CEE 4606 - Capstone II Structural Engineering

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CEE 4606 - Capstone IIStructural Engineering

• Lecture 5 Gravity Load Design (Part 1)

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Outline

1. Review of Progress Report 1 Presentations
2. IBC Concrete Design Requirements
3. Beam One Way Slab Design
4. Slab Thickness Considerations
5. Load Path and Framing Possibilities
6. Connection Analysis Issues
7. Seismic Detailing Requirements
8. Work Tasks

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Progress Report 1 Comments

• Overall, a very good job
• Comments on presentations
• Timing good
• Dont worry about the intro stuff next time
• Know where our site is located you have
  coordinates that are accurate to within 3 miles!!!

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Progress Report 1 Comments

• Range of values
• 100 to 150 mph design wind speed
• Seismic Design Category D (unanimous)
• 2000 to 2800 psi concrete strength
• 49000 to 53400 psi steel yield strength

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IBC Concrete Design Requirements

• IBC Chapter 19
• Mimics ACI 318 Code
• IBC 2000 version based on 1999 ACI 318
• IBC 2003 will use 2002 version of ACI 318
• First seven sections (1901 1907) correspond to
  ACI 318 Chapters 1 to 7

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IBC Concrete Design Requirements

• Section 1908 gives specific modifications to ACI
  318
• Deals with meat of ACI Code
• Sections 1909 1916 deal with specialized areas
• Sec. 1910 Seismic Design Requirements
• Sec. 1912 Anchorage to Concrete
• Get to know this document!!!

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Load Path / Framing Issues

• Building Frame System
• Frame for gravity load
• Shear walls for lateral load
• Consider support of the chapel gravity loads
• Where do the columns go?
• What beams do I need?
• How do I design my slab?

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Beam One Way Slab Design Review

• We presumably know how to do the following from
  CEE 3422
• Design a rectangular beam of unknown
  cross-section size
• Design a rectangular beam of known cross-section
  size
• Design a simply supported one way slab

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Beam One Way Slab Design Review

• We presumably know how to do the following from
  CEE 3422
• Design a T-beam for positive moment
• Design a T-beam for negative moment
• Design a doubly reinforced beam (beam with
  compression reinforcement)
• Design a beam for shear

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Design of Continuous Beams and Slabs
Gap

• You know how to design cross-sections for
  positive or negative moment
• Reinforcement follows the moment diagram
• Why continuous spans?
• Moments
• Deflections

Two Simple Spans
Continuous over Center Support
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Design Moments (Uniform Dist. Loading)

• Simple Spans
• wL2/8
• Continuous Spans
• Analysis far more complicated
• What type of fixity do we actually have?
• Must consider effects of patterned loading
• Formation of plastic hinges allows for moment
  redistribution

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Design Moments Continuous Spans

• We have four analysis options
• Elastic Analysis (preferably STAAD)
• Elastic Analysis w/ Moment Redistribution
• Approximate Frame Analysis
• ACI Approximate Moment Coefficients
• See McCormac text Chapter 13

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Slab Thickness Considerations

• What governs the thickness of a slab?
• Flexural Strength
• Shear
• Deflections
• Usually, deflections will govern the thickness
  requirements for a one-way slab
• Size slab based on deflection requirements
• Check shear
• Design reinforcement for flexure

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Slab Thickness Considerations

• Review McCormac text, Ch. 5 (serviceability) and
  Ch. 3 (one-way slabs)
• Review notes from CEE 3422, lectures on one-way
  slab design and serviceability
• ACI Sec. 9.5.2.1

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Slab Thickness Considerations(such that we do
not need to compute deflections)

• For simply-supported beams, total beam depth h
  must be at least L/16
• A 16 ft. long simply supported beam must be at
  least 12 in. deep.
• For simply-supported one-way slabs, total slab
  thickness h must be at least L/20
• A 10 ft. long simply supported one-way slab must
  be at least 6 in. deep.
• You will have to look up other values!!!

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Slab Thickness Considerations

• Something to keep in mind.
• Your material properties!
• These tables are based on normal strength
  concrete
• You may wish to consider creative ways to adjust
  tables for your low concrete strength
• Hint Think about what the key concrete material
  property related to deflections is

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Load Path / Framing Possibilities

• Now we can begin to develop a framing plan for
  our structure
• Typical practice on site is a 5 in. thick slab
• We have a methodology to determine how far a slab
  of a given thickness can span
• Do our material properties have any effect?
• Lets look at a plan view of the two-story
  section

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Note columns automatically placed at each wall
end or corner
Think well need some additional framing
members???
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Framing Concepts

• Lets use a simple example for our discussion
• Column spacing
• 30 ft. on center
• Think about relating it to your design as we
  discuss

Plan
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Framing Concepts

• We can first assume that well have major girders
  running in one direction in our one-way system

Plan
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Framing Concepts

• If we span between girders with our slab, then we
  have a load path, but if the spans are too long

Plan
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Framing Concepts

• We will need to shorten up the span with
  additional beams

Plan
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Framing Concepts

• But we need to support the load from these new
  beams, so we will need additional supporting
  members

Plan
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Framing Concepts

• Now we have a viable plan
• Lets think back through our load path now to
  identify our heirarchy of members

Plan
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Framing Concepts

• One-Way Slab (continuous)
• Beams
• Interior (T-beams)
• Exterior (L-beams)
• Girders
• Interior (T-beams)
• Exterior (L-beams)

Plan
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Framing Concepts

• Note that by running the one-way slab in this EW
  direction, we are actually making the EW running
  beams our major girders
• The NS running beams simply transfer the load out
  to these girders (or directly to a column)

Plan
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Framing Concepts

• Now lets go back through with a slightly
  different load path

Plan
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Framing Concepts

• We again assume that well have major girders
  running in one direction in our one-way system

Plan
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Framing Concepts

• This time, lets think about shortening up the
  slab span by running beams into our girders.
• Our one-way slab will transfer our load to the
  beams.

Plan
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Framing Concepts

• With this approach, we have already established
  our heirarchy
• The only difference is in the direction of our
  load path
• 90 degree rotation

Plan
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Framing Concepts - Conclusions

• Either load path will work
• In this case, they are identical
• With a rectangular bay (instead of a square) bay,
  there will be a difference
• Tradeoff is usually in number of supporting
  members vs. span of supporting members

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Two Load Path Options
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Framing Concepts - Considerations

• For your structure
• Look for a natural load path
• Identify which column lines are best suited to
  having major framing members (i.e. girders)
• Assume walls are not there for structural
  support, but consider that the may help you in
  construction (forming)

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Connection / Analysis Issues

• With continuous reinforced concrete framing
  systems, connections are a major issue with
  respect to
• Detailing of reinforcement at these congested
  areas
• Assumptions regarding fixity of beams and slabs

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Connection / Analysis Issues

• Lets first consider our continuous one-way slab
  (12 strip shown) framing into an exterior
  (spandrel) beam

Plan
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Slab-Exterior Beam Connection

• Slab is a six span continuous system
• Some fixity at end of slab due to torsional
  rigidity of exterior beam, but what happens when
  beam and slab crack?
• Do we want to count on fixity?
• Also, if we design slab for negative moment here,
  we must develop reinforcement (like a cantilever)

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Slab-Exterior Beam Connection

• Typical assumptions
• Simple support at end
• No moment in slab at end
• Place some reinforcement at top of slab to
  control cracking
• Design exterior beam for minimal torsion

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Connection / Analysis Issues

• Now lets consider our beam-girder joints

Plan
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Beam-Girder Connection

• Beam is a two span continuous system
• Similar situation some fixity at end of beam due
  to torsional rigidity of exterior girder, but
  what happens when beam and girder crack?
• Do we want to count on fixity?
• Also, if we design beam for negative moment here,
  we must develop reinforcement (like a cantilever)

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Slab-Exterior Beam Connection

• Typical assumptions
• Simple support at end
• No moment in beam at end
• Place some reinforcement at top of beam to
  control cracking
• Design exterior girder for minimal torsion

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Analysis One-Way Slab T-Beams

• For the simple elements just described, where
  supports are provided by beams and girders,
• Supporting elements have some stiffness, but it
  is fairly small
• Assumption of treating one-way slabs and T-beams
  as continuous beams is valid
• A frame analysis is not needed since there are no
  columns involved
• Simple analysis methods can be used if all
  assumptions are met (i.e. ACI moment coefficients)

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Connection / Analysis Issues

• Finally, lets look at beam-column and
  girder-column joints
• Three situations
• Interior column
• Exterior column
• Corner column

Plan
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Interior Column Connection

• Girders framing in to a column
• Columns will provide some rigidity
• Moments will depend upon distribution of
  stiffness
• Frame analysis is warranted to determine these
  moments
• Unbalanced loading (patterned live load) must be
  considered
• Goal Determine moments in girders (they will not
  necessarily be equal), as well as axial load
  moment combinations for columns
• Beam/girder reinforcement must be continuous
  through joint

Plan
M cu
M2
M1
M cl
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Exterior Column Connection

• Same basic situation
• Columns will provide some rigidity
• Moments will depend upon distribution of
  stiffness
• Frame analysis is warranted to determine these
  moments
• Unbalanced loading (patterned live load) must be
  considered
• Goal Determine moments in girders (they will not
  necessarily be equal), as well as axial load
  moment combinations for columns
• Beam/girder reinforcement must be developed for
  negative moment

Plan
M cu
M1
M cl
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Corner Column Connection

• This is essentially the same situation as an
  exterior column
• Note that where we have beams (not girders)
  framing into columns, the same principles apply
• However, these moments are typically very small
  and will usually not control the design

Plan
M cu
M1
M cl
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Analysis Girders Beams Framing Into Columns

• For these elements, support is provided by
  columns
• Columns have substantial stiffness and will
  attract some moments
• Assumption of treating these girders and beams as
  continuous beams is not valid
• A frame analysis is needed to determine the
  appropriate distribution of moments
• Elastic analysis is recommended (STAAD, PCABeam)

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Seismic Detailing Requirements for Reinforced
Concrete - Introduction

• IBC Section 1910
• ACI 318-99 Chapter 21
• These two sections, together, identify specific
  detailing requirements related to seismic design
  of concrete structures
• Level of detailing required is based on Seismic
  Design Category

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Work Tasks

• Determine final loads on the structure
• Gravity loads (dead, live)
• Lateral loads (seismic, wind)
• Truss analysis on roof design of roof members
• Detailing of roof-to-structure connection
• Develop a load path (framing plan) to support the
  gravity loads associated with the second story
  chapel

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Work Tasks

• Look into how the selection of Seismic Design
  Category D will affect concrete design detailing
  requirements for your beams, columns, and slab
• Work on design of one-way slab, beams, and
  girders
• We will discuss design for shear and torsion next
  time!

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Assignment for Tuesday

• Submit a detailed sketch showing your framing
  plan (load path for gravity loads) for the second
  story chapel
• Identify all columns, beam, and girder locations,
  and specify a slab thickness
• Summarize on one sheet how the selection of
  Seismic Design Category D will affect the
  detailing of your structure
• Use a bullet item / list format to identify
  specific detailing requirements for your beams,
  columns, and slab
• Dont consider shear walls for now (they will be
  masonry)