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Lecture 12 Pan Joist and Pattern Loads

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Lecture 12 - Pan Joist and Pattern Loads. September 27, 2002. CVEN 444. Lecture Goals ... Pattern Loading. One way Slabs. Pan Joist Floor ... Pattern Loads ... – PowerPoint PPT presentation

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Title: Lecture 12 Pan Joist and Pattern Loads


1
Lecture 12 - Pan Joist and Pattern Loads
  • September 27, 2002
  • CVEN 444

2
Lecture Goals
  • Pan Joist
  • Load Factors
  • Pattern Loading
  • One way Slabs

3
Pan Joist Floor Systems
View of Pan Joist Slab from Below
Walter P. Moore Assoc.
4
Pan Joist Floor Systems
Walter P. Moore Assoc.
View of Double Skip Joist Slab from Below
5
Pan Joist Floor Systems
Placing Reinforcement for a Pan Joist Slab
Walter P. Moore Assoc.
6
Pan Joist Floor Systems
General framing layout of the pan joist system.
7
Pan Joist Floor Systems
Pouring a Pan Joist Slab
Walter P. Moore Assoc.
8
Pan Joist Floor Systems
  • Definition The type of slab is also called a
    ribbed slab. It consists of a floor slab,
    usually 2-4 in. thick, supported by reinforced
    concrete ribs. The ribs are usually tapered and
    uniformly spaced at distances that do not exceed
    30 in. The ribs are supported on girders that
    rest on columns. In some ribbed slabs, the
    space between ribs may be filled with permanent
    fillers to provide a horizontal slab soffit.

9
One-Way Joist Construction
Definition Pan joist floor systems are series
of closely spaced cast-in-place T-beams or joists
used for long-span floors with relatively light
loads. Typically removable metal forms (fillers
or pans) are used to form joists.
MacGregor, Fig. 10-28
10
One-Way Joist Construction
Details of ribbed floor with removable steel pans.
Ribbed-floor cross sections.
11
One-Way Joist Construction
The design of a ribbed floor with steel pan forms
and average weight of the floor.
12
One-Way Joist Construction
The design of a ribbed floor with steel pan forms
and average weight of the floor.
13
One-Way Joist Construction
Joist Details
14
Pan Joist Floor Systems
  • ACI Requirements for Joist Construction
  • (Sec. 8.11, ACI 318-02)
  • Slabs and ribs must be cast monolithically.
  • Ribs must be spaced consistently
  • Ribs may not be less than 4 inches in width

15
Pan Joist Floor Systems
  • ACI Requirements for Joist Construction (cont.)
  • (Sec. 8.11.2, ACI 318-02)
  • Depth of ribs may not be more than 3.5 times the
    minimum rib width
  • Clear spacing between ribs shall not exceed 30
    inches.
  • Ribbed slabs not meeting these requirements
    are designed as slabs and beams.

16
Pan Joist Floor Systems
  • Slab Thickness
  • (ACI Sec. 8.11.6.1)
  • t 2 in. for joints formed with 20 in. wide
    pans
  • t 2.5 in. for joints formed with 30 in. wide
    pans (1/12 distance)

17
Pan Joist Floor Systems
  • Slab Thickness (cont.)
  • Building codes give minimum fire resistance
    rating
  • 1-hour fire rating ¾ in. cover, 3-3.5 slab
    thickness
  • 2-hour fire rating 1 in. cover, 4.5 slab
    thickness

18
Pan Joist Floor Systems
  • Standard Removable Form Dimensions
  • Note the shapes

19
Pan Joist Floor Systems
  • Standard Removable Form Dimensions
  • Standard Widths 20 in. 30 in. (measured at
    bottom of ribs)
  • Standard Depths 6, 8, 10, 12, 14, 16 or 20 in.

20
Pan Joist Floor Systems
  • Standard Removable Form Dimensions (cont.)
  • End Forms one end is closed (built-in) to form
    the supporting beam
  • Tapered End Forms provide additional shear
    capacity at ends of joists by tapering ends to
    increase rib width.

21
Pan Joist Slabs
Standard Pan Joist Form Dimensions Ref. CECO
Concrete Construction Catalog
22
Pan Joist Slabs
Standard Pan Joist Form Dimensions Ref. CECO
Concrete Construction Catalog
23
Pan Joist Floor Systems
  • Laying Out Pan Joist Floors
  • Rib/slab thickness
  • Governed by strength, fire rating, available
    space
  • Overall depth and rib thickness
  • Governed by deflections and shear

24
Pan Joist Floor Systems
  • Laying Out Pan Joist Floors (cont.)
  • Typically no stirrups are used in joists
  • Reducing Forming Costs
  • Use constant joist depth for entire floor
  • Use same depth for joists and beams (not always
    possible)

25
Pan Joist Floor Systems
  • Distribution Ribs
  • Placed perpendicular to joists
  • Spans lt 20 ft. None
  • Spans 20-30 ft. Provided a midspan
  • Spans gt 30 ft. Provided at third-points
  • At least one continuous 4 bar is provided at top
    and bottom of distribution rib.
  • Note not required by ACI Code, but typically
    used in construction

26
Member Depth
  • ACI provides minimum member depth and slab
    thickness requirements that can be used without a
    deflection calculation (Sec. 9.5 ACI 318)
  • Useful for selecting preliminary member sizes

27
Member Depth
  • ACI 318 - Table 9.5a
  • Min. thickness, h (for beams or ribbed one-way
    slab)
  • For beams with one end continuous L/18.5
  • For beams with both ends continuous L/21
  • L is span length in inches
  • Table 9.5a usually gives a depth too shallow for
    design, but should be checked as a minimum.

28
Member Depth
ACI 318-99 Table 9.5a
29
Member Depth
  • Rule of Thumb
  • hb (in.) L (ft.)
  • Ex.) 30 ft. span -gt hb 30 in.
  • May be a little large, but okay as a start to
    calc. DL
  • Another Rule of Thumb
  • wDL (web below slab) 15 (wSDL wLL)
  • Note For design, start with maximum moment for
    beam to finalize depth.
  • Select b as a function of d
  • b (0.45 to 0.65) (d)

30
Pattern Loads
  • Using influence lines to determine pattern loads
  • Largest moments in a continuous beam or frame
    occur when some spans are loaded and others are
    not.
  • Influence lines are used to determine which spans
    to load and which spans not to load.

31
Pattern Loads
  • Influence Line graph of variation of shear,
    moment, or other effect at one particular point
    in a structure due to a unit load moving across
    the structure.

32
Pattern Loads
  • Quantitative Influence Lines
  • Ordinate are calculated (exact)

MacGregor (1997)
33
Pattern Loads
  • Qualitative Influence Lines
  • Mueller-Breslau Principle
  • Used to provide a qualitative guide to the shape
    of the influence line

34
Pattern Loads
  • Qualitative Influence Lines (cont.)
  • For moments
  • Insert pin at location of interest
  • Twist beam on either side of pin
  • Other supports are unyielding, so distorted shape
    may be easily drawn.
  • For frames, joints are assumed free to rotate,
    assume members are rigidly connected (angle
    between members does not change)

35
Qualitative Influence Lines
The Mueller-Breslau principle can be stated as
follows If a function at a point on a structure,
such as reaction, or shear, or moment is allowed
to act without restraint, the deflected shape of
the structure, to some scale, represents the
influence line of the function.
36
Pattern Loads
Qualitative Influence Lines
Fig. 10-7 (b,f) from MacGregor (1997)
37
Pattern Loads
  • Frame Example
  • Maximize M at point B.
  • Draw qualitative influence lines.
  • Resulting pattern load
  • checkerboard pattern

38
Pattern Loads
  • Arrangement of Live Loads (ACI 318-02, Sec.
    8.9.1)
  • It shall be permitted to assume that
  • The live load is applied only to the floor or
    roof under consideration, and
  • The far ends of columns built integrally with the
    structure are considered to be fixed.

39
Pattern Loads
  • Arrangement of Live Loads ACI 318-99, Sec.
    8.9.2
  • It shall be permitted to assume that the
    arrangement of live load is limited to
    combinations of
  • Factored dead load on all spans with full
    factored live load on two adjacent spans.
  • Factored dead load on all spans with full
    factored live load on alternate spans.

40
MomentEnvelopes
The moment envelope curve defines the extreme
boundary values of bending moment along the beam
due to critical placements of design live loading.
Fig. 10-10 MacGregor (1997)
41
Approximate Analysis of Continuous Beam and
One-Way Slab Systems
  • ACI Moment and Shear Coefficients
  • Approximate moments and shears permitted for
    design of continuous beams and one-way slabs
  • Section 8.3.3 of ACI Code

42
Approximate Analysis of Continuous Beam and
One-Way Slab Systems
  • ACI Moment and Shear Coefficients - Requirements
  • Two or more spans
  • Approximately Equal Spans
  • Larger of 2 adjacent spans not greater than
    shorter by gt 20
  • Uniform Loads
  • LL/DL 3 (unfactored)

43
Approximate Analysis of Continuous Beam and
One-Way Slab Systems
  • ACI Moment and Shear Coefficients - Requirements
    ( cont.)
  • Prismatic members
  • Same A, I, E throughout member length
  • Beams must be in braced frame without significant
    moments due to lateral forces
  • Not state in Code, but necessary for coefficients
    to apply.
  • All these requirements must be met to use the
    coefficients!

44
Approximate Analysis of Continuous Beam and
One-Way Slab Systems
ACI Moment and Shear Coefficients Methodology
wu Total factored dead and live load per
unit length Cm Moment coefficient Cv Shear
coefficient ln Clear span length for span in
question for Mu at interior face of exterior
support, Mu and Vu ln Average of clear span
length for adjacent spans for Mu at interior
supports
45
Approximate Analysis of Continuous Beam and
One-Way Slab Systems
  • ACI Moment and Shear Coefficients
  • See Section 8.3.3 of ACI Code

46
Homework
ETABS Problem. (Due 10/4/02) It will be up
later this afternoon.
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