jar546
Forum Coordinator
Here is part of an article from the American Wood Council and 5 questions to answer after you read it. This is just for fun and may be a learning experience for others. Here you go:
Strength and stiffness are equally important. For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems.
Stiffness of structural members is limited by maximum allowable deflection. In other words, how much a joist or rafter bends under the maximum expected load. Only live loads are used to calculate design values for stiffness.
Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120″/360 = 1/3 inches under maximum design loads. Drywall attached to the underside of this system is not expected to crack when the floor joist system deflects 1/3″.
Typical deflection limits referenced in code books are L/360, L/240 or L/180. These limits are based on live loads and activities experienced in specific rooms of a house. Examples of code-prescribed deflection limits and live load values are:
To determine the dead load value for a given floor or roof system, the weight of all permanently installed materials in a given component are added together. For a floor system you can find the individual weights of drywall, strapping, floor joists, subfloor, underlayment and carpet in an architectural handbook like Architectural Graphic Standards. But for most cases there is a cookbook solution. Simply reference Span Tables for Joists and Rafters published by the American Wood Council (AWC). AWC’s Appendix A lists a variety of live and dead load combinations for floors, ceilings and rafters. For example, Appendix A indicates that one type of clay tile roof system has a live load value of 20 psf and a dead load value of 15 psf.
Questions:
Residential Loading
The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads.Strength and stiffness are equally important. For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems.
Stiffness of structural members is limited by maximum allowable deflection. In other words, how much a joist or rafter bends under the maximum expected load. Only live loads are used to calculate design values for stiffness.
Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120″/360 = 1/3 inches under maximum design loads. Drywall attached to the underside of this system is not expected to crack when the floor joist system deflects 1/3″.
Typical deflection limits referenced in code books are L/360, L/240 or L/180. These limits are based on live loads and activities experienced in specific rooms of a house. Examples of code-prescribed deflection limits and live load values are:
- Living room floors L/360 & 40 psf
- Bedrooms and habitable attic floors L/360 & 30 psf
- Attic floors with limited storage L/240 & 10 psf
To determine the dead load value for a given floor or roof system, the weight of all permanently installed materials in a given component are added together. For a floor system you can find the individual weights of drywall, strapping, floor joists, subfloor, underlayment and carpet in an architectural handbook like Architectural Graphic Standards. But for most cases there is a cookbook solution. Simply reference Span Tables for Joists and Rafters published by the American Wood Council (AWC). AWC’s Appendix A lists a variety of live and dead load combinations for floors, ceilings and rafters. For example, Appendix A indicates that one type of clay tile roof system has a live load value of 20 psf and a dead load value of 15 psf.
Questions:
- What is the function of beams, studs, joists and rafters in a house?
- What is the difference between strength and stiffness of structural members?
- How are maximum deflection limits expressed in building codes?
- What are some examples of code-prescribed deflection limits and live load values for specific rooms in a house?
- How can the dead load value for a floor or roof system be determined?
