1 / 47

Lecture 12 - Pan Joist and Pattern Loads

iram
Download Presentation

Lecture 12 - Pan Joist and Pattern Loads

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

    1. Lecture 12 - Pan Joist and Pattern Loads February 11, 2002 CVEN 444

    2. Lecture Goals Pan Joist Load Factors Pattern Loading

    3. Pan Joist Floor Systems

    4. Pan Joist Floor Systems

    5. Pan Joist Floor Systems

    6. Pan Joist Floor Systems

    7. Pan Joist Floor Systems

    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

    10. One-Way Joist Construction

    11. One-Way Joist Construction

    12. One-Way Joist Construction

    13. One-Way Joist Construction

    14. Pan Joist Floor Systems ACI Requirements for Joist Construction (Sec. 8.11, ACI 318-99) 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, ACI 318-99) 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

    17. Pan Joist Floor Systems Slab Thickness (cont.) Building codes give minimum fire resistance rating: 1-hour fire rating: ľ 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

    22. Pan Joist Slabs

    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 < 20 ft.: None Spans 20-30 ft.: Provided a midspan Spans > 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) Useful for selecting preliminary member sizes

    27. Member Depth ACI 318 - Table 9.5a: Min. thickness, h 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

    29. Member Depth Rule of Thumb: hb (in.) ~ L (ft.) Ex.) 30 ft. span -> 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”) See Fig. 10-7(a-e)

    33. Pattern Loads Qualitative Influence Lines Mueller-Breslau Principle Figs. 10-7(f), 10-8, 10-9 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

    36. Pattern Loads

    37. Pattern Loads

    38. Pattern Loads ACI 318-99, 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. ** For the project, we will model the entire frame. **

    39. Pattern 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. ** For the project, you may use this provision. **

    40. Project: Load Cases for Beam Design DL: Member dead load (self wt. of slab, beams, etc.) SDL: Superimposed dead load on floors LLa1: Case a1 LL (maximize +Mu/-Mu in 1st exterior beam) LLa2: Case a2 LL (optional) (maximize +Mu/-Mu in 2nd exterior beam – symmetric to 1st exterior beam)

    41. Project: Load Cases for Beam Design LLb: Case b LL (maximize +Mu in interior beams) LLc1: Case c1 LL (maximize -Mu in beams 1st interior support) LLc2: Case c2 LL (optional) (maximize -Mu in beams at 2nd interior support – symmetric to LLc)

    42. Project: Factored Load Combinations for Beam Design Factored Load Combinations: U = 1.4 (DL+SDL) + 1.7 (LLa1) U = 1.4 (DL+SDL) + 1.7 (LLa2) U = 1.4 (DL+SDL) + 1.7 (LLb) U = 1.4 (DL+SDL) + 1.7 (LLc1) U = 1.4 (DL+SDL) + 1.7 (LLc2) Envelope Load Combinations: Take maximum forces from all factored load combinations

    43. Moment Envelopes

    44. 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

    45. 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 > 20% Uniform Loads LL/DL 3 (unfactored)

    46. 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!**

    47. Approximate Analysis of Continuous Beam and One-Way Slab Systems

    48. Approximate Analysis of Continuous Beam and One-Way Slab Systems ACI Moment and Shear Coefficients See Section 8.3.3 of ACI Code

More Related