×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Vertical Loads
2

Vertical Loads

Vertical Loads

(OP)
On page 59 of the april edition of modern steel construction magazine, does anyone know why the flat roof snow load is multiplied by cos^2 instead of just cosine to get the sloped roof equivalent, like the dead load?

RE: Vertical Loads

2
Fatalxception,

The span of a parallel chod sloped joist is the length along the slope.    The gravity loads have two components, one perpendicular to the joist that produces the moment and shears, and the other parallel to the slope that produces axial force in the top chord.

The dead load is given over the slope length of the joist, and is multiplied by the cosine of the slope to obtain the load perpendicular to the joist.

The live and snow loads are given over the horizontal projection of the slope, and are multiplied by cos^2; the first cosine to spread the load over the sloped span, and the second cosine to obtain the load perpendicular to the joist.

AEF

RE: Vertical Loads

(OP)
I guess I'm missing something, but if I have a snow load I have calculated with a dead load, I would multiply each one by the cosine one time to find the perpendicular component on a sloped joist.

RE: Vertical Loads

(OP)
Wait, I think I see what you mean now.It's like the snow height is getting shorter as the joist span increases with increasing slope. It's more like they give you a total snow load and it gets spread over a longer length like you said. thanks.......

RE: Vertical Loads

Fatal, I think it has nothing to do with the snow load being less for a given higher slope.

You start with q psf of snow adequate for the situation on one notional horizontal plane.

The first multiplication by cos slope gets the horizontal load spreaded along the length.

But then, if you are to calculate the forces with the span along the slopes, the vertical forces are not orthogonal to your inclined beam. By vectorial decomposition of the force, the component parallel to the beam does not cause any bending, and only the perpendicular to the beam, got by multiplying again per cos slope does.

So it is that you multipliy by cos^2 slope.

The effect you refer to, the snow loads being lesser for roofs of higher slope is cared of at the establishment of the initial load as if projected on one horizontal surface.

RE: Vertical Loads

Snow is applied in force per unit length of horizontal projection... plf.  But you may want pounds per length "along the span".  Two different "feets".

RE: Vertical Loads

(OP)
I sent this email before I posted on this forum, here is the reply:

 
We apologize for the delay in responding to you, as we had to contact the author.  The author of that article left me a voice message regarding your inquiry.  He mentioned that there are errors in both Figure 19 and the SL calculation.
 
In Figure 19, it appears that "LL x cos^3" should really be "LL x cos^2".  In the text, the SL calculation uses a cos^2 term, which according to the author, should be just cos.
 
I have forwarded this message to Modern Steel Construction, as they may want to confirm these typos with "(I deleted name here)" and possibly print an editorial correction sometime in the future.
 
Regards,
"(I deleted name for anonymity here also)", Ph.D.
AISC Solutions Center

American Institute of Steel Construction, Inc.
One East Wacker Drive, Suite 3100
Chicago, IL  60601-2001

This document has been prepared in accordance with recognized engineering principles and information made available to AISC at the time of its preparation. While it is believed to be accurate, it has not been prepared for conventional use as an engineering or construction document and should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability and applicability by a licensed engineer, architect or other professional. AISC disclaims any liability arising from the unauthorized use of the information contained in this document.

 
 

-----Original Message-----


On page 59 of the april edition of modern steel construction, why is the snow load multiplied by cos^2 instead of just cosine like the dead load?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close