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Introduction

and

Building Loads

CE 636 - Design of Multi-Story Structures

T. B. QuimbyUAA School of Engineering

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Course Objective The objective of the course is to give entry level

structural engineers an understanding of the

principles associated with the structural design ofbuilding systems.

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Expected OutcomesAt the conclusion of this course, the students will have:

an understanding of the engineering design process as it relates to buildingstructural design, including an appreciation for:

the iterative nature of the design process

the concept that there are more than one way to solve most engineering problems

an understanding of structural loads and their determination.

a basic understanding of the behavior and use of various structural systems

a basic understanding of what is required in a set of construction drawings

a basic understanding of what is required in a set of construction specifications

a recognition of the need for continual learning as a professional

an understanding of the need for professional registration

an understanding of professional and ethical responsibility the basic ability to:

identify, formulate, and solve building structural design problems

produce a set of construction drawings.

produce a set of construction specifications

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Course Content

The emphasis of the course will be slightly different than

the text. We will be considering all multi-story structures,

not just Tall buildings.

Load computations

Preliminary calculation methods

Computer modeling

Different GFRS and LFRS Calculations and Contract Documents

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Tall Buildings

Author: A tall building .... is one that, because of its

height, is affected by lateral forces due to wind or

earthquake actions to an extent that they play an important

role in the structural design.

History

Defense

Ecclesiastical

Commercial (from 1880 to current)

Residential (from 1880 to current)

Maximize use of high cost land

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Factors Affecting Development

Materials

Timber & Masonry limit to ~ 5 stories

Wrought Iron & Steel in mid 1880s

Structural Concrete after 1900 The Elevator

Made upper stories attractive to rent

Made tall buildings financially viable

Construction Technology Increase Speed

More efficient equipment

Improved methods

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Office vs Residential

Office/Commercial buildings

Large entrances and open lobbies

Reconfigurable space (large column free open

areas)

Residential buildings

Partitions are frequent and the same from storyto story

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The Design Team

Consists of:

Owner

Architect

Structural Engineer

Services Engineer (Mechanical & Electrical)

Team should collaborate EARLY to agree on a form of

structure to satisfying the conflicting requirements.

Structural system is subservient to the architectural

requirements.

Compromise is inevitable.

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The Design Process Design is an evolutionary (iterative) process.

Do preliminary sizing of members for gravity loads using

approximate analysis.

Check lateral strength and deflections, adjust members

sizes and configuration as necessary.

Make alterations to original layout as owner and architect

refine the design. May require radical rearrangement and

complete review of structure.

Make a rigorous final analysis using a refined analytical

model and verify deflections and member strengths.

Include the effects of movements due to creep, shrinkage,

temperature differentials, and foundation settlement.

Complete Construction Documents

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Design Criteria

Architectural

Internal layout to meet functional requirements

Aesthetic qualities

Structural

Strength (Elastic vs. Plastic)

Serviceability (deflections, vibrations, etc....)

Services

Power

Ventilation

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Limit State Design

A probabilistic approach Structural properties

Loading conditions

When a LIMIT STATE is reached, the structures said to

have failed. Strength Limit States

Exceedance of these limit states endanger lives and/or cause

serious financial loss.

Probability of material failure and instability must be low.

Serviceability Limit States

Fitness of the building for normal use

Probability of failure may be higher since failure is not

catastrophic.

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Loading

Buildings are designed to carry all gravity loads and lateralloads to be seen during construction and service.

Must consider sequential loading (particularly duringconstruction) in buildings where the sequence is important.

Types of Loading: Dead

Occupancy (Live)

Impact

Snow

Wind

Seismic

1997 UBC Chapter 16

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Strength & Stability

the building structure should have adequate strength to

resist, and to remain stable under, the worst probable load

actions that may occur during the lifetime of the building,

including the period of construction.

Consider probable load combinations (1997 UBC 1612)

Second order affects

Progressive collapse

Differential movement (shrinkage, creep, settlement,

temperature)

Overturning

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Stiffness and Drift Limitations

Deflections under gravity loads must be with in tolerable

limits for the occupancy.

Deflections under lateral load must be small enough to

satisfy

Second order effects (P-delta)

Avoid distress to the structure (cracking, redistribution

of loads to partitions, etc....)

Human comfort (acceleration, period, amplitude, visualand acoustical cues, past experience)

Serviceability

Lateral drift requirements (1997 UBC 1630.10)

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Tributary Areas

Useful for determining member forces due

to UNIFORMLY APPLIED loads (dead,

live, pressure, etc....) on SIMPLYSUPPORTED members.

Use structural analysis theory to find the

path that loads take as they find theirway down to the foundation through the

structural members.

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Example #1 Applied load

is uniformlydistributed.

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Tributary widths of beams

supporting joists coming in at

odd angles

w

ws

Reaction of Joist

Trib. width of joist along beam

Lj 2 p

s cos

Lj

2p

cos

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Beam AB

Lj =L1

L2

is 0 at end B.

w =L1

2 L2p

w =L1

2 L2L2 p =

L1

2pmax

x

x

x

1

1

1

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Beam BC

Lj =L1

L2

is 0 at end B.

w =L1

2 L2cos p

w =L1

2 L2

L2

coscos p =

L1

2cos p

2

max

2

cos

x

x

x

2

2

2

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Column Tributary Area

For a triangular load, the reaction

at B is 1/3 of the total load on the

beam. This means that the

column supports 1/3 of the area.

For a triangular load, this means

that the column at B support

L/sqrt(3) of the length of the

beam.

TotalLoad wL

wL xwx

L

xL

1

2

13

12

12

3

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Example #2 Identify the

Tributary Areasfor:

For each beam

For each column

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Example #2 Beam Areas

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Example #2 Column Areas

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Example #3: Multi-Story

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Outline Tributary Areas for column at C2

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Areas Trib. to column at C2

Lecture #1, Multi-Story Building

Supported AreasLevel

ft 2239Roof4

ft 2478Roof, 4th floor3

ft 2717Roof, 3rd & 4th flr2

ft 2956Roof, 2nd, 3rd, 4th1

These columns probably

support exterior wall sections as

well. Depends on details. Gravity loads tend to

accumulate linearly as you go

down the building.

Live loads may be reduced.

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Example #4

Have fun with this

one!

Find area supported

by beams on radial

grids.

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Dead Load Calculations

Dead loads are the weights of all items

permanently attached to the structure.

Roof, Floor, and Wall dead loads aretypically expressed in terms of unit loads

(the weight per unit of surface area).

Permanently attached equipment andmachinery are generally treated as point

loads or uniform loads over a limited area.

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Unit Load Calculations

All unit load calculations should be accompanied by a

sketch or reference a drawing showing a typical

calculation.

Each item is expressed in terms of its weight per unit

surface area.

Must compensate for slopes over 4:12.

Final result should be not include decimals! (your overall

estimate is not any more accurate than three significant

figures (if that!)

Should add an appropriate Misc.. amount for minor

items not specifically accounted for in itemized

calculation.

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DL Calculation #1

Floor Dead Load:

psf1.51/4" Linoleum

psf1.51/2" Underlayment

psf1.85/8" Plywoodpsf3.32x12 @ 16" O.C.

psf2.85/8" GWB

psf1.8Metal suspended ceilin

psf10Partitions

psf2.3Misc

psf25Total

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DL Calculation #2

Roof Dead Load

AdjustedSlopeFlatSlope/12

psf2.81.122.56Asphalt Shingles

psf2.01.121.865/8" Plywood

psf3.71.123.362x12 @ 16" O.C.psf4.01.123.66Insulation

psf3.11.122.865/8" GWB

psf1.81.001.80Suspended Ceil ing

psf1.5Misc

psf19.0Total

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Live Loads Live loads are any loads that are not permanently attached

to the structure. Live loads may be expressed in term of area loads or point

loads.

Live loads are placed for maximum effect.

Tabulated code values result from experience and typicalfield surveys.

See 1997 UBC 1606 & 1607

Live loads may be reduce for design of members that have

large tributary areas. 1994 UBC 1607.5

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UBC Floor Live Load Reduction

Use when:

Member supports more than 150 ft2

Live load not greater than 100 psf

Member does not support a place of public assembly

Use the lessor of:

R = 0.08(A-150)

R = 23.1(1+DL/FLL)

R = 40% for members receiving load from one level

only, or 60% for members receiving load from more

than one level.

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Alternate Floor Live Load

Reduction As an alternative, the following equation may be used for

member with an influence area greater than 400 ft2.

(New with the 1994 UBC)

L = L0(.25+15/sqrt(AI))

Maximum reduction is 50% for members supporting one

level and 60% for members supporting multiple levels.

AI is the influence area. For a column AI is four times the

trib. area. For a beam, AI is two times the trib. area. For a

2-way slab, AI equals the panel area. For a precast T-

beam, AI is the span times the full flange width.

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UBC Roof Live Loads

1997 UBC 1607.4 & Table 16-C If unbalanced loading causes maximum effects, it must be

considered.

Snow loads must be considered where they exceed the

values for the roof live loads. (See 1997 UBC Appendix tochapter 16, Div. I - Snow Load Design)

When analyzing for snow loads, must consider unbalancedloading and drifting.

Snow Loads may be reduced with increasing roof slope.

RS = S/40 - .5

RS = snow load reduction (psf) per degree slope over 20

S = total snow load (psf)