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A guide for understanding footing size calculations in post-supported structures
with details on how the FootingPad® calculator works I. Introduction II. Determining the footing size needed for post-in-the-ground structures III. FootingPad size chart IV. Load Calculation formula applied to post frame (pole barn) construction IV. Example – Post Frame footing calculation V. Footing Load Calculation formula applied to deck construction VI. Basic deck components: A top view of a rectangular deck VII. Example – Deck post footing size calculation Appendix 1: Example - Post Frame footing calculation, side wall door Appendix 2: Deck footing calculation, snow load higher than 40lbs/sq. ft
Footing Size Calculations in Post Foundation Structures
I. Introduction
Footings are an important structural component of any post supported structure. Some simple math is needed to ensure the footing is the correct size. Armed with the knowledge in this guide, anyone can understand and comfortably discuss the footing sizes needed for their project with an inspector or other building official. The methodology in this guide applies to determining the footing size needed, regardless of the material from which it is made. Isolated Footing
FootingPad diameter Surface area of FootingPad (sq. ft) 10”. 12”. 16” 1. 20” 2. 24” 3. FootingPad post foundations are round, and the surface area of any circle is determined by the formula: Pi r 2
Or: 3.14 x (radius x radius)
The surface area of a square or rectangle is determined by length x width. For example, a 12” x 12” square has a surface area of 1 sq. foot.
III. Surface Area of FootingPad and other round footings
III. Load Calculation formula applied to post frame (pole barn) construction Post frame construction footing size calculations are easily determined because of its simple load bearing structure. Typical post frame construction involves a series of posts opposite each other with a truss or rafters spanning the distance. The ultimate load is the defined by area of the roof carried by each post. The farther apart the posts are spaced, both in building width and side wall post spacing, the more load on each post. As typical post frame construction will have sidewall posts spaced equally, each post carries approximately the same weight. There is little weight on the end walls, so to calculate the load on the footing, the only data points needed are the building width and post spacing. The FootingPad calculator pre-fills the dead weight load of a common steel-roofed at 5 lbs. per square foot. Top View – Post Frame Structure = Post = Truss/rafter = Roof Surface = Section of roof carried by one post = Roof ridge Side View
FootingPad diameter Surface area of FootingPad (sq. ft) 10”. 12”. 16” 1. 20” 2. 24” 3. Using the chart below, any footing larger than 1.33 square feet will work in our example. The 16” FootingPad will be recommended by our calculator using the data supplied in this example
V. Footing Load Calculation formula applied to deck construction To determine the footing size needed for a deck post, the same formula is used: A = B/C Where A = Surface area of the footing B = Load on the footing in pounds C = Load carrying capacity of the soil in pounds per square foot Because decks are not uniform in construction, defining the load carried by each footing, called the tributary area (TA), is dependent on design of the deck, especially the placement of beams and posts. In our footing size calculator, the data you input is used to identify the footing which has the largest tributary area, and subsequently, the largest load. This allows the calculator to provide a FootingPad large enough for all posts. In practice, once you understand the calculations, you can easily determine the footing size needed for each individual post for each project.
Using the same deck as on prior page, we show how to identify the load on any given footing, which we use to determine footing size. To determine the tributary area of each post/footing, you must know the distance between posts and between beams. The TA is a measure of the length and the width that a post and footing carry. By taking ½ the distance between one post in each direction you can determine how much surface area of the deck that post and footing will carry. 20 feet (width of deck) House 1 4
15.5 feet (Projection of deck) 1.5 feet 2 8 feet 8 feet feet 2 feet 8 feet 6 feet TA= 8’ x 7’ = 56 sq. ft Red arrows = distance between posts Blue Arrows = distance between beams/overhang Dotted lines = Trib Area of each post/footing Yellow outline = Largest Trib Area TA= 6’ x 7’ = 42 sq. ft = post / footing (^) = beam = ledger
VII. Example – Deck post footing size calculation
Side View Top View TA= 8’ x 5.5’ = 44 sq. ft
Note this deck has a ledger (green), which means the deck is connected to the house, which will carry part of the load. In our example, we can look at any post/footing to determine its tributary area. We will examine the calculations for Post 1 and Post 2. A single post will have one footing to support its load. They are identified in our example by the red circle with a number. In this example Footing 1 carries half the distance to the house (half of 6’ = 3’) in one direction and half the distance to the post under the nearest beam (post # 4) (half of 8’ = 4’) in the other. So post # carries 7 feet (3’ + 4’) in one direction (projection of the deck away from the house) In the other direction (width of the deck), Footing # will carry the overhang (the distance between the post and the edge of the deck = 2’) and half the distance to post #2 (half of 8’ = 4’). For a total of 6’ Therefore the tributary area of Footing #1 = 6’ x 7’ or 42 square feet. This is 6’ of width and 7’ of projection. Normal decks are considered to have a dead load of 10 lbs. per square foot and a live load of 40 lbs. per square foot. This example applies to areas where the snow load is LESS THAN 40 pounds per square foot (psf). See Appendix 2 for an example of a snow load HIGHER THAN 40 psf The 42 square feet of Footing 1 multiplied by 50 lbs./per square foot = 2100 lbs. of total load on Footing #1. See next page for calculations of the footing size. With the data we collect on the footingpad.com calculator, we need to identify the largest tributary area (TA) of the deck. In the example, that TA is identified by a yellow border carried by Footing #2. As the beam that post/footing #2 supports is 6’ from the house and the next beam is 8’ away the projection carried by #2 is 7’ (½ of 6 feet + ½ of 8 feet). The width of post 2’s TA is dependent on the spacing of the posts along the beam. Since they are 8’ apart, post #2 carries 4’ in both directions, for a total of 8 feet. The TA of post #5 is 7’ projection + 8’ width or 8 x 8 = 56 square feet. Often, as in this example, an interior footing will have the largest tributary area.
8 feet Appendix 1: Load Calculation formula for post frame door on SIDE wall Many post frame buildings have overhead or other large doors on the end walls, which carry little structural weight. If your post frame building has a door on the SIDE wall, and that door is WIDER than your post spacing, you will have extra weight on the posts on either side of the door. This will require a separate footing size calculation. Because of the wider spacing of the posts on either side of the wide garage door, there is more roof carried by those posts which increases the load. Therefore the calculation changes slightly. See next page. Top View – Post Frame Structure with side wall door = Post = Truss/rafter = Roof Surface = Section of roof carried by one post = Roof ridge 8 feet 16 feet 8 feet 8 feet 8 feet = Garage Door on side wall Side View
Top View – one side Y Y^1 Y Y^ Y Using the same data from the earlier post frame example: Building Width: 40 feet Soil Capacity: 3000 psf Post Spacing (Y): 8 feet Post Spacing – door (Y^1 ): 16 feet Snow Load 20 lbs. / square foot Surface area of footing (in sq. ft) = Total load (in lbs.) on footing / Soil Capacity Total load = (½ building width x (½ Y + ½ Y^1 Post spacing) x (5 lbs. dead load^1 + snow load^2 ) = 20’ x (1/2 of 8’ + ½ of 16’) x (5 + 20 lbs. / square foot) = 20’ x (4’ + 8’) = 240 square feet x 25 lbs. / square foot = 6000 lbs. Surface area of footing (in sq. ft) = 6000 / 3000 Surface area of footing (in sq. ft) = 2. As the 20” FootingPad is larger than the required 2.0 square feet, this size is needed for those posts on either side of the door. FootingPad diameter Surface area of FootingPad (sq. ft) 10”. 12”. 16” 1. 20” 2. 24” 3.
