Design Loads
Dead Loads o
o
An engineered design approach involves calculating a demand due to loads, and comparing the demand with the capacity of the member or element under consideration. Gravity (dead) loads are a natural starting starting point dead load is an ever-present load
It is necessary to determine the magnitude of the vertical loads before the design seismic loads can be estimated. 2 /45
Dead Loads o
o
An engineered design approach involves calculating a demand due to loads, and comparing the demand with the capacity of the member or element under consideration. Gravity (dead) loads are a natural starting starting point dead load is an ever-present load
It is necessary to determine the magnitude of the vertical loads before the design seismic loads can be estimated. 2 /45
Dead Loads Weights of all materials permanently attached to the structure including the following: o weight of the roof or wood floor system o sheathing o framing o insulation o ceiling a utomatic fire sprinkler, sprinkler, ducts o piping, automatic o fixed equipment o etc.
Table 204-2 Minimum Design Dead Loads (kPa) [use actual when available] CEILINGS
COVERINGS, Roof and Wall
Acoustical Fiber Board . . . . . . . . . . . . . . . . . . 0.05
Asphalt singles . . . . . . . . . . . . . . . . . . . . . . . . .0.10
Gypsum Board (per (per mm thickness) . . . . . . . 0.008
Cement tile . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.77
Mechanical duct allowance allowance . . . . . . . . . . . . . . 0.20
Clay tile (for mortar add 0.48 kPa)
Plaster on tile or concrete . . . . . . . . . . . . . . . 0.24
Book tile, 50 mm . . . . . . . . . . . . . . . . . . . . 0.52
Plaster on wood lath . . . . . . . . . . . . . . . . . . . . .0.38
Book tile, 75 mm . . . . . . . . . . . . . . . . . . . . . 0.96
Suspended steel channel system system . . . . . . . . . . 0.10
Ladowici . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.48
Suspended metal lath lath and cement plaster plaster . . 0.72
Roman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.57
Suspended metal lath and gypsum plaster . . .0.48
Spanish . . . . . . . . . . . . . . . . . . . . . . . . . . . . .0.91
Wood furring suspension system . . . . . . . . . . 0.12
Composition Three-ply ready roofing . . . . . . . . . . . . . . . .0.05 Four-ply felt and gravel gravel . . . . . . . . . . . . . . . .0.26 Five-ply felt and gravel . . . . . . . . . . . . . . . . .0.29 Copper or tin . . . . . . . . . . . . . . . . . . . . . . . . . . .0.05 Corrugated asbestos-cement asbestos-cement roofing . . . . . . .0.19 Deck, metal 20 gage . . . . . . . . . . . . . . . . . . . . .0.12 Deck, metal 18 gage . . . . . . . . . . . . . . . . . . . . .0.14 Fiberboard, 13 mm . . . . . . . . . . . . . . . . . . . . .0.04
Sample Dead Load Calculation Roof Dead loads
D in psf
Roofing (5 –ply with gravel)
6.5
Reroofing
2.5
½” - in. plywood (3 psf x ½”.)
1.5
Framing (estimate 2 x 12 @ 16” o.c.)
2.9
Insulation
0.5
Suspended ceiling (acoustical tile)
1.0 Roof dead load D 14.9 ~ 15.0 psf
Sample Dead Load Calculation Floor Dead loads
D in
Floor covering (lightweight concrete 1½ “ at 100 pcf)
psf
12.5
1-1/8” plywood (3 psf x 1-1/8” )
3.4
Framing (4” x 12” @ 4’ o.c.)
2.5
Ceiling supports (2” x 4” @ 24” o.c.)
0.6
Ceiling ( ½“ drywall, 5 psf x ½“)
2.5 Floor dead load D 21.5 ~ 22.0 psf
Live Loads o
o
o
Lr – roof live load, including any permitted live load reduction L – live load, including any permitted live load reduction For the reduction of both Lr and L, the area contribution load to the member under design consideration is taken into account. for roof live load ( Lr ), the tributary area is used
for floor live load ( L), the influence area is used
NSCP Live Loads Table 205-1 – Minimum Uniform and Concentrated Live Loads Uniform Load2
Concentrated Load
Description
KPa
KN
Call Centers and BPO
2.9
9.0
Lobbies and ground floor corridors
4.8
9.0
Offices
2.4
9.02
Building corridors above ground floor
3.8
9.0
Press rooms
7.2
11.02
Composing and linotype rooms
4.8
9.02
Basic floor area
1.9
06
Exterior balconies
2.94
0
Decks
1.94
0
Storage
1.9
0
Use or Occupancy
Category
13. Office
14. Printing plants
15.
Residential 8
NSCP Live Loads Table 205-2 – Special Loads Use or Occupancy
Vertical Load (kPa)
Lateral Load (kPa)
Walkway
7.2
-
Canopy
7.2
-
Seats and footboards
1.75
-
Catwalks
1.9
-
Follow spot, projection and control rooms
2.4
-
Over stages
1.0
-
All uses except over stages
0.5
-
Category
1. Construction, public access at site (live load) 2. Grandstands, reviewing, stands bleachers, and folding and telescoping seating (live load0 3. Stage accessories (live load)
4. Ceiling framing (live load)
Description
Tributary Area, AT o
o
o
o
Tributary area - the area assumed to load a given member generally measured from midway between members on one side of the member under consideration to midway between members on the other side. for members spaced a uniform distance apart, the tributary width is equivalent to the spacing between members since tributary areas for adjacent members do not overlap, all distributed loads are assumed to be supported by the nearest structural member Note: tributary area approach should only be used when the loading is uniform
Influence Area, K LL AT o
In contrast to the tributary approach, the influence area recognizes that the total load experience by a structural member maybe influenced by loads applied outside the tributary area of the member
NSCP 205.6 Alternate Floor Live Load Reduction o
o
The influence area ( A I in m2) is four times the tributary area for a column Two times the tributary area for a beam, equal to the panel area for a two-way slab
Tributary Area
Tributary Area Calculations AT (ft2)
K A (ft2) LL T
Joist J1
2 x 12 = 24
2 x 24 = 48
Joist J2
2 x 14 = 28
2 x 28 = 56
Girder G1
(12/2 + 14/2)20 = 260
2 x 260 = 520
Girder G2
(12/2 + 14/2) 24 = 312
2 x 312 = 624
Interior col. C1
(12/2 + 14/2) (20/2 + 24/2) = 286
4 x 286 = 1144
Exterior col. C2
(20/2 + 24/2) 12/2)= 132
4 x 132 = 528
Corner col. C3
(14/2)(20/2) = 70
4 x 70 = 280
Reduction of Floor Live Loads 205.6 Alternate Floor Live Load Reduction
The unit live loads maybe reduced in accordance with the equation shown on any member, including flat slabs, having an influence area of 40 m2 or more
=
1
0.25 + 4.57
Ai = Influence area, m2 L = reduced design live load per square meter of area supported by the member Lo = unreduced design live load per square meter of area supported by the member (Table 205-1)
Note: The reduced live load shall not be less than 50% of the unit live load for members receiving load from one level only, nor less that 40 percent of the unit live load for other members.
Seatwork Determine the following: (a) tributary and influence areas are for J1, J2, G1, G2, C1 and C2 (b) axial force required for the design of the interior column C1 due to a dead load of 0.2 kPa and a floor live load L = 1.9 kPa.
Roof Live Loads o
o
o
o
The live load on a roof is usually applied for a relatively short period of time during the life of the structure This is normally of no concern in the design of structures other than wood However in wood structures, the length of time for which a load is applied does have an effect on the capacity (resistance) of the member. The standard roof live load for small tributary areas on flat roofs is 20 psf (1 kPa)
Reduction of Roof Live Loads
=
0
1
2
and 12
≤ ≤ 20
≤ − ≥ ≤ − ≥ 200
1
1
=
1.2
0.001
200 <
0.6
= 1.2
0.05
0.6
< 600
600
2
4
1
2
2
4<
< 12
12
AT = tributary area supported by structural member, ft 2 F = the number of inches or rise per foot for a sloped roof Lo = minimum uniform live load per ASCE 7-10 Table 4-1
2
NSCP Roof Live Loads Table 205-3 – Minimum Roof Live Loads Method 1
Method 2
Tributary Area (m2)
ROOF SLOPE
0 to 20
20 to 60
> 60
Uniform Load (kPa)
Rate of Reduction, r
Maximum Reduction R (percentage)
Uniform Load (kPa) 1. Flat or rise less than 4 units vertical in 12 units horizontal (33.3% slope). Arch and dome with rise less than one-eight of span.
1.00
0.75
0.60
1.00
0.08
40
0.75
0.70
0.60
0.75
0.06
25
3. Rise 12 units vertical in 12 units horizontal (100% slope) and greater. Arch of dome with rise threeeights of span or greater.
0.60
0.60
0.60
0.60
4. Awnings except cloth covered.
0.25
0.25
0.25
0.25
5. Greenhouses, lath houses and agricultural building.
0.50
0.50
0.50
0.50
2. Rise 4 units vertical to less than 12 units vertical in 12 units horizontal (33.3% to less than 100% slope). Arch and dome with rise one-eight of span to less than three-eights of span.
No reduction permitted
Example Problem Determine the uniformly distributed roof loads (including dead load and roof live load) for the purlins and girders in the building shown. Also determine the total load on column C1. Assume that the roof is flat (except for a minimum slope of ¼ in/ft for drainage). Roof dead load D = 0.38 kPa.
6.10 m
4.88 m
4.88 m
1.22 m
6.10 m
Example Problem Determine the uniformly distributed roof loads (including dead load and roof live load) for the purlins and girders in the building shown. Also determine the total load on column C1. Assume that the roof is flat (except for a minimum slope of ¼ in/ft for drainage). Roof dead load D = 8 psf.
Combined Dead and Live on Sloping Roof Lr from NSCP
L2
Equivalent total roof loads ( D + Lr ):
Load on horizontal plane: Horizontal
Roof D along roof slope
1
=
+
2
Load along roof slope:
=
+
2
1
Combined D
L on sloping roof
Example Problem The building is a standard residential occupancy. The rafters are sloped at 6 in./ft, and the roof covering consists of cement asbestos shingles. Determine the shear and moment for the rafters under dead plus live load if they are spaced 4 ft o.c. Roof dead load D has been estimated as 14 psf along the roof surface.
Lr = 62 psf 18’
18’
12 6
D = 14 psf
q
Rafters @ 4’ o.c.
Ridge beam
Other Loads o
Soil Loads and Hydrostatic Pressure (H) soil lateral loading most commonly occurs at retaining walls
where retaining walls are provided, it is possible to develop hydrostatic pressure
it is also possible to have upward hydrostatic pressures on adjacent floor slabs
pressure can also be due to storage of grain, aggregates, or other bulk solids that can exert lateral pressures
Other Loads o
o
Loads due to Fluids (F ) not intended to address flood loads or hydrostatic pressure when fluids are contained in non-building structures or nonstructural components, other standards might be applicable and can provide guidance in the design Rain Loads (R) primarily applicable to low slope roofs that are surrounded by parapets need not be considered for sloped roofs that cannot develop water buildup
Other Loads o
o
o
o
Flood Loads (F ) Section 211 of the NSCP applies to buildings and other structures located in areas prone to flooding as defined on a flood hazard map Snow Loads (not applicable) Self-straining Loads loads arising from temperature change, moisture change, creep, movement due to differential settlement or combinations thereof Earthquake Loads NSCP Section 208 will not be considered in this course
Other Loads o
Wind Loads (NSCP Section 207) Method 1 – simplified procedure can be used for the large majority of wood frame buildings
Method 2 – analytical procedure can be used for determining wind loads on structures of all sizes, configurations and exposure conditions requires defining more variables than Method 1
Method 3 – wind tunnel procedure for complex buildings that might be anticipated to have unusual dynamic behavior Limited to a small group of buildings for which the time and expense of a detailed study can be justified
Method 1 – Simplified Method o
o
Main wind-force-resisting system (MWFRS) - is a system providing wind resistance for the overall structure.
In wood frame buildings, the MWFRS most commonly consists of Shear walls – sheathed walls that resist in-plane loads
Roof and floor diaphragms – sheath floor and roof assemblies that transmit in-plane loads to the shearwalls
Method 1 conditions (NSCP 207.4.1.1)
conditions for the MWFRS to be designed using Method 1 o simple diaphragm building Both windward and leeward loads are transmitted through the floor and roof diaphragms to the same MWFRS o
building is low rise has a mean roof height h less than or equal to 18 m
mean roof height h does not exceed least horizontal dimension
Method 1 conditions (NSCP 207.4.1.1)
conditions for the MWFRS to be designed using Method 1 o building is enclosed meets requirements for wind-borne debris protection, if applicable o
o
building is regular has no unusual geometrical irregularity in spatial form building is not classified as a flexible building has a fundamental frequency greater than 1 hertz (fundamental period less than 1 second)
Method 1 conditions (NSCP 207.4.1.1)
conditions for the MWFRS to be designed using Method 1 o building does not have response characteristics that create unusual loading (such as galloping or vortex shedding) o
o
o
not sited in a location where unusual wind load effects might occur building has an approximately symmetrical cross section in each direction has a flat roof or a gable or hip roof with slope less than or equal to 45 degrees (12 in 12 pitch)
Method 1 conditions (NSCP 207.4.1.1)
conditions for the MWFRS to be designed using Method 1 o has a flat roof or a gable or hip roof with slope less than or equal to 45 degree o
Building is exempted from the torsional load cases as indicated in Note 5 of Figure 207-10, or the torsional load cases defined in Note 5 do not control the design of any of the MWFRSs of the building
NSCP 207.4.2.1 For the design of MWFRS the basic formula for calculating design wind pressure p s is :
=
l =
adjustment factor for building height and exposure from Figures 207-2A and 207-3 I w = Importance factor K zt = topographic factor as defined in Section 207.5.7 p s9 = simplified design wind pressure for Exposure B at h = 9m and I w = 1.0 from Figure 207-2 kPa
9
For components and cladding, the basic formula for calculating design wind pressure pnet is
=
9
pnet 9 = net design wind pressure for Exposure B at h = 9 m and I w = 1.0 from Figure 207-3 kPa
Adjustment Factor for Building Height and Exposure l Mean roof height (m)
Exposure
B
C
D
4.5
1.00
1.21
1.47
6.0
1.00
1.29
1.55
7.5
1.00
1.35
1.61
9.0
1.00
1.40
1.66
11.0
1.05
1.45
1.70
12.0
1.09
1.49
1.74
13.7
1.12
1.53
1.78
15.2
1.16
1.56
1.81
16.8
1.19
1.59
1.84
18.0
1.22
1.62
1.87
Design Wind Pressures on Walls and Roofs of Enclosed Buildings Basic Wind Speed (kph)
150
Roof Angle (o)
Load Case
0 to 5
Horizontal Pressures, kPa
Vertical Pressures, kPa
Overhangs
A
B
C
D
E
F
G
H
E oh
Goh
1
0.66
-0.34
0.44
-0.21
-0.79
-0.45
-0.55
-0.35
-1.11
-0.87
10
1
0.75
-0.31
0.50
-0.18
-0.79
-0.48
-0.55
-0.37
-1.11
-0.87
15
1
0.83
-0.28
0.55
-0.16
-0.79
-0.52
-0.55
-0.40
-1.11
-0.87
20
1
0.92
-0.24
0.61
-0.13
-0.79
-0.55
-0.55
-0.42
-1.11
-0.87
1
0.83
0.13
0.60
0.14
-0.37
-0.50
-0.27
-0.40
-0.69
-0.59
2
0.00
0.00
0.00
0.00
-0.14
-0.27
-0.04
-0.18
0.00
0.00
1
0.74
0.51
0.59
0.41
0.06
-0.45
0.02
-0.39
-0.26
-0.30
2
0.74
-0.08
0.59
0.41
0.29
-0.22
0.25
-0.16
-0.26
-0.30
25
30 to 45
vertical pressures are net design wind pressure for Exposure B at h = 9 m and I w = 1.0 from Figure 207-3 kPa
Design Wind Pressures on Walls and Roofs of Enclosed Buildings Basic Wind Speed (kph)
200
Roof Angle (o)
Load Case
0 to 5
Horizontal Pressures, kPa
Vertical Pressures, kPa
Overhangs
A
B
C
D
E
F
G
H
E oh
Goh
1
1.18
-0.62
0.79
-0.36
-1.42
-0.81
-0.99
-0.63
-2.00
-1.57
10
1
1.34
-0.56
0.89
-0.32
-1.42
-0.87
-0.99
-0.67
-2.00
-1.57
15
1
1.49
-0.49
0.99
-0.28
-1.42
-0.93
-0.99
-0.71
-2.00
-1.57
20
1
1.64
-0.43
1.10
-0.24
-1.42
-0.99
-0.99
-0.75
-2.00
-1.57
1
1.48
0.24
1.08
0.24
-0.66
-0.90
-0.48
-0.72
-1.23
-1.05
2
-
-
-
-
-0.25
-0.49
-0.07
-0.31
-
-
1
1.34
0.91
1.06
0.73
0.11
-0.81
0.04
-0.69
-0.47
-0.54
2
1.34
0.91
1.06
0.73
0.51
-0.40
0.45
-0.29
-0.47
-0.54
25
30 to 45
vertical pressures are net design wind pressure for Exposure B at h = 9 m and I w = 1.0 from Figure 207-3 kPa
Design Wind Pressures on Walls and Roofs of Enclosed Buildings Basic Wind Speed (kph)
250
Roof Angle (o)
Load Case
0 to 5
Horizontal Pressures, kPa
Vertical Pressures, kPa
Overhangs
A
B
C
D
E
F
G
H
E oh
Goh
1
1.84
-0.95
1.22
-0.57
-2.21
-1.26
-1.54
-0.97
-3.09
-2.42
10
1
2.07
-0.86
1.38
-0.50
-2.21
-1.35
-1.54
-1.04
-3.09
-2.42
15
1
2.31
-0.77
1.54
-0.44
-2.21
-1.44
-1.54
-1.10
-3.09
-2.42
20
1
2.54
-0.67
1.70
-0.37
-2.21
-1.54
-1.54
-1.17
-3.09
-2.42
1
2.31
0.37
1.67
0.38
-1.03
-1.40
-0.74
-1.12
-1.91
-1.63
2
-
-
-
-
-0.39
-0.76
-0.11
-0.49
-
-
1
2.07
1.41
1.65
1.13
0.16
-1.26
0.05
-1.08
-0.73
-0.83
2
2.07
1.41
1.65
1.13
0.79
-0.62
0.69
-0.44
-0.73
-0.83
25
30 to 45
vertical pressures are net design wind pressure for Exposure B at h = 9 m and I w = 1.0 from Figure 207-3 kPa
Notes 207.1.4.1. Main Wind-Force Resisting System The wind load to be used in the design of the MWFRS for an enclosed or partially enclosed building or other structure shall not be less than 0.5 kPa multiplied by the area of the building or structure projected onto a vertical plane normal to the assumed wind direction. 207.4.2.1.1 Minimum pressures The load effects of the design wind pressures from Section 207.4.2.1 shall not be less than the minimum load case from Section 207.1.4.1 assuming the pressures, p s, for zones A, B, C and D are all equal to +0.50 kPa, while assuming zones E, F, G and H all equal to 0 kPa.
notes o
The dimension is a defined as 10% of least horizontal dimension 0.4hmean whichever is smaller. However a may not be taken less than 0.9m (3’) or less than 4% of least horizontal dimension.
Referenced Wind Zone Map and Importance Factor Table 207-3 Importance Factor, I w (Wind Loads)
Occupancy Category2
Description
I w
I
Essential
1.15
II
Hazardous
1.15
III
Special Occupancy
1.15
IV
Standard Occupancy
1.00
V
Miscellaneous
0.87
1
see Table 103-1 for types of occupancy under each category
Figure 207-24 Referenced Wind Zone
Topographical Factor K zt 207.5.7 Topographic Effects o accounts for significantly higher wind speeds at sites located on the upper half of an exposed hill, ridge or escarpment o
o
K zt = 1.0 except for very exposed sites NSCP provides a series of five (5) criteria, all of which must be met in order to require evaluation for a K zt of other than 1.0
Topographical Factor K zt 207.5.7.2 Topographic Factor wind speed-up effect shall be included in the o The calculation of design wind loads by using the factor K zt
= 1+
1
2
2
3
where K 1, K 2 and K 3 are given in Figure 207-4. o
If site conditions and locations of structures do not meet all the conditions specified in Section 207.5.7.1 the K zt = 1.0.
Topographical Factor K zt
Example Problem Wind Forces for MWRFS Determine the design wind pressures based on the simplified method for the primary Lateral Force Resisting System (LRFS). This is a gable structure that uses a system of diaphragms and shear walls for resisting lateral forces. The building is a standard occupancy enclosed structure located in Zone 2. Exposure C is to be used, K zt is 1.0. Wind forces for designing MWFRS are obtained based on p s9 from NSCP. End zone and interior zone locations to be considered for horizontal pressures on the vertical projection of the building surface
Wind pressure zones on vertical and horizontal projections of building surfaces for MWFRS; wind direction parallel to transverse walls (end walls). Zone A (wall end zone) Zone B (roof end zone) Zone C (wall interior zone) Zone D (roof interior zone)
Example Problem Wind Forces for MWRFS Determine the design wind pressures based on the simplified method for the primary Lateral Force Resisting System (LRFS). This is a gable structure that uses a system of diaphragms and shear walls for resisting lateral forces. The building is a standard occupancy enclosed structure located in Zone 2. Exposure C is to be used, K zt is 1.0. Wind forces for designing MWFRS are obtained based on p s9 from NSCP. End zone and interior zone locations to be considered for horizontal pressures on the vertical projection of the building surface
Wind pressure zones on vertical and horizontal projections of building surfaces for MWFRS; wind direction perpendicular to transverse walls (end walls). Zone E (windward roof end zone) Zone F (leeward roof end zone) Zone G (windward roof interior zone)
44 /45
Wind Forces for MWRFS o
o
o
o
wind Speed = 200 kph (Zone 2, p. 2-72) Exposure C (No overhangs) total height of building = 5.8 m eave height = 3.6 m p sF = -1.21 kPa
p sE = -1.73 kPa
Wind pressures on vertical and horizontal projections of building surfaces; wind direction parallel to transverse walls (end walls).
End zone p sG = -1.21 kPa
p sH = -0.92 kPa
Interior zone a = 2.56 m
p sD = -0.29 kPa
p sB = -0.52 kPa
2.2 m
3.6 m
Wind Leeward
Windward
direction p sC = 1.34 kPa
12.8 m
p sA = 2.00 kPa
Wind Forces for MWRFS p sH = -0.92 kPa p sG = -1.21 kPa
p sD = -0.29 kPa p sF = -1.21 kPa p sC = 1.34 kPa p sE = -1.73 kPa p sB = -0.52 kPa
p sA = 2.00 kPa
Wind Forces for MWRFS o
o
o
o
wind Speed = 200 kph (zone 2, p. 2-72) Exposure C (No overhangs) total height of building = 5.8 m eave height = 3.6 m
Wind pressure on vertical and horizontal projections of building surfaces; wind direction perpendicular to transverse walls (end walls).
-1.73 kPa
-1.21 kPa
End zone -1.21 kPa
-0.92 kPa
Interior zone 2.00 kPa
1.34 kPa
a = 2.56 m
2.2 m
Wind direction
3.6 m 12.8 m
