PDD NCARB Handbook Sample Question #1 / Structures

Follow

Christine Hernandez

August 08, 2021 20:12

I am having trouble understanding a concept related to a sample question in the NCARB ARE 5.0 Handbook for PDD.

 

The sample question below asks: if the design snow load is 60 psf, what is the total snow load per linear foot on the ridge beam?

 

When I initially tried solving this, I used the total span and calculated: 20 FT x 60 LB/SF = 1200 LB/FT

 

However, the NCARB Handbook says to use half the length (the length of one rafter) and calculate: 10 FT x 60 LB/SF = 600 LB/FT

 

Why use the length of one rafter instead of the total length?

Looking for a different explanation to help me understand.

 

Thank you.

 

0

• 

  mermann August 09, 2021 03:26

  Half the load (5 feet worth) of each rafter is taken up by the exterior wall and the other half by the ridge beam…

  5

  Comment actions Permalink
• 

  Bankole Folorunso August 09, 2021 04:21 (Edited August 10, 2021 04:24)

  Hi Christine,

  One thing to get out of the way first is that loads on structures are two types.

  1. Gravity Loads; effected by virtue of gravity, i.e, they act vertically downwards on structure's live and dead loads.

  2. Lateral Loads; effected by forces acting perpendicular to the structures vertical planes, sometimes with tendency to cause uplift, i.e on walls, windows and roofs. Hurricanes, tornadoes and earthquakes are examples.

  The question is asking about gravity load, thus we read dimension linearly, not parallel to the rafter (10ft not 14ft). So,10' between supports, thus 5ft on either sides (5+5=10) each support bears the rafter load midway (tributary load). The question states that the load is 60psf, so we have 10x1x60=600. 

  You may want to read more in Bldg Constr. Madan Mehta, check out chapter 3.

  Hope this helps,

  Good luck.  

  0

  Comment actions Permalink
• 

  Xijue Wang September 28, 2021 13:54

  Hi Christine, When I was studying related topic, I found this youtube video I think it's really helpful. https://youtu.be/guik2K0inzw

  0

  Comment actions Permalink
• 

  Li Xing October 06, 2021 03:36 (Edited October 06, 2021 04:35)

  Hi, Christine:

  All the above comments are very useful. I also have one important  tip for tackling this issue.

  For long structure elements like beam,ridge beam and handrail, we need to use the uniformly distributed load per linear foot（abbreviated as plf）for measuring tributary load per linear foot. The sample question is testing the understanding for this.  So here we will use 60 psf*10' to get the answer 600 plf.

  Personally I feel I master the spirit of the structure study after I fully understand the difference between psf and plf abbreviated for uniformly distributed load.

  Hope this help

  0

  Comment actions Permalink

Please sign in to leave a comment.