot problem so there is only one correct answer.

 

[SlideRuleEra]

Agree with BAretired, 413 lb:

I don’t like the problem at all. If it’s a dead load then the arrows on the sloped load should be pointed down. They also don’t give the support type (pinned or roller).

IMHO, this problem is presented in an unusual way but is a good one for the PE exam:

A student is given initial conditions need to solve a problem (so they can learn).
An engineer does not necessarily have the initial conditions, or the info may not be obvious or straightforward.

In "Design of Wood Structures" 3rd Ed., Donald Breyers covers two methods for solving rafter loads:
"Sloping Beam Method" and "Horizontal Plane Method". See the attachment (Note: I consider this use of the book's example to be "fair use" for copyright purposes).

Live Load is presented in "Horizontal Plane Method" format.
Dead Load is presented in "Sloping Beam Method" format.
Type of support - An engineer qualified to take the PE exam should be able to figure that out.

http://www.SlideRuleEra.net

 

[gte447f]

It's 413 for a dead load.
Bonus question. What would the vertical reaction be if it really was acting perpendicular to the member (like a wind load)?