There are circumstances when the deflection requirements for a specific structure that utilizes floor trusses as structural members are questioned. The current residential and commercial building codes provide minimum design requirements for loads and for deflection of structural floor members, which also apply to floor trusses. The building designer may specify more stringent requirements. This guide will guide the designer in complying with manufacturer or trade association deflection requirements for a number of floor topping/covering related products.

Step-by-Step: 

Intro

Introduction

  • Current building codes provide minimum design requirements for floor deflection
  • Manufacturers or trade associations often recommend more stringent deflection requirements to address serviceability or appearance for a variety of products
  • This presentation gives a step by step approach to ensure the proper application of floor structural member deflection limits when using certain types of floor toppings

Step 1

Check Applicable Building Codes

  • Deflection of structural members is covered by the building codes.
  • IBC Table 1604.3 contains deflection limits for floor structural members, including creep
  • The IRC also provides deflection limits in Table R301.7, although creep is not addressed

Step 2

Identify Minimum Deflection Limits

  • ANSI/TPI 1 (referenced by the IBC and IRC) includes:
    • Additional information required on the construction documents in order to effectively design trusses for serviceability
    • Detailed information regarding deflection and creep in floor trusses

Step 3

Check Trade Association Guidelines

  • Trade associations or other groups often recommend more stringent deflection limits than code minimum requirements.
  • Additional recommendations may apply, including:
    • Reduced on-center spacing of structural members
    • Specific subfloor & underlayment requirements
    • Maximum dimensions of topping or variation in thickness
    • Other detailing requirements

Step 4

Creep

Total Load Deflection = Live Load Deflection + Dead Load Deflection

TL = ∆LL + ∆DL  

 

Time dependent deformation under long term loading (Creep)

LongTerm = KCR x ∆LT + ∆ST

 

  KCR =   Creep factor

         >   2.0 for trusses using seasoned lumber used in dry service conditions

         >   3.0 for trusses using green lumber or for wet service conditions

LongTerm = Total long term deflection due to immediate deflection of both short term and long term loads and creep deflection of long term loads

LT = Immediate deflection due to the long term component of the design load (immediate deflection due to the portion of load considered to be present over a sustained time period, typically dead load or a portion of the dead load

ST = Deflection due to short term or normal component of the design load (deflection due to transient loads, typically live loads)

 

For purposes of deflection limitations in accordance with the IBC, trusses using only seasoned lumber used in dry service conditions shall determine the deflection for the total load check as follows:

CR =   Deflection due to live load plus creep component of deflection due to dead load

 

                = ∆LL + (KCR -1) x ∆DL

  KCR =   Creep factor

         >   2.0 for trusses using seasoned lumber used in dry service conditions

         >   3.0 for trusses using green lumber or for wet service conditions

 

Step 5

Verify in Design Software

  • Truss design software typically addresses code deflection requirements by default but the designer needs to verify the settings are correct for their design.
  • The building designer may specify more stringent criteria.
  • Given complete and accurate loading and serviceability information, truss designs will account for all the considerations and include pertinent information on the Truss Design Drawing.