Minimum Design Loads for Building and Other Structures-To ASCE 7

7/22/2019 Minimum Design Loads for Building and Other Structures-To ASCE 7

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Lecture notes for CE 248 Behavior of plastic design of steel structures

Topic: Loading

Prepared by: T.Y. Yang

Date: August 31st2006

Text: ASCE 7 02 (or 05): Minimum Design Loads for Building and Other Structures.

Scope:

This standard provides minimum load requirements for the design of buildings

and other structures that are subject to building code requirements. Loads and appropriateload combinations, which have been developed to be used together, are set forth for

strength design and allowable stress design. For design strengths and allowable stress

limits, design specifications for conventional structural materials used in buildings andmodifications contained in this standard shall be followed.

Provide the minimumload requirements. Used for LRFD or ASD.

Allowable Stress Design (ASD):

Used by engineers for more than 150years.

Best estimate of the load are applied toa linearly equivalent model.

Stress of the model is calculated andchecked against Allowable stress(usually portion of the yield stress).

For example:o Allowable tensile stress for gross steel cross section = 0.6 Fy. This is

equivalent of providing a safety factor of 1.67 (Fy/0.6Fy = 1.67).o Different material (steel, concrete) and action (tension, shear, bending,

torsion) will have different safety factor.

Load and Resistant Factor Design (LRFD):

Routinely used by engineers in thedesign office.

Loads are factored to calculate theultimate load, where the load factors arebased on statistical interpolation of

measured condition.

o For example the load factor forthe dead load is greater than the

live load.

The applied load is then applied to a linearly elastic model.

(CourtesyofProf.Moehle)

(CourtesyofPro

f.Moehle)


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The component capacities are calculated assuming some inelastic behavior of thecross section.

Different strength reduction factor are assigned for different failure mode. Forexample: = 0.9 (for bending), = 0.85 (for shear). This is because we want to

be more conservative in shear failure than flexure failure.

Simultaneously consider the uncertainties in both the Load and ResistingStrength. => i i nQ R .

Basic requirements:

1. Strength:a. ASD => stress check.b. LRFD => check the force between demand and capacity.

2. Service ability:a. Drift, vertical deflection, vibration,, ect.b.

Typical deflection limits: LL=> L/360, DL+LL=> L/240 ~ L/180c. Figure 5.7 for vibration as source of discomfort and damage.

3. Self-strained force:a. Creep. dt, , ect.

4. Counter acting structural action:a. Overturning, sliding, up lifting, , ect.b. For example, design of isolators. Need to consider the friction force

needed to prevent sliding.

System integrity:

Ductility, energy dissipation, progressive collapse. Redundancy, Redundancy, Redundancy!!!

Classification of Buildings and other structures:

Category I, II, III, IV. (Higher the number the higher importance). Selected base on:

o Number of occupancy and Hazardous content in the building. Minor storage facility => Category I. Typical residential single unit house => Category II. Schools, Offices with more people involved => Category III. Hospitals, Federal facilities => Category IV. Your own house => Category V. (Just kidding, there is no such category). Check ASCE 7-02 Table 1-1 (pg 4) for more detail information.


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Load combination factors (LRFD):

1. 1.4(D +F)2. 1.2(D +F + T ) + 1.6(L +H) + 0.5(Lr or S orR)3. 1.2D + 1.6(Lr or S orR) + (L or 0.8W)4. 1.2D + 1.6W +L + 0.5(Lr or S orR)5.

1.2D + 1.0E +L + 0.2S6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H* Note there are some exceptions to watch. Please refer to ASCE 7-02 Section 2.3.2.

Load combination factors (ASD):

1. D + F2. D + H + F + L + T3. D + H + F + (Lr or S or R)4. D + H + F + 0.75(L + T ) + 0.75(Lr or S or R)5. D + H + F + (W or 0.7E)6.

D + H + F + 0.75(W or 0.7E) + 0.75L7. 0.75(Lr or S or R)

8. 0.6D + W + H9. 0.6D + 0.7E + H

* Please refer to ASCE 7-02 Section 2.4.2for more details.

Notations:

D = dead load

Di = weight of iceE = earthquake load

F = load due to fluids with well-defined pressures and maximum heightsFa = flood load

H = load due to lateral earth pressure, ground water pressure or pressure of bulk materials

L = live loadLr = roof live load

R = rain load

S = snow load

T = self-straining forceW = wind load

Wi = wind-on-ice determined in accordance with Chapter 10


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Sometimes the maximum loading for the member is not to assign live loads.

For example:

Maximum negative moment at support B. Maximum reaction at support B.

Live loads

Dead loads

A B C D

Maximum positive moment at span AB and CD.

Dead loads

A B C D

Live loadsLive loads


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Dead loads:

3.1 Definition. Dead loads consist of the weight of all materials of constructionincorporated into the building including, but not limited to, walls, floors, roofs,ceilings, stairways, built-in partitions, finishes, cladding, and other similarly

incorporated architectural and structural items, and fixed service equipment

including the weight of cranes. CheckASCE 7-02 Table C3-1 (pg 246) for minimum design dead loads. CheckASCE 7-02 Table C3-2 (pg 252) for material weight.

Live loads:

4.1 Definition. A load produced by the use and occupancy of the building or otherstructure that does not include construction or environmental loads, such as wind

load, snow load, rain load, earthquake load, flood load, or dead load.

0L , ASCE 7-02 Table 4-1(pg 12) - minimum uniformly distributed andconcentrated live loads.

Live load reduction: ASCE 7-02 Section 4.8(pg 10)o Since the probability for elements with large influence area to have all its

live load acting on the element at the same time is small, most building

code allows live load reduction.

o 0 150.25LL T

L LK A

= +

ASCE 7-02 Equation 4-1(pg 10)

TA = tributary area [ 2ft ]. LLK = live load element factor. ASCE 7-02 Table 4-2(pg 15) Note: LL TK A = IA = Area influence. Explain: how to calculate LLK

using ASCE 7-02 Figure C4(pg 258)

For element carrying live load for more than one story, add IA forall the stories together.

00.5L L for member supporting one story. 00.4L L for membersupporting more than one story.

o Exceptions:1. Heavy live loads (> 100 psf): => no reduction. Except member

support more than one story, 00.8L L=

2. Car garage: => no reduction. Except member support more thanone story, 00.8L L=

3. Public assembly occupancy (< 100 psf): => no reduction.4. One-way slab. 21.5T bA L , where bL = span length.


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Roof live loads:

4.1 Definition. A load on a roof produced (1) during maintenance by workers,1 1R = equipment, and materials and (2) during the life of the structure by movable

objects, such as planters or other similar small decorative appurtenances that are

not occupancy related.

Minimum roof live loads:ASCE 7-02 Section 4.9(pg 11)o

1 220rL R R= where 12 rL 20 [psf].

o2

2 2

1

2

1 for 200

1.2 0.001 for 200 600

0.6 for 600

t

t t

t

A ft

R A ft A ft

A ft

= < 70 degrees => no snow load.

< 10 degrees

crown

eave

> 70 degrees

no snow loads


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For folded plate, saw tooth and barrel vault roofs, sC = 1 (no reduction). Thisroof collect snows in the valley.

Unbalanced snow loadso When 15 degrees roof slope 70 degrees, snow from the windward

side might be blow to the leeward side of the rood by wind, hence it is

important to consider the unbalanced snow load. Balance and unbalancedsnow loads should be considered separately.

o Unbalanced snow load = 1.5 sp / eC .

roof

Balanced snow loads

,roof

Unbalanced snow loads

o For curved roof and multiple fold plate, saw tooth and barrel vault roof seeASCE 7-02 Section 7.6.2 and Section 7.6.3 (pg 79)

o Remember to consider the worst combination for the member loading!


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Minimum Design Loads for Building and Other Structures-To ASCE 7