Sizing of Wood Beams Using Span Tables

Posted on January 27, 2017
Category: Structural

The sizing of wood beams can be accomplished in several ways. There is a great deal of information available on the web such as calculators and tabulated data to help you select appropriate sized wood members for simple loading conditions. This article will focus on using American Wood Council (AWC) Span Tables in conjunction with the Massachusetts State Building Code to size sawn lumber joists. This information is most relevant to residential dwellings.

Needed Information:

Span tables are a very effective and simple way of sizing joists. You will need the following information to determine joists sizes:
1. Length of span (distance between face of end supports)

 
2. Uniform live load to be supported

Typical Minimum Live Load Values:

(From Mass Building Code Seventh Edition – 780 CMR Table 5301.5)

Use Live Load (psf)
Attics without Storage 10
Attics with storage 20
Sleeping rooms 30
Rooms other than sleeping rooms 40
Decks 40
Exterior balconies 60

 

3. Deflection limit
Typical Minimum Deflection Limits:

(From Mass Building Code Seventh Edition – 780 CMR Table 5301.5)

Structural Member Deflection Limit
Rafters with slope > 3/12 with no attached finished ceiling L/180
Floors and plastered ceilings L/360
All other structural members L/240

 

4. Uniform dead load to be supported

The following tables provide typical dead load values for common building materials. The dead load values from these table should be added together depending on the construction. When in doubt, it is always better to overestimate.

Typical Minimum Design Dead Load Values – Wood Joist Floor with ¾” Plywood Subfloor:

(From ASCE 7-10 Table C3-1)

  Design Dead Load (psf)
Joist Size (in.) 12-in. Spacing 16-in. Spacing 24-in. Spacing
2 x 6 6 5 5
2 x 8 6 6 5
2 x 10 7 6 6
2 x 12 8 7 6

 

Typical Minimum Design Dead Load Values – Floor Finishes:

(From ASCE 7-10 Table C3-1)

Floor Finish Design Dead Load (psf)
Concrete fill finish (per inch thickness) 12
Hardwood flooring, 7/7-in. 4
Linoleum or asphalt tile, ¼” 1
Slate (per mm thickness) 15
Solid flat tile on 1-in. mortar base 23

 

Typical Minimum Design Dead Load Values – Ceilings Finishes:

(From ASCE 7-10 Table C3-1)

Ceiling Finish Design Dead Load (psf)
Acoustical fiber board 1
Gypsum board (per 1/8-in. thickness) 0.55
Plaster on wood lath 8
Suspended metal lath and cement plaster 15
Suspended metal lath and gypsum plaster 10
Wood furring suspension system 2.5
Plywood (per 1/8-in. thickness) 0.4

 

Typical Minimum Design Dead Load Values – Other Dead Loads for Consideration:

(From ASCE 7-10 Table C3-1)

Other Components Design Dead Load (psf)
Mechanical Duct Allowance 4
Cellular glass insulation (per 1-in. thickness) 0.7
Fibrous glass insulation (per 1-in. thickness) 1.1
Fiberboard insulation (per 1-in. thickness) 1.5

 

Sizing the Beam:

After you have selected the appropriate loads and deflection requirements from the tables above, it is now time to size your beam using the Span Tables from the AWC. A free download of the span tables can be found at this link:

http://www.awc.org/codes-standards/spantables

To use the span tables, you first need to find the right table. As an example, let’s say that I am sizing a floor joist with the following criteria:

Length of Span = 14 feet

Uniform Live Load = 40 psf

Deflection Limit = L/360

Uniform Dead Load = 20psf

Use the table of contents to find the appropriate table based on your design criteria. My example corresponds to table F-5. Let’s say that based on dimensional requirements for my project, I want to use a 2×10 spaced at 16” on center.

Table1

In the span table find where it says 2×10 for the joist size and go to the row for 16” spacing. Move across the row to the right until you come to a column with a span length that is greater than or equal to what you need. In this case, it was exactly 14’-0”. At the top of the column you will find the required modulus of elasticity for the beam to meet deflection requirements. In this case, it is 1,000,000 psi. Next go the bottom portion of the table, labeled as “Fb”, in the same column and row level as your selected span to find the required bending stress. In this case, it is 1,101 psi.

Now that you have the required elastic modulus and bending stress you can find the appropriate species and grade of lumber in the AWC National Design Specification (NDS) Supplement. This information can be found at this link:

http://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2015-Supplement-ViewOnly-1411.pdf

For this example, let’s assume that Spruce–Pine-Fir is available to us. Find the Spruce–Pine– Fir section of Table 4A in the NDS Supplement.

Table2

Recall that from the span tables we required a minimum elastic modulus value of 1,000,000 psi to meet deflection requirements. Based on Table 4A shown above, all grades of Spruce-Pine-Fir meet the minimum requirement. The minimum bending stress value from the span tables is 1,101psi. From Table 4A, only the Select Structural Grade of Spruce-Pine-Fir meets the minimum requirement for bending strength therefore a Select Structural – Spruce Pine-Fir 2×10 spaced at 16” on-center would be acceptable for use.

Closing Notes:

The span tables from NDS are an easy way of sizing wood beams for simple situations. More complex situations may require the use of the NDS design formulas and more in depth engineering. When in doubt it is always best to contact an engineer. Note that the example shown above is only one way to size a beam using span tables. In the example, we started with the beam size because we knew what beam size we wanted to use. You could always start with the span length and find the appropriate size beam. An iterative process is sometimes required to find a combination that will work for your project.

Duct Sizing Methodologies

Posted on December 18, 2016
Category: HVAC, Mechanical

Duct sizing is a challenge contractors, designers, and engineers face every day. There are three primary methods used sizing ductwork and each has specific applications and uses. Duct sizing can be complicated, but in many situations all you have to do is take a step back and consider the application and specific goals. Today we’re going to break down the three primary methods used for sizing commercial ductwork and provide a couple common applications for each. In the end it’s up to the person responsible to determine the appropriate method and if you need specific advice about an application contact us for professional guidance.

 

1) Velocity method
Using duct velocity to size ductwork is simple and can be quickly calculated in the field. Different environments and applications require different velocities. For example, medium pressure duct mains over corridors and other occupied spaces should be limited to 1,500 feet per minute (FPM). Below is a table with common duct velocities that can be used for many applications; however, it is up to the designer of record to make the final velocity determination for each specific application.
  Commercial Systems Industrial Systems High Velocity Systems
  FPM   FPM   FPM
Mains   1000-1500   1500-2400   1700-3250
Primary Branches   600-1000   1000-1500   1100-2000
Branches and Runouts   200-500   600-1000   800-1300
2) Equal friction loss method

The equal friction loss method is the most commonly used method because it is simple, flexible, and accurate. Most commercial, industrial, and even residential systems are sized using this method. Duct systems sized using this method generally use the least amount of space and have the lowest initial cost. The equal friction method is based upon a constant pressure loss per unit length (i.e. 0.08 in.wc. per 100 ft.) plus the pressure loss through each fitting. The fan is selected based upon the airflow and the pressure drop through the duct run with the highest pressure drop. When using this method the system is not self-balancing and requires a testing, adjusting, and balancing technician to balance the system. Below is a table with common friction losses that can be used for many applications; however, it is up to the designer of record to make the final determination for each specific application.

  Commercial Systems Industrial Systems High Velocity Systems
  in.wc.  /100 ft.   in.wc.  /100 ft.   in.wc.  /100 ft.
Mains   0.15   0.20   0.30
Primary Branches   0.10   0.15   0.20
Branches and Runouts   0.06   0.10   0.12
Many contractors, designers and engineers will us a combination of the first two methods by limiting the velocity and limiting the friction loss.
3) Static regain method
The final method is the most complicated and as a result it’s not used that often. However, it is more energy efficient and is self balancing when designed properly. The most common application is in a gymnasium or hockey arena, where manually balancing the system is not practical due to height or space limitations.  When using the static regain method ducts are designed so that the available static pressure is used to offset the friction loss on the subsequent section of duct. This ensures that the static pressure available at the end of each branch is equal throughout the system resulting in equal airflow at each terminal. More complicated calculations, procedures and diagrams are required to ensure this method is executed properly. There is also little room for error because the system cannot be manually balanced or corrected after construction.