Roof Renovation

[BTAboy]

Can someone concisely explain how "L", the live load, and "L-sub-r", the roof live load are employed in load combinations at paragraph 2.3.2 of ASCE "Minimum Design Loads for Buildings and Structures" (second printing incorporating errata identified through Apr 6, 2011)?

At para 4.1 of that same ASCE publication, the Live Load and Roof Live Loads are defined thus:

LIVE LOAD: A load produced by the use and occupancy of the building or other strucure that does not include construction or environmental loads, such as wind load, snow load, rain load, earthquake load, flood load, or dead load.

ROOF LIVE LOAD: A load on a roof produced (1) during maintenance by workers, equipment, and materials and (2) during the life of the structure by movable objects, such as planters or other similar small decorative appurtneances that are ot occupancy related.

In the same ASCE publication Table 4-1, titled "Minimum Uniformly Distributed Live Loads, L-sub-0, and Minimum Concentrated Live Loads", sets the minimum live load for ordinary flat, pitched, and curved roofs at 20 lb/sq ft. But when calculating the load combinations defined at para 2.3.2 is this 20 lb/sq ft the "L" for the roof, or is it the "L-sub-r", or is it both?

Would appreciate some clarification if you're able to provide it.

Thanks!

 

[JAE]

L_o is the unreduced general live load (either floor - L or roof - Lr).

If you look at sections 4.7 and 4.8, it takes the L_o for either floors (section 4.7) or roofs (section 4.8) and reduces it to either L or Lr for use in the load combinations.

 

[BTAboy]

JAE, thanks for your reply.

I understand the load reduction sections, in particular the area and slope reductions at paragraph 4.8 for roofs.

What I'm not understanding is the use of the minimum roof live load specified at Table 4-1 in the para 2.3.2 load combinations that call for both "L" (live load) and "L-sub-r" (roof live load). Specifically:

comb 2 -- 1.2D + 1.6L + 0.5(L-sub-r or S or R)

comb 3 -- 1.2D + 1.6(L-sub-r or S or R) + (L or 0.5W)

comb 4 -- 1.2D + 1.9W + L + 0.5(L-sub-r or S or R)

The commentary paragraph C4.8 further confuses the question:

"C4.8.2 Flat, Pitched, and Curved Roofs -- The values specified in Eq. 4-2 [please note that "change bars" at the current edition's para 4.2 indicate that paragraph has been replaced in its entirety. No equation 4.2 appears there anymore, thus the reference to equation 4.2 at para C4.8.2 now appears to be errata.] that act vertically upon the projected area have been selected as minimum roof live loads, even in localities where little or no snowfall occurs. This is because it is considered necessary to provide for occasional loading due to the presence of workers and materials during repair operations.

C4.8.3 Special Purpose Roofs -- Designers should consider any additional dead loads that may be imposed by saturated landscaping materials in addition to the live loads required in Table 4-1. Occupancy related loads on roofs are live loads (L) normally associated with the design of floors rather than roof live loads (L-sub-r), and are permitted to be reduced in accordance with the provisions for live loads in Section 4.7 rather than Section 4.8."

Which still leaves me with my original quandry. Specifically, is the roof live load at Table 4-1 "L", or is it "L-sub-r", or is it both?

-- if it is "L", then what is "L-sub-r"? Is "L-sub-r" for an additional live load, if any, beyond the Table 4-1 minimum floor load? If so, then what type and magnitude would such an additional load be -- a construction and repair load of say, 20 lb/sq ft (sort of implied by the roof live load definition at para 4.1)?

-- if it is "L-sub-r", then what is "L"? Does "L" become zero anytime that one is investigating a load combination for a roof, unless the owner is contemplating using the roof as a traffic way, or an outdoor restraunt, or some other use that will call for a floor-type loading on the roof? If so, then could one reasonably select a floor loading value from Table 4-1 for the "L"?

This is just not clear to me. I'm hoping someone can clarify.

Thanks!

 

[SLTA]

L is for floors and decks. Lr is for roofs. You may have both or either. When you're talking about loading on a roof L would not come into play, only Lr. If you had a rooftop terrace or bar, it would still be Lr, but a much higher load.

Does that help?

 

[JAE]

Yes - what slta says. L is for floors...and for roofs if you have human occupancy like a roof deck. Lr is for roofs without human occupancy/decks.