1	Post Frame Snow Load Design and Commentary on ASCE Unbalanced Snow Load Design
2	Todays Topics Discuss Design factors used in IBC snow load design Review the history of ASCE-7 and how its changes have affected unbalanced snow load design. Show where this design standard is moving to in 7-05
3	Michigan Ground Snow Load Map (IBC)
4	MRC Ground Snow Load Map
5	Adjustment Factors to Obtain Roof Snow Load P_f = .7 * C_e* C_T* I * P_ground .7 = 70% on roof C_e = Roof Exposure Factor (.9, 1.0, 1.1, 1.2) C_T= Temperature Factor (1.0, 1.1, 1.2) I = Importance Factor (.8, 1.0, 1.2) P_ground= Map values P_f = flat roof snow load design value
6	C_e = Snow Exposure Factors
7	Sheltered Condition ??
8	C_T= Temperature Factor 1.0 1.1 1.2 All except as indicated below (heated, R< 25) Structures kept above freezing, ventilated roofs in which the R between the ventilated space and the heated space exceeds 25. (i.e. 12” insulation, R38) Unheated structures (see IBC pp 308- table 1608.3.2)
9	C_T= Temperature Factor Ct= 1.2 unheated Ct = 1.1 heated with attic and r= 38 insulation in ceiling
10	Building Importance Factor
11	Snow load Comparisons for Unbalanced Loading
12	C_s Factor - P_s= P_f* C_s C_s (roof slope) factor is Multiplied times the P_f (flat roof snow load) and this yields the final design roof snow load (P_s) It is a function of roof insulation and Roof temperature It is favorable to Steel roofs (range=.7- 1.0- generally about~.95 for 3 and 4 /12 roof pitches with cold roofs) Less favorable to shingle roofs( Equal to 1.0 for roof pitches up thru 12/12- 45 degrees cold roofs) Steel roofs with snow guards are to be treated like shingle roofs. This factor becomes smaller with higher roof pitches- thus it reduces the design roof snow load.
13	Cs Factor
14	Graphs for Determining Roof Slope Factor Cs
15	Cs Factor
16	Results of Average Adjustment Factors to Obtain Roof Snow Load P_f = .7 * C_e* C_T* I * P_ground .7 = 70% on roof C_e = Roof Exposure Factor (.9, 1.0, 1.1) C_T= Temperature Factor (1.0, 1.1, 1.2) I = Importance Factor (.9, 1.0, 1.1) P_ground= Map values P_f = flat roof snow load design value P_f= .71.01.21.0P_g= .84*P_g
17	Michigan Ground Snow load Map
18	Other Roof Load Design Considerations Snow Drift Sliding snow Drifting snow Roofing Conditions Steel or shingles Snow guards
19	Snow Drift on Lower Roofs ASCE 7-98 Figure 7-8
20	Drift
21	Snow Drift Areas
22	ASCE- 7 Snow Loads
23	Unbalanced Snow Loads
24	ASCE 7-98 Unbalanced Snow Load Design Requirements
25	ASCE 7-98 (adopted by 2000 IBC)- Cases 1, 2 and 3
26	ASCE 7-98 case 4
27	The 7-98 Unbalanced Design Requirement put Considerable Extra Cost in the Building Design it Affected Footings Possibly columns Sidewall headers Truss to column connections Trusses purlins
28	Foundations Footing Sizes Affected by Unbalanced Roof Loads
29	Truss Design with Balance Snow Load (42 PSF)
30	Truss Design with Unbalanced Snow Load for Same Truss (13/76 PSF)
31	Purlin Design Comparisons with Unbalanced Snow Loads
32	Industry / State Actions In 2003 Wisconsin was one state that understood the ramifications of this extreme change in unbalanced snow load design and adopted an “alternate unbalanced snow load standard”, allowing the use of the Canadian Building Code design procedure. In the meantime ASCE 7-02 was relaxing some of their unbalanced snow load design provisions. These were then adopted by IBC-2003
33	Wisconsin Alternate Standard (adopted 2003)
34	Summary of Unbalanced Reactions
35	Wisconsin Second “Alternate Standard” The current 2003 IBC adopts the use of this same EP-486 for design of Post frame buildings.
36	EP-486 Soil Pressure Increases as Follows
37	Comparison of EP-486 Calculated Values vs. 2000 and 2003 IBC Base Presumptive Values * 5’ H.D with an 18” footing.
38	Footing Requirements Comparisons
39	ASCE 7-02 Unbalanced Snow Load Eliminated case 4
40	ASCE 7-02 Unbalanced Snow Load Design Requirements
41	ASCE 7-02(adopted by 2003 IBC) Cases 1, 2 and 3
42	Summary of Unbalanced Reactions
43	IBC Codes and ASCE -7 Standards
44	Early model of Unbalanced Snow Loads
45	Proposed Draft of ASCE 7-05 on Unbalanced Load Standard
46	ASCE 7-05 Proposed Formulas for Unbalanced Snow Loads
47	ASCE 7-05 (adopted by 2006 IBC ) Cases 1, 2 and 3
48	Summary of Unbalanced Reactions
49	Summary of Unbalanced Reactions
50	ASCE 7-05 Unbalanced Snow Load Provision NFBA supports the 7-05 unbalanced snow load provision. History tells us that buildings designed under ASCE 7-93, 7-95 have shown good performance and 7-02 and 7-05 are moving back toward these results.
51	Thank You