Combined Roof and Floor Loading
- by Moe Ghaedi -
 

It is not uncommon to design trusses with combined roof and floor loading. It is very important to make sure that the loading of this type of component is accurate and the structure is not under-designed or overdesigned. An under-designed structure could prove to be dangerous and over-designing a component could be inefficient.

The following two methods can simplify the loading procedure when designing a component with combined floor, wall, and roof load.

 

Option I: (Simple, One Load Case)

·         In the "Standard" load case, run the truss with roof live/construction load and roof dead loads.

·         Add all the live and dead loads from floor systems to this load case.
·         Add all the dead loads from walls (if applicable) to this load case.
·         Lumber and plate duration (Stress Increase) for this load case must be set at 0%.

·         Note this option may be a bit more conservative for plates and lumber depending on proportions of roof and floor live loads.

 

 
Option II :( Two Load Cases)

    Standard Load Case   

·         In the "Standard" load case, run the truss with roof live/construction load and roof dead loads.

·         Add all the live and dead loads from floor systems to this load case.
·         Add all the dead loads from walls (if applicable) to this load case.
·         Lumber and plate duration (Stress Increase) for this load case must be set at 25% or 15%.
 
Load Case #1
·         Create a "Load Case #1" by copying the "Standard" load case, and change the load duration factor for this load case to 0% for both lumber and plates.

·         Remove all roof live/construction load components from "Load Case 1".

·         "Load Case #1" shall include all floor live loads and all the dead loads from roof, floor, and walls.

 

 
 
 

Additional notes:

1.    Deflection limits criteria for floor loading controls. Set live load deflection to L/d = 360 or as required by building designer. Set the total deflection to L/d =240 or as required by building designer.

2.    Top chord of wall girders, as is required of all other trusses, must be laterally braced if not connected to structural rigid sheathing.

3.    Wind loading must always be applied to trusses with combined roof and floor loading.

4.    Load duration factor or stress increase factor is 15% for snow regions and 25% for non-snow regions.  
5.    Unbalanced loads are automatically considered based on the "Standard Load Case" for trusses in the 4x2 orientation with chase openings, cantilevers or multiple bearings.  For trusses in the 2x_ orientation, the user must click on the "Unbalanced Partial Loading" button to consider unbalanced loading. 

 

Please feel free to contact our office for further discussion of this subject.

                                                       

- Employee Spotlight - 
Moe Ghaedi

Moe was born and raised in Iran and finished his primary education there.  In 1966, he attended the Artisan School of the National Iranian Oil Company.  Following that, he joined the work force at the Abadan refinery of Iran working as an operator in several departments including distillation and utilities.  Moe moved to Tampa, Florida in 1978 and began working in the retail truss and lumber industry.  Moe earned a Civil Engineering degree from the University of South Florida in 1999 and joined the Robbins Engineering staff.

Moe just recently celebrated his 30th wedding anniversary to his lovely wife Lynda and they have one daughter, Laila.  Moe loves soccer, movies, and animals and enjoys the company of his 3 dogs, 3 cats, and 4 fish.

 

  

 
 
6904 Parke East Blvd Tampa Florida 33610
Phone: 813-972-1135  Fax: 813-971-6117  Email: TPA-Information@mii.com