Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base foundationsupports the weight of its branches column and bends as it is loaded by the wind cantilever beam. A complete analysis of wood's mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables.

Let's start by taking a broad view. The structural goal of a house is to safely transfer building loads weights through the foundation to the supporting soil. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal "reaction load". If, when the loads of the house are combined, the house weighs more than the soil can support - the house will sink until it reaches a point at which the soil can support the load.

This article will focus on how simple beams like joists and rafters react to loading. The house acts as a structural system resisting dead loads weight of materialslive loads weights imposed by use and occupancylike 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. 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:.

Strength of a material is obviously important. Joists, and rafters must be strong enough not to break when loaded. Unlike stiffness, live loads and dead loads are added together to determine minimum design values for strength. 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. 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.

Many factors influence how a system responds to loading. It is important to realize that the way you select and use materials will control costs and performance.

Alright, so now you want to use this information. First you need to get a few things: Code book; AWC's Span Tables for Joists and Rafters this assigns allowable spans to various combinations of E and Fb ; and a copy of Design Values for Joists and Rafters this has Fb and E values for various species, sizes and grades of dimension lumber.

The code book can be purchased through your local code official. Building codes provide you with information about required grades, spans, bearing, lateral support, notching, etc.Some information contained in it may be outdated.

Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use. Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base foundationsupports the weight of its branches column and bends as it is loaded by the wind cantilever beam.

The structural goal of a house is to safely transfer building loads weights through the foundation to the supporting soil. Remember when your science teacher said: every action has an opposite and equal reaction? If, when the loads of the house are combined, the house weighs more than the soil can support — the house will sink until it reaches a point at which the soil can support the load.

This article will focus on how simple beams like joists and rafters react to loading. The house acts as a structural system resisting dead loads weight of materialslive loads weights imposed by use and occupancylike 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.

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. 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:. Strength of a material is obviously important. Joists, and rafters must be strong enough not to break when loaded.

Understanding Loads and Using Span Tables

Unlike stiffness, live loads and dead loads are added together to determine minimum design values for strength. 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.Low-slope roofs—i. For these projects, rain and ponding loads should be a design criteria and, when appropriate, factored into the roof design loads. When using prefabricated wood roof trusses, roof loadings noted on the structural construction drawings should indicate magnitudes of ponding loads.

Although IBC Table This is especially important for long-span roof members where deflections permitted in the code can exceed 1 inch. For example, when a long-span roof truss has an initial dead load deflection of 0. If the trusses are supported by long beams, the net deflection at the center of the truss bay is even higher. Section Roofs shall be designed for the maximum possible depth of water that will pond thereon as determined by the relative levels of roof deck and overflow weirs, scuppers, edges or serviceable drains in combination with the deflected structural elements.

In determining the maximum possible depth of water, all primary roof drainage means shall be assumed to be blocked. The maximum possible depth of water on the roof shall include the height of the water required above the inlet of the secondary roof drainage means to achieve the required flow rate of the secondary drainage means to accommodate the design rainfall rate as required by Section The required stiffness of all roof framing members should be indicated on the construction drawings, which should state the assumed loads plus either the design properties of the framing or maximum allowed deflection.

Required camber, if applicable, should also be noted on the drawings. IBC Table See IBC for footnotes. When there is the possibility of water ponding, which may cause excessive loads and additional progressive deflection, each component of the roof system, including decking, purlins, beams, girders, or other principal structural supports, should be designed accordingly.

Continuous or cantilevered components should be designed for balanced or unbalanced load, whichever produces the more critical condition. Adequate drainage capacity and proper construction details should be provided. The spring constant of a roof can be expressed as the deflection in inches per arbitrary unit of load.

It is convenient to express spring constant as inches of deflection per 5 lb of load per square foot because the weight of a 1 inch depth of water is approximately equal to a load of 5 psf. The actual weight of a 1 inch depth of water is 5. Roofs with large tributary drainage areas are, in general, more susceptible to ponding than roofs with smaller areas. Ponding problems may occur in all parts of the country.

They appear to be greater in areas where small design live loads are used. Ponding failures have occurred in semi-arid regions because rainstorms of high intensity occur, although the annual rainfall in these regions is small.Log In. Thank you for helping keep Eng-Tips Forums free from inappropriate posts.

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Are you an Engineering professional? Join Eng-Tips Forums! Join Us! By joining you are opting in to receive e-mail. Promoting, selling, recruiting, coursework and thesis posting is forbidden. Students Click Here. Related Projects. Home Forums Structural Engineers Activities Structural engineering general discussion Forum deflection of steel truss and failure deflection distance 2. Was recently called to look at a building with 43 foot steel trusses.

Trusses are on 11 foot centers. This was measured after roof was shoveled off. Any one have any idea what the total deflection for safety or roof collapse would be? While large deflections doesn't necessarily mean member failure, other factors could be influenced by the large deflections such as roof ponding. I guess this roof was in no danger of collapse at this time.

It sounds to me based upon the minimal data provided that the roof trusses may have been underdesigned from the beginning. The top chord seems too small to be much more than a chord size of 1 or 2, which for a 24" deep joist does not even seem to conform to SJI requirements.

roof truss deflection limits

And the 11' spacing seems excessive for the size and span. Without accurate data, it seems that the roof trusses may only be capable of supporting a TOTAL load of less than 20 psf, which doesn't seem correct at all.

I suggest you obtain accurate measurements of the various components and attempt to ascertain which joist configuration actually exists at the facility, using the method described in the Steel Joist Institute's 60 year manual. According to Cornell's snow load data. Medium density snow of which this was weighs twelve pounds per square foot and with almost thirty six inches on the roof it figures to I do agree that I will have to look a little more closely at it.

It is not an engineered building,so anything is possible. Also with bracing eight feet in from both sides the actual clearspan is 27 feet which should help considerably. More data always helps. When you originally wrote bracing at 8', I thought you meant lateral bracing. If the bracing is providing vertical support, it changes the situation entirely. The configuration of the bracing is critical though.There are many roles played in the design and delivery of residential wood roof trusses.

Engineers can play various roles in this process, and it is essential to understand which role you play. This article discusses the scope of work required of the various roles as defined by the various codes and standards for residential roof truss.

If a building falls within the IRC, all roles can be played by non-engineers, unless the jurisdiction requires the construction documents to be prepared by a Registered Design Professional.

It is a prescriptive code. For those elements that fall outside of the prescriptive criteria, engineering design i. The IRC does not have prescriptive provisions for the design and installation of prefabricated wood trusses, but they are allowed per Section R The applicability limits for trusses are found in Section R These must be followed in order to stay within the purview of the IRC.

The limits that apply when snow loads control the design are:. The requirements for wood Trusses in the IBC Some of the differences include:. It establishes the minimum requirements for the design and construction of metal-plate-connected wood Trusses.

Chapter 2 of this Standard defines the roles and responsibilities of the various players Owner, Building Designer, Truss Manufacturer, and Truss Designerand it is essential to know which role you are playing. Section 2. It is important to understand the bracing details in this document.

This article intends to educate engineers about the roles and division of responsibilities for residential wood Trusses. The Truss Designer is responsible for individual Truss Design Drawings using loading information obtained from the Truss Manufacturer, who gets information from the Contractor in the form of selected information from the Construction Documents.

The Building Designer is responsible for ensuring that the Truss loads given to the Truss Designer are accurate. The Building Designer is also responsible for ensuring that all Trusses act together as a roof system.

9. Setting Deflection Limits for Floors and Roofs

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roof truss deflection limits

Get help. Civil Engineering Community. Home Latest Posts. Ramp and Stair Modeling in Prota Structure. ProjectWise and MicroStation -Collaboration. Calculation and Analysis Civilax Premium Resource. The Truss Design Drawings shall be prepared by a Registered Design Professional where required by the statutes of the jurisdiction in which the project is to be constructed in accordance with Section RA primary focus of truss design is ensuring each truss component has sufficient capacity or strength to safely support design loads.

Designing components with adequate strength is very important and a primary requirement of the building code.

roof truss deflection limits

Another design requirement of equal importance is deflection and stiffness. These serviceability issues rarely affect life-safety and are sometimes marginalized by strength design.

However, the vast majority of truss complaints received by component manufacturers CMs relate to serviceability performance. An awareness of the typical serviceability issues that can adversely affect truss performance is crucial. Many potential complaints and problems can be alleviated by a truss technician during the design phase.

Section 2. Subsection g identifies six serviceability issues that are often part of the project specifications. These include:. Dead load, live load and in-service creep deflection criteria for roofs subject to ponding loads. Criteria for differential deflection from truss to truss or truss to adjacent structural member s.

Dead load, live load and in-service creep deflection criteria for floor trusses supporting stone or ceramic tile finishes. Corrosive potential from wood preservatives or other sources that can be detrimental to the trusses. Although this information may be absent from some construction documents, an astute truss design technician with a strong understanding of the various situations that cause or contribute to each of these issues can implement design practices to mitigate potential in-service problems.

In fact, many serviceability issues can be avoided by paying close attention to deflection. Deflection is the amount that a member displaces or sags under the influence of loads that create resistance forces in truss members. With the exception of scissors trusses, the performance or deflection of a truss is generally based on the amount of vertical movement relative to a horizontal line think a horizontal plane such as the ceiling plane.

The building code and industry standards provide guidance and limitations for acceptable vertical movement relative to member span. Truss deflection analysis compares calculated displacement to a code-prescribed limit.

A calculated displacement less than the prescribed limit is generally considered acceptable. However, establishing a deflection limit based solely on member span does not ensure satisfactory performance. As a span becomes longer, the corresponding deflection limits become larger i.

Subsequently, some building codes adopted stiffer floor performance parameters within code tables or by referencing an industry standard.

Tending to Deflection: Improving the Performance & Serviceability of Trusses

Table 7. These defection limits are consistent with typical minimum building code requirements but do not preclude a truss design technician from using stiffer criteria when the in-service application warrants.

Two examples help place deflections in perspective:. Some truss design software permits the truss design technician to limit the truss deflection to a specified maximum amount. This feature becomes invaluable for long truss spans or when large loads are present.

Truss design technicians can mitigate potential truss performance issues and call-backs by accounting for the magnitude of the deflection up front.

Skip to main content. The trusses were carrying almost 70, pounds at this point in the test. About the Authors: Scott Coffman, P. He is currently employed by Construction Science and Engineering as a forensic engineer specializing in construction defects.

roof truss deflection limits

Jim Vogt, P. View Digital Edition. Continuous Improvement Can Prevent Disaster.There are many roles played in the design and delivery of residential wood roof trusses. Engineers can play various roles in this process, and it is essential to understand which role you play.

This article discusses the scope of work required of the various roles as defined by the various codes and standards for residential roof truss.

If a building falls within the IRC, all roles can be played by non-engineers, unless the jurisdiction requires the construction documents to be prepared by a Registered Design Professional.

It is a prescriptive code. For those elements that fall outside of the prescriptive criteria, engineering design i. The IRC does not have prescriptive provisions for the design and installation of prefabricated wood trusses, but they are allowed per Section R The applicability limits for trusses are found in Section R These must be followed in order to stay within the purview of the IRC.

The limits that apply when snow loads control the design are:. The requirements for wood Trusses in the IBC Some of the differences include:. It establishes the minimum requirements for the design and construction of metal-plate-connected wood Trusses. Chapter 2 of this Standard defines the roles and responsibilities of the various players Owner, Building Designer, Truss Manufacturer, and Truss Designerand it is essential to know which role you are playing. Section 2. It is important to understand the bracing details in this document.

This article intends to educate engineers about the roles and division of responsibilities for residential wood Trusses. The Truss Designer is responsible for individual Truss Design Drawings using loading information obtained from the Truss Manufacturer, who gets information from the Contractor in the form of selected information from the Construction Documents.

The Building Designer is responsible for ensuring that the Truss loads given to the Truss Designer are accurate. The Building Designer is also responsible for ensuring that all Trusses act together as a roof system.

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