Introduction and Building Loads

1
Introductionand Building Loads

• CE 636 - Design of Multi-Story Structures
• T. B. Quimby
• UAA 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 of building systems.

3
Expected Outcomes

• At the conclusion of this course, the students
  will have
• an understanding of the engineering design
  process as it relates to building structural
  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 story
  to 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 lateral loads to be seen during construction
  and service.
• Must consider sequential loading (particularly
  during construction) 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,
  visual and 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 SIMPLY SUPPORTED members.
• Use structural analysis theory to find the path
  that loads take as they find their way down to
  the foundation through the structural members.

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Example 1

• Applied load is uniformly distributed.

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Tributary widths of beams supporting joists
coming in at odd angles
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Beam AB
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Beam BC
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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.

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Example 2

• Identify the Tributary Areas for
• 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

• 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 are typically
  expressed in terms of unit loads (the weight
  per unit of surface area).
• Permanently attached equipment and machinery 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 412.
• 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
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DL Calculation 2
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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
  typical field 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(1DL/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(.2515/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 to chapter 16, Div. I - Snow Load
  Design)
• When analyzing for snow loads, must consider
  unbalanced loading 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)