CME 342: Floor Framing

Floor Framing Notes

Introduction

  • Course: CME 342

  • Instructor: Paul Crovella

Floor Systems Overview

  • Platform Framing

    • Based on platform framing with many years of using balloon framing.

    • Key Features:

    • Allows building of 3 or more stories from short (8’) lumber.

    • Safe fire construction methods employed.

    • Wall heights fit modular materials (e.g., 4 x 8 sheathing).

    • Utilizes the least expensive lumber available.

Historical Context

  • Balloon Framing

    • Used from mid-1800s to mid-1900s.

    • Constructed first and second story walls simultaneously with floors hung on the sides.

    • Fire Hazards:

    • Allowed rapid fire spread within the walls.

    • Required high-quality 16’ lumber.

    • Walls typically sheathed with solid boards.

Components of a Platform Frame

  • Key components include:

    • Sill Plate (also referred to as mudsill):

    • Pressure treated lumber that is bolted to the foundation.

    • Anchor bolts are set every 4’ or 6’ apart.

    • Can be leveled if the foundation is not level, sealed with sill sealer or caulk.

    • Girder:

    • Supports floors spanning more than 16’.

    • Typically made of Steel, LVL (Laminated Veneer Lumber), or built-up solid sawn wood.

    • Size of girder specified on the plans.

    • Floor Joists.

    • Bridging.

    • Sheathing (Subfloor).

Structural Considerations

Support Points

  • Girders, interior basement walls, exterior foundation walls, and columns serve as support points for floor frame sections.

On-Center Spacings

  • Floor joists must be placed to support both edges of an 8’ (96”) sheet of plywood.

  • Allowable joist spacings include:

    • 12”, 16”, 19.2”, and 24”.

  • Note: When starting the layout of joists from the edge of the floor, the first center-to-center distance is ¾” less than the standard spacing.

Sizing Floor Joists

  • NYS Building Code provides span tables based on:

    • Dead Load: 10 or 20 psf.

    • Live Load: 30 or 40 psf.

    • Joist Spacing: 12, 16, 19.2, or 24”.

    • Species of Wood: Douglas fir-larch, Hem-fir, Southern Pine, or Spruce-pine-fir.

    • Grade: SS, #1, #2, #3.

  • These factors determine the acceptable span for different sizes of joists: 2x6s, 2x8s, 2x10s, or 2x12s.

Examples of Span Tables

  • Table R502.3.1(1): Maximum floor joist spans for common lumber species, under residential sleeping areas with a live load of 30 psf and a dead load of 10 psf:

    • Douglas Fir-Larch:

      • SS: 2x6 (12'-6"), 2x8 (16'-6"), 2x10 (21'-0"), 2x12 (25'-7")

      • #1: 2x6 (12'-0"), 2x8 (15'-10"), etc.

      • #2: 2x6 (11'-10"), 2x8 (15'-7"), etc.

    • Hem-Fir and Southern Pine tables with similar configurations.

Floor Layout Example

  • Room dimensions include:

    • Master Bedroom: 12'-5" x 12'-7"

    • Dining Room: 8'-5" x 12'-10"

    • Bedroom #2: 10'-0" x 11'-5"

  • Layout questions:

    • (1) Where will the girder be placed?

    • (2) Depth of floor joists if spacing is 12” o.c. with SPF #3.

Special Framing Techniques

  • Extra Framing Requirements:

    • Framing around openings for stairs, chimneys, and mechanical ducts should include extra support.

    • Under walls parallel to joists and under bathtubs, additional framing is necessary.

    • Cantilevered joists should be modified to ensure adequate support.

    • Bridging must be installed on joists that extend over 8’.

Openings Framing

  • Framing for Openings:

    • Doubling of joists on each side of openings to provide increased support.

    • Perpendicular members referred to as “Headers” and parallel members as “Trimmers.”

    • Locations for full joists are filled with “tail joists.”

Cantilevered Joists

  • Used in applications such as:

    • Bay windows, second and third-floor decks, and steep slope sites.

  • Joists should extend back into framing three times the distance they extend out.

  • When bearing walls are present, cantilevers can extend the depth of the joist.

  • Blocking (not bridging) should be installed between joists at the support point.

Framing Around Bathtubs

  • When walls run parallel to joists and support a bathtub, joists must be doubled for extra support.

  • Spacing between joists allows for plumbing and electrical installations.

Bridging Details

  • Purpose of Bridging:

    • Supports floor joists to prevent twisting under load (reduce lateral torsional buckling), and aids in load sharing.

    • Generally required at spans exceeding 8’.

  • Types of Bridging Include:

    • Wood cross-bridging.

    • Wood solid bridging.

    • Metal cross-bridging.

Cutting Joists

  • Cutting of joists by other trades (electrical, plumbing, HVAC) is common.

  • Holes must not significantly weaken the joists, especially crucial for I-joists.

Advanced Framing Techniques

  • Designs requiring joists to span over 16’ often utilize I-joists and open-web joists (parallel chord truss).

  • Engineered wood products are favorable for spans approaching 40’.

Safety and Building Codes

  • Joists must be installed following the manufacturer's guidelines, and no alterations are allowed without approval from a registered professional.

  • Case Study:

    • A tragedy occurred involving a volunteer Deputy Fire Chief who fell through a floor hole during fire response on June 25, 2006.

    • Recommendations for fire departments include:

    • Acknowledging that unprotected pre-engineered I-joist systems fail more rapidly than solid wood joists in fire situations.

Engineered Wood Products Details

  • Components include:

    • TJI Rim Joist (9-1/2" and 11-7/8").

    • Specific blocking panels for additional structural integrity.

    • Use of squash blocks and web stiffeners where appropriate.

Floor Sheathing

  • Tongue and Groove Subflooring:

    • To be installed with construction adhesive for improved durability and reduced noise.

    • Available in both veneered plywood and composite materials.

    • Thickness based on span rating.

Estimating Materials

  • Components & Calculations Include:

    1. Sill Plate (pressure treated)

    2. Joists based on spacing (standard framing: 12, 16, 19.2, or 24” o.c.): length based on increments from 8’ to 16’.

    3. Special framing includes extra joists for parallel walls and openings.

    4. Bridging calculation.

    5. Header joists calculated at two times the long dimension of floor.

    6. Subflooring estimated based on area divided by sheet dimensions (4’x8’).

  • Example Calculation:

    • Using Hem-Fir #1 spaced at 19.2” o.c., consider:

    1. Perimeter length of sill plate.

    2. Number of standard framing joists.

    3. Quantity of joists for special framing.

    4. Total bridging request.

    5. Total header jots requirements

    6. Total sheets of T&G subfloor based on area.

Total Material Estimation Example

  • Example for a floor frame:

    1. Sill Plate = 2 x (28’ + 36’)

    2. Number of standard joists from table R502.3.3(1) spans

    3. Calculate support for parallel walls, tubs, stair openings and bridging requirements

    4. Total area calculation for subflooring (divide total floor area by 32).