Structural Engineering

Things to consider when considering the structural integrity of the outdoor classroom range from the weight and general conditions around the classroom. To better understand these ideas, I met with structural engineer Joe Otto of Ireland Engineering. Through some deliberation, Joe provided six areas of importance in understanding how to construct the classroom. To provide more structural integrity, joe suggested a series of 2x8 rafters made of wood which supported staggered ½ inch sheets of plywood. These sheets would then be the platform on which the corrugated polycarbonate roofing would be attached. However, to better understand the load capacity of the roof, there are some calculations that must be made about the existing members that form the structure of the classroom.
List of Things to Consider
Earthquake Loads
Wind Loads
Gravity Loads (dead and live roof loads)
Connection Details
Member Sizing
Dead and Live Load Calculations
Calculating the involved weight factors associated with the structural integrity of the classroom involves better understanding of the concepts of dead loads and live loads. The dead load of the classroom is the weight of the structure itself. Based on the dead load, there are different amounts of suggested foundation. For 1500 pounds per square foot, you need 1 square foot of foundation. Posts in contact with the soil must also be preservative treated. Furthermore, there must also be a calculating for thee loads calculated on individual members such as the bending moment, the shear, and max deflection loads.
To begin the process of understanding loads, we first must calculate the dead load derivation for each of the materials included in the structure of the classroom. This will give the dead load quantity to use in calculations for the subsequent members used to support the classroom.
The dead load derivation is calculated as follows:
Weight per square foot
Plastic Roofing
1.19 pounds per square foot
Vertical Beams
53.41 pounds / 23.11 square feet = 2.31 pounds per square foot
50.17 pounds / 14.83 square feet = 3.38 pounds per square foot
Horizontal Beam
40.7 pounds / 17.67 square feet = 2.30 pounds per square foot
Joists (2x8x16 rafters made from redwood)
54.87 pounds / 26.88 square feet = 2.04 pounds per square foot
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The dead load of the entire structure is equaled to 1.19 + 2.31 + 3.83 + 2.30 + 2.04 = 11.67 pounds per square foot. The next step in this process is calculating the total load applied by a possible live load of someone walking on the roof. This structural calculation must be made using a CBC guideline of 20 pounds per square foot for a live load; this accounts for any load placed on the roof itself. Using the plan that the cross joists on the roof will be placed 2 feet apart, there will be a final calculation of the dead load plus the live load(11.67+20) multiplied by the gap between members (2 feet). This equals the pounds per lineal foot. For this structure, it will be equaled to 63.34 pounds per lineal foot. This quantity is equaled to “W”. “L” is equaled to the span of a member. There are three different forces you must calculate for each member that already exists using the derived weight per lineal foot.
Formulas for Member Calculations
Bending Moment: M = WL^2/8
Shear: V= WL/2
delta(max deflection) = 5l^4/1728*E*I

“E” is equal to the modulus of elasticity and “I” is equal to the moment of inertia.
Calculations for Members
Bending: M = 63.34 pounds per square foot * 7.16 feet^2 / 8 = 405.89
Shear: V = 63.34 pounds per square foot * 7.16 feet / 2 = 226.75
delta(max deflection) = 5 *7.16^4/ 1728*1,400,000*76.26 = 0.000000071
Bending: M = 63.34 pounds per lineal foot * 16 feet^2/8 = 2026.88
Shear: V = 63.34 pounds per square foot * 16 feet / 2 = 506.72
delta(max deflection) = 5*16^4/1728*1,400,000*12.51 = 0.000011
Horizontal Beams
Bending: M = 63.34 pounds per lineal foot * 13.1 feet^2/8 = 1358.72
Shear: V = 63.34 pounds per lineal foot*13.1 feet / 2 = 414.877
delta(max deflection) = 5*13.1^4/1728*1,400,000*12.51=0.0000048
Certain websites were used in calculating the bending, shear, and max deflection forces acting on a ceiling joist and after comparing multiple wood species, the best suggested wood is Douglas fir for it has the longest span and therefore the ability to provide the most overhang for the roof. Here is a table summary of inputted values and the suggested total span for the length of joists. Furthermore, the values for the redwood were smaller and did not pass the requirement to be considered as roof members for total length. Furthermore, the members of the roof will not be held between beams, but instead will be placed on top of the structure to transfer the load to the two columns which are hopefully supported by some sort of foundation.
Values provided by American Wood Council Calculator
Douglas Fir
Douglas Fir
Member Size
No. 2 & Btr
No. 1 Open Grain
Member Type
Cieling Joist
Cieling Joist
Deflection Limit
24 inches
24 inches
Exterior Exposure
wet conditions
wet conditions
Live Load
20 psf
20 psf
Dead Load
10 psf
10 psf
Modulus of Elasticity
1,600,000 psi
99,000 psi
Bending Strength
1,173 psi
1,069.5 psi
Bearing Strength
625 psi
284.75 psi
Shear Strength
180 psi
155.20 psi
maximum horizontal span
13.5 feet
12.083 feet
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Earthquake and Wind Proofing
Furthermore, there also need to be calculations made to consider the potential for earthquakes and wind resistance. To account for proofing against natural weathering, there are two things that must be done. The first thing that must be done is to add knee braces on the frame of the classroom facing outward. These two diagonal supports help to keep the classroom somewhat stable when it is subject to higher wind speeds. It shouldn't be incredibly hard to implement, and will be complimented by a series of ties to help connect the classroom and the frame; by connecting both of these parts together, it will help the classroom to move as one unit during the chance of an earthquake.
Therefore, when the frame and the chicken shed which it is connected to move together, it will be a more stable structure. Both the proposed knee braces and the suggested use of ties should allow for the classroom to be a more stable structure when it reacts against natural weathering. While the load of the roof may be slightly large, the knee braces and light weight of the roof should allow for the structure to remain relatively strong. The calculations above should be compared to table values to determine the weight of the members and whether the load of the entire roof can be supported by the current structure.
Foundation Considerations
Depending on the amount of foundation already utilized to prop up the structure, the load of the roof can be increased. The current calculations for the structure suggest that the total load per lineal foot would be about 60 pounds per foot. This load is the calculated extreme for the roof and with the added live loof road, is meant to calculate the load if someone walks on top of the roof. This is likely never to happen and therefore the more consistent load will be much smaller. For this reason, the light nature of the roof and the probable existing foundation should be enough to support the frame of the classroom.
Aside from the considerations of earthquake/wind proofing and the general guidelines surrounding load distribution, there also needs to be consideration surrounding providing foundation for the posts of the classroom. Based on CBC guidelines, it is approximated that you need approximately 1 square foot of foundation for 1500 pounds per square foot in pressure. For this reason, we should most probably have about 1 square foot of foundation for both posts. Another consideration for the posts is possibly treating them with some sort of chemicals to prevent wood rotting.
Diagrams and Structural Integrity Calculations Provided by Structural Engineer
Beam and Ceiling Joist Calculations
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Beam Calculations
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Joist Calculations
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Conclusion of Structural Analysis

Furthermore, it would be easier to use smaller members such that they are a lighter weight but can still provide structure to the coveted roofing for length of the classroom. After testing multiple members sizes, using a series of 2 x 2 members could possibly allow for better roof structure and more consistent roofing. Members would only be spread apart by one foot and they could possibly support the load of the full plastic corrugated roofing. The only difference in creating this structure will be that there will need to be a third vertical member that hangs from the barn to the edge of the columns. With all of these factors considered, there are a series of new requirements for the classroom that must be implemented into the new design. They are as follows in the provided text.
Rafters for the roof will now be spaced two feet apart and will begin at the edge of the vertical members of the classroom. There will be two added knee braces and some ties to help connect the frame of the outdoor classroom to the tool shed. Furthermore there needs to be some sort of accommodation for about two square feet of foundation in total for the classroom. Using the provided load calculations and additional CBC guidelines, this plan for the classroom should be finalized.
Finalized Plan Provided by Structural Engineer
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