REVIEWED BLD2023-0886+Calculations+8.24.2023_8.20.23_AM+3744560REVIEWED
BY
CITY OF EDMONDS
DANIEL C. SMITH, PE
Consulting Engineers
5068 Isabella Cannon Dr
Raleigh NC 27612
P 919-844-6050 ext 1
decksOdansmithpe.com
August 23, 2023
Project: 23-D-036 WA Catalina Mathieu Edmonds, WA
Subject: Load Calcs for Ftg Diameter; Wind Uplift; Framing; Lateral Load; Guardrail
Introduction:
BLD2023-0886
RECEIVED
Aug 24 2023
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
The footing calculations determine the minimum required footing diameter and depth based upon
deck height, frost depth, and uplift forces. This value is determined by the force on the girder posts
acting against the soil bearing capacity.
Wind Speed = 110 Wind Exposure = C
Footing Diameter Calculation:
For primary case condition, we use a tributary area of 36 square feet, and a total load (combined
dead load and live load) of 70 pounds per square foot. We reach a maximum force on the footing of
2520 pounds.
DCSPE utilizes soil formulas to account for both soil bearing pressure and friction resistance along
sides of footings. The side friction component is only used on footings at least 24" deep Thus, for a
24" deep footing, DCSPE used an equivalent soil bearing pressure of 2400 pounds.
DCSPE calculated required footing sizes of 13.83" (16" minimum) for primary condition. DCSPE
specified footings of 16", diameter in the appropriate locations on the plans. 16" > 14" OK
Wind Uplift Calculation for Footing Depth:
The Default 7-10 calculated wind uplift = -3.43 psf. This uplift force is calculated as (DL - (0.6 x
WL)), with DL =10 psf and WL = -22.39 psf, with the negative value indicating uplift. Total uplift
force applied to the deck is (Deck Area x Wind Uplift) = 587 lbs.
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The total uplift force is compared against the weight of the footings, which is calculated as (130 x
((3.14 x diameterA2)/(4 x 144))). With 4 footings on this project, the total weight of the footings must
exceed-586.21 lbs, which averages to 147 lbs per footing.
Using conservative concrete weight estimate of 130 lbs/cf, each footing must have at least 1.13 cf of
concrete. Thus, the worst case, smallest footing (16" diameter aka 1.40 so requires a concrete depth
of 10". DCSPE uses a minimum depth of 12" in all cases. DCSPE specified footings 24" deep. 24"
x 12" OK
As an alternative calculation, the worst case, smallest footing (16" diameter aka 1.40 so at 24" deep
has 2.79 of of concrete, resulting in a conservative concrete weight of 363 lbs. 363# x 147# OK
Framing Calculation:
The attached deck is long overall by wide overall and 9.17high. Loads are 60 LL,10 LL, and 70
psf total load. Framing lumber is pressure treated HF #2.
DCSPE calculates framing members used on each deck. Design values from the American Wood
Council calculator are used for floor joist equations. Girders equations use BeamChek design
70# DL+LL Allowable Spans (ft)
HF#2
FJ @ 16" o.c. Girder w/ 7 ft TW
2x 4
2x 6
2x 8
5.75
2x10
12.83 7.25
1 Unshaded = OK
2x12
14.83
8.25
This framing plan specifies HF #2 floor joists as a minimum of 2x10 @ 16" o.c. for a span of 11.00'
and HF #2 girder as a minimum of (2) 2x 8 for a span of 5.50'.
Lateral Load/Seismic Calculation:
Seismic is not an issue for the way DCSPE designs decks in the vast majority of situations. Only
exception would be for decks greater than 11 feet above grade, in which case, one or more posts
would be buried into the ground.
The attached deck is long overall by wide overall at 15 DL conservative (< 10 DL design value).
The deck would have a total dead load of 2,565 lbs.
For a wood connected structure, the typical Cs value would be around 0.06 of the deck load. A
conservative seismic load would be Cs = 0.12 (Cs = Sds / (R/I)) of the total load, or for the example
deck 2,565 lbs x 0.22 = 564 lbs of lateral movement form the entire deck.
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Resistance: Shear at post or columns is equal to 1 posts x 2 bolts x 440 lbs shear per bolt = 880 lbs.
The shear resistance of the deck with (4) columns is structurally adequate and meets Code.
Lateral Resistance: DCSPE specifies tension ties by USP LTS19-TZ, which requre (8) 10d nails 0.148"
dia. x 3" long into joist and (1) 2-3/8" dia. lag bolt into the top plate or band board. The allowable
tension of each strap is listed as 1,200 lbs, mimimum of (4) straps provides 4,800 lbs resistance.
DCSPE calculates the seismic force = 564 lbs < 4,800 lbs OK
Guardrail Post Calculation:
Railing Guardrail Post has a allowed 200 lb concentrated load.
See attached (2) sketches of the guardrail post. A Finite Element Model was made to evaluate the
forces in the bolts. The loads are shown in sketch 2 of 2.
Steel bolts are typically 50,000 psi yeild. The area of a 1/2" diameter bolt is 0.19 sq in.
The calculated stress is 1431 lb force / 0.19 = 7,531 psi < .6(50,000) = 30,000 psi Allowable Pressure on
the wood - standard 1/2" bolt washer is 1.37" diam. =1.47 sq in
We subtract the shaft of the bolt 1.47 - 0.19 =1.28 sq in
Hem -Fir compression perp to grain is 405 psi, Cd = 2.0
1431 lbs / (1.28 sq in x 2) = 558 psi > 405 psi. The band board will crush a little bit.
The calculated deflection is 1/4 inch of the 48" max. tall guardrail post. OK
Summary:
The engineer certifies that the structural design specified on the plans is structurally adequate to
support the deck loads.
Respectfully Submitted,
Daniel C. Smith, P.E.
Footing Diameter
PASSED
Wind Uplift
PASSED
Framing
PASSED
Lateral Loads
PASSED
Guardrail Post
PASSED
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