Stairs Terminology and Design Rules

Why Stairs Important consideration in a structure design function Accommodate traffic flow from one level to another Key to circulation in a plan Code requirements based upon safety established by occupant count

Stair Groupings based on Material 1--Wood 2--Steel 3--Concrete

Types of Stairs Basic types straight run right angle or “L” double “L” reverse or “U” winder spiral or curved

Straight Run does not change direction typical width is 3’-0” to 3’-6” (residential) typical width for commercial construction is set by code simple popular

Right Angle or “L” Intermediate platform between floors A 90 degree directional change occurs, usually near the center Platform at directional change of stairs Platform provides a resting area between flights of stairs Platform

Double “L” Same as “L” but with multiple platforms Platform Platform

Reverse or U A 180 degree directional turn occurs, usually near center Platform at direction change Platform length is 2 (x) stair width Platform

Winder Wedge shape steps Steps continue in place of landing Unsafe at the small of the wedge The design: arc in the area of the winder is half the stair width and the tread design is 10” 10” tread width

Spiral and Curved Spiral Stairs Curved Stairs Used as an additional stair--not used as the main stairs, follow code requirements Used for unique design with requirements such as limited or tight spaces aesthetics extra/addition stairs Curved Stairs

Stair Terminology Total Rise vertical distance between floors Run/Unit Run (design run) horizontal distance from riser to riser, horizontal member Total Run horizontal distance between first and last riser Rise/Unit Rise (design rise) vertical distance between stairs/treads, vertical member Rise total rise Run total run

Stair Terminology Landing “floor” at the bottom and top of a flight of stairs Headroom vertical distance from outside edge of step (nosing) to ceiling headroom stringer landing Stringer/Carriage/Stair Jack member on which risers and treads rest, provides structural support for risers and treads

Stair Terminology Nosing Tread Riser projection of tread beyond riser usually 1 1/8” Tread equal to unit run + nosing Riser equal to unit rise

Stair Terminology Plain Stringer Closed/Housed Stringer notched out 2x12 to support the treads and risers Closed/Housed Stringer stairs ends at stringer and are not exposed, stringer usually routed so the treads and risers will fit into it

Stair Terminology at a glance Run (Unit Run) Rise (Unit Rise) Total run Total rise Tread Riser Nosing Stringer (carriage) Headroom Clearance Stairwell Landing (fin fl) Kick board

Stairs Framing Terms double header double trimmer stringers joist hangers stairwell

Design Considerations Stairs should be comfortable to climb 30 - 35 degree angle is optimum to prevent fatigue Max of 16 steps between landings Design rules have been developed with years of study and experimentation

Design Considerations Every step should be same size Absolute minimum ratio of riser to tread 4” riser with 11” tread Absolute maximum ratio of riser to tread 7” riser with no maximum tread Optimum ratios 7” riser (unit rise) 11” tread (unit run) includes nosing

Design Rules Riser + Tread = 17” - 17 1/2” [book] (17”- 18” usually) 2 (x) Riser + Tread = 24” - 25” Riser (x) Tread = 70 - 75 sq inches

Design Rule Example Normal conditions 7 + 10=17 (required 17 to 18) If: Riser = 7 and Tread = 10 7 + 10=17 (required 17 to 18) 2 x 7 + 10 = 24 (required 24 to 25) 7 x 10 = 70 (required 70 to 75) All conditions conform to rules

Design Rule Example A minimum condition If: Riser = 5 and Tread = 14 5 + 14 = 19 (required 17 to 18) 2 (x) 5 + 14 = 24 (required 24 to 25) 5 (x) 14 = 70 (required 70 to 75) Only two rules are satisfied, okay since at least one rule works

Stair Calculations Necessary to determine exact riser height and total run 1. determine vertical distance between finished floors 2. divide vertical distance by approximate desired riser height to set approximate number of risers 3. round to number of whole risers 4. divide vertical distance by number of risers to get actual riser height 5. number of unit runs = number or unit rise minus 1 6. Total run: use design rule to determine unit run size then multiply by the number of unit runs

Stair Calculations Example Given: vertical distance = 102” 1--approx # unit rise = 102 / 7.5 = 13.6 2--round 13.6 to 14 3--unit rise height = 102 / 14 = 7.286” 4--14 unit rise (-) 1 = 13 unit runs 5--design rule: R + T = 17.286 (therefore) 7.286 + T = 17.286(then) T = 10” 6--total run = 10 * 13 = 130” = 10’-10” Solution: 14 R @ 7.286” and 13 T @ 10”

Example of a Wood Stair Section

Example of an Exterior Concrete Stair Section

Stair Section A A--Section View B1 B--Structural Connection Details B2 Scale of sectional drawing (1/2” = 1’-0”) Coordinate sectional view with details (notations) B--Structural Connection Details Scale of detail drawings: 1”= 1’-0” Details to draw 1) top of stair run 2) intermediate landings/platforms 3) bottom of stair run (More) B1 B2 B3

Stair Section View Add height and width of platform Add 2x4 kick board Add 2x6 ledger @ platform (down steps area) Determine number and size of Riser and Tread include on your drawing in a note Show total run on each flight of stairs (More)

Structural Connection Details scale 1”=1’-0” Detail at Top of Stair connection note use of joist hanger

Structural Connection Details Stringer to Landing connection note use of ledger Add platform joist spacing 16”OC 2x6 Ledger 2x4 ledger beyond (hidden lines) (More)

Structural Connection Details Bottom of Stair connection note 2 x 4 plate (kick board) & notched stringer Joist are 2x8 @ 16” OC

ASSIGNMENT On sheet A-7 create the stair cross section A/A-7 and B/A-7 Develop a landing detail 1/A-7 Develop a bottom of stair detail 2/A-7 Develop a top of stair detail 3/A-7 Include handrail information on any or all of the above