Chapter VIII - Structural Evaluation & Analysis of Load Combinations

Once the design criterion has been defined a structural evaluation can proceed, by the building code if possible, or using reasonable methods of analysis or testing. The design analysis shall apply design loads and load combinations as determined in the prior section. It is first necessary to clearly define terms normally associated with loads and load combinations in residential structures

Load Combinations

Design load combinations that are based upon Allowable Stress Design (ASD) or Load and Resistance Factor Design (LRFD) should comply with the generalized load combinations in the following table.

ASD Load Combinations

LRFD Load Combinations

D+H+L+0.3 (Lr or S)

1.2D+1.6(L+H)+0.5(Lr or S)

D+H+(Lr or S) +0.3L

1.2D+1.6(Lr orS) +0.5 (L or 0.8W

D+(W or 0.7E) +.05L+0.2S

1.2D+1.6+0.5L+0.5(Lr or S)

0.6D+W

1.2D+1.OE+0.5L+0.2S

0.6D+0.7E

0.9D+1.6W

 

0.9D+1.0E

 

 

Where:  D=Dead Load,  H=Soil Lateral Load,  L=Live Load,  Lr=Uniform Roof Live Load, S=Snow Load, W=Wind Load, E=Seismic Load



Load combinations typically used for the design of residential building components and systems.

Component or System

ASD Load Combinations

LRFD Load Combinations

Foundation wall

(Gravity & Soil Lateral Loads)

D+H

D+H+L+0.3(Lr or S)

D+H+(Lr or S) +0.3L

1.2D + 1.6H

1.2D + 1.6H + 1.6L + 0.5 (Lr or S)

1.2D + 1.6H + 1.6L(Lr or S)+0.5L

Headers, Girders, Floor Systems, Interior Load Bearing Walls, Footings (Gravity Loads)

D+L+0.3(Lr or S)

D+(Lr or S) + 0.3L

1.2D + 1.6L + 0.5 (Lr or S)

1.2D + 1.6(Lr or S) + 0.5L

Exterior Load Bearing Walls

(Gravity & Wind Lateral Loads

Same as Above plus,

D+W

D+0.7E+0.5L+0.2S

Same as Above plus,

1.2D + 1.6W

1.2D + 1.0E +0.5L + 0.2S

Roof Rafters, Trusses, Beams,

Roof & Wall Sheathing

(Gravity & Transverse Loads)

D+(Lr or S)

0.6+Wu

D+W

 

1.2D + 1.6(Lr or S)

0.9D + 1.6 Wu

1.2D + 1.6W

Floor Diaphragms & Shear Walls

(Lateral & Overturning  Loads)

0.6D+W

0.5D+0.7E

0.9D + 1.6 W

0.9D + 1.0E

 

 

 

 

 

 

Where:  D=Dead Load,  H=Soil Lateral Load,  L=Live Load,  Lr=Uniform Roof Live Load, S=Snow Load, W=Wind Load, E=Seismic Load, Wu=Wind Uplift on Roof or Suction Load

Load combinations in these tables should be applied uniformly for residential building design using material design specifications for materials including steel structural members.

Combined load proportioning of the ASD load combinations has been done in a manner consistent with the proportioning of loads in the LRFD format, resulting in a more realistic application of ASD load combinations.

Dead Loads(D)

The dead load of a structure is the total of the weight of the permanent components of the structure such as beams, floor slabs, columns and walls. These components will produce the same constant 'dead' load during the lifespan of the building. Dead loads are exerted in the vertical plane.

Dead load for a particular component is calculated by multiplying the volume of the structural member by the unit weight of the material.

Once an accurate dead load is determined for each component the different components can be added together to determine the dead load for the entire structure.

With steel construction a specification of weight per foot is included for each element in the design, which is helpful when calculating dead load. In ISBU construction arriving at a dead load is also  relatively easy since the weights of a 40’ and 20’ containers are known (20’ = 4,850 lbs, 40’ = 8,223 lbs).  In addition it will be necessary to add the weight of any roofing materials, flooring materials, interior and exterior wall sheathings such as drywall, as well as doors and windows, and other materials permanently attached to your structure.

Typical dead load values for common residential construction materials based on typical practice.

ROOF CONSTRUCTION

 

Asphalt shingles, metal roofing. Or wood shake or shingles.

15  psf

Build up roll roofing, tar & gravel

18psf

Light weight tile or 1.4” slate

20 psf

Conventional clay tile, concrete tiles, or 3/8” slate

25 psf

FLOOR CONSTRUCTION

 

Carpet or Vinyl flooring

10 psf

Wood flooring

12 psf

Ceramic tile

15 psf

1/2 “ slate or ceramic tile with ½” mortar bed

20 psf

Live Loads

Live loads consist of moving or movable external loads on a structure; includes the weight of people, furnishings and portable equipment, etc. The floor live load is based on type of occupancy. Residential live loads are usually 40psf for living areas and 3psf for bedroom areas. The live loads listed in the following table represent typical practice but may vary relative to local building code requirements.

APPLICATION

UNIFORM LOAD

CONCENTRATED LOAD

Roof

 

 

Slope  ³ 4:12

15 psf

250 lbs

Slope  < 4:12

20 psf

250 lbs

 

 

 

Attics

 

 

Without storage

10 psf

250 lbs

With storage

20 psf

250 lbs

 

 

 

Floors

 

 

Bedroom Areas

30 psf

300 lbs

Other Areas

40 psf

300 lbs

 

 

 

Garages

40 psf

2,000 lbs

Stairs

40 psf

300 lbs

Decks &  Balconies

60 psf

300 lbs

Concentrated live loads shall be applied to a small surface area consistent with the application and shall be located and directed to produce the maximum possible load effect on the element or assembly under consideration. Concentrated live loads shall not be required to be applied simultaneously with uniform live loads.

Soil Lateral Load(H)

The equivalent fluid density (Rankine Method) of determining soil lateral loads is relatively simple and effective for shallow residential foundation walls and soil retaining structures. For typical residential foundations a minimum of 30 pcf equivalent fluid densities has been the long-term norm. The value of Ka in the table below shall be used to determine equivalent fluid density value for well-drained, lightly compacted soils in accordance with the following equation.

q=KaW

Where q= soil equivalent fluid density, Ka=active soil pressure coefficient, and W= soil unit weight

For saturated soil conditions such as encountered in a flood plain or in poorly drained soil, an equivalent fluid density value of 85 pcf shall be used.

VALUES OF Ka, soil unit weight, and equivalent fluid density by soil type.

Type of Soil

Active Pressure Coefficient (Ka)

Soil Unit Weight

(pcf)

equivalent fluid density (pcf)

Sand or Gravel

(GW,GP,GM,SW,SP)

0.26

115

30

Silty Sand, Silt, and Sandy Silt

(GC,SM)

0.35

100

35

Clay-silt, Silty clay

(SM-SC,SC,ML,ML-CL

0.45

100

45

Clay

(CL,Mh,CH)

0.6

100

60

In areas subject to hydrodynamic loads due to moving floodwater or hydrostatic loads due to standing floodwater, the provisions of ASCE 7-98, accepted engineering practice, or local requirements for design of building foundations in flood hazard areas shall be followed. In general, in costal flood zones, elevated foundations such as pile or pier foundations should be used to minimize or avoid hydrodynamic flood loads from fast moving waters.

Foundations shall be adequately protected against frost heave or bear on soils at a depth equivalent to the locally prescribed frost depths.


Air-Freezing Index  ( °F days)

Footing Depths

250 or less

12

500

18

1,000

24

2,000

36

3,000

48

4,000

60

Wind Loads(W)

For the purpose of calculating wind loads, the sites basic wind speed shall be based on the gust wind speed as provided in IRC wind map. Wind exposure shall be designated as one of the following categories.

OPEN – Exposed open terrain with few, well-scattered obstructions having heights generally less than 30 feet; it includes flat open country, grasslands, and direct costal exposures.

SUBURBAN – Urban, suburban and mixed wooden areas, or other terrain with many obstructions having the size of single-family dwellings or larger, scattered open areas and fields are included.

PROTECTED – Densely wooded terrain with the building not extending above the average height of surrounding obstructions (i.e. trees or buildings) and with the site design wind speed less than 130 mph.

The suburban exposure is often used as a reasonable default condition for residential design. However, when homes are located in predominately open terrain such as an isolated home on the plains or oceanfront property, the open exposure condition must be used.

Using the site’s basic wind speed, the basic velocity pressure (suburban Exposure) shall be determined in accordance with the next table.

Basic Wind Speed

(MPH peak gust)

One-,Two-, & three-story Buildings

85

12

90

13

100

16

110

19

120

23

130

27

140

31

150

36

FOR OPEN WIND EXPOSURE MULTIPLY THE TABLE VALUES BY 1.4.

FOR PROTECTED WIND EXPOSURE MULTIPLY THE TABLE VALUES BY 0.8

FOR TWO-STORY BUILDINGS MULTIPLY TABLE VALUES BY 0.9

FOR ONE-STORY BUILDINGS MULTIPLY TABLE VALUES BY 0.8

Determine the lateral wind pressures on the main wind force resisting system (MWFRS) of a building by multiplying the appropriate lateral pressure coefficients from table below by the basic wind velocity pressure from the table above. These pressures shall be applied to the vertical projection of the roof and walls for two orthogonal directions of loading.

APPLICATION

 

Lateral Pressure coefficients

Roof projected area (by slope)

 

Flat to 6:12

0.5

7:12

0.6

8:12 to 12:12

0.7

Wall projected area

1.1

APPLICATION

 

Lateral Pressure coefficients

Roof projected area (by slope)

 

Flat to 6:12

0.5

Note: These values are composite pressure coefficients, which include the effect of positive pressures on windward Face of the building and negative (suction) pressures on leeward face of the building.

To determine wind pressures on components and claddings, multiply the appropriate pressure coefficients from by the basic wind velocity pressure. With the exception of the roof uplift coefficient, all pressure calculated using these coefficients shall be applied perpendicular to the actual building surface area tributary to the component under consideration. The roof uplift pressure coefficient shall be used to determine a single wind pressure to be applied to the horizontal projected area of the roof assembly to determine roof tie-down connection forces and to evaluate the roof uplift contribution to building overturning forces as shown in the next table.

Pressure Coeficients For Building Components, Cladding and Various Assemblies


APPLICATION

PRESSURE COEFFICIENTS1,2

 

ROOF

 

trusses, Roof Beams, Ridge & Valley Rafters

-0.9, =0.4

Rafters & Truss Panel Members

-1.2, +0.7

Roof Sheathing (panels, boards, or purlins)

-2.2, +1.0

Skylights & Glazing

-1.2, +1.0

Roof Uplift3

-1.0 (hip roof with slope less than 3:12)

-0.8 (hip roof with slope between 3:12 and 6:12)

-0.4 (hip roof with slope greater than 6:12)

-1.0 (gable roof of any slope)

Windward Overhang4

+0.8

 

 

WALL

 

 

All framing members

-1.2, +1.1

Wall Sheathing (panels, boards, or girts)

-1.3, +1.2

Windows, Doors, & Glazing

-1.3+1.2

Garage Doors

-1.1, +1.0

 

 

Notes: 1-All coefficients include internal pressure in accordance with an enclosed building condition (i.e. no openings). Higher internal pressures shall be considered and table values adjusted in accordance with Section 6.7. 2- Positive and negative signs represent pressures acting inward and outward, respectively, from the building surface. A negative pressure is a suction or vacuum. Both pressure conditions shall be considered. 3-The roof uplift pressure coefficient is used to determine uplift pressures that are applied to the horizontal projected area of the roof for the purpose of determining uplift connection forces. Additional uplift force on roof connections due to windward roof overhangs shall also be included. The uplift force must be transferred through a continuous load path to the foundation or to a point where it is adequately resisted by the factored dead load of the building. 4-The windward overhang pressure coefficient is applied to the underside of a windward roof overhang and acts upward on the bottom surface of the roof overhang. If the bottom surface of the roof overhang is also the roof sheathing, then the overhang pressure shall be additive to the roof sheathing pressure.;

Snow Loads(S)

Ground snow loads shall be based on approved local climate data.

The uniform roof snow load shall be determined in accordance with the following formula.

p = CeCspg

where p=uniform roof snow load, pg=uniform ground snow load and the values for Ce (roof snow load exposure factor) and Cs (roof slope factor) are as follows:

Ce = 0.8 for windy areas with open exposure

= 1.0 for typical suburban areas

= 1.2 for sheltered or wooded areas

Cs = 1.0 for slopes ≤ 6:12

= 0.9 for 7:12 slope

= 0.8 for 8:12 slope or greater

\

An off-balance snow load of 0.8p on one side of the roof and 1.2p on the opposite side of the roof shall also be considered.

Earthquake Load(E)

Because the likelihood of seismic activity in the United States varies considerably by region and the fact that the most active regions (California & Alaska) have adapted very specific seismic requirements in their building codes and it is beyond the scope of this book to explore them fully. If you live in an active seismic zone further information can be gathered at your local building department. The following information will cover seismic load only as it is covered in the IRC. The site design ground motion shall be based on the short period spectral response acceleration, Ss, provided for your region in the IRC appendix. The total seismic shear (lateral) load for each level of the building shall be determined by the following formula:

E = 0.8 é(Ss )(F a)ùWg[ R]

Values for Fa and Rs can be obtained below Site Amplification factor (Fa) for typical firm soil.

Ss

£0.25

0.50

0.75

1.00

³1.25

Fa

1.6

1.4

1.2

1.1

1.0

Seismic Response modifiers - R

Building System

R

Light-frame walls (wood or Cold-formed steel)

  • With wood structure panel sheathing<
  • Stucco and wire lath
  • Gypsum wall board

 

6.0

4.0

2.0

Masonry Walls

-Unreinforced

-Reinforced

 

1.5

3.5

Concrete walls

-Unreinforced

-reinforced

 

2

4.5

The story shear load shall be distributed to and resisted by shear walls in a manner that does not induce unacceptable torsional response or overloading due to differences in stiffness of various structural systems or building configuration. Acceptable methods to distribute seismic story shear load to supporting shear walls or other vertical shear resisting elements include the use of tributary building weight (dead load) or stiffness-based procedures. Seismic story shear loads shall be considered in separate directions acting parallel to each major axis of the building. Adequate direct shear, torsional, and overturning resistance shall provide stability.

Scroll down for next chapter


Next Chapter