PDD NCARB Handbook Sample Question #1 / Structures



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    Michael Ermann

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

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    Bankole Folorunso (Edited )

    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.  

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    Xijue Wang

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

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    Li Xing (Edited )

    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

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