Valley frame load atop existing trusses
Valley frame load atop existing trusses
(OP)
From Thread337-99026 Ann1971 (Structural) writes;
A building built in the 60's is having an addition put on. It will require that overframing be put on the existing wood trusses. What exactly am I required to check before the overframing can be put on. I was going to verify the connection of the existing truss to the wall and if this does not hold to the current spec. then I was going to increase it. Thanks in advance.
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pmkPE (Structural) responds;
You need to check the resultant stresses from the increased dead load and potential new live loads. This can be very difficult without having the original design drawings to go by in regards to foundation capacities as well as members that are hidden from view. Also, their are no doubt existing stresses on your structure; they must be lkess than the allowable stresses for the existing materials resulting in a reserve capacity. Is the reserve capacity sufficient for the new and potentially increased loads. Also, do you local governing codes require you to bring the structure up to date - could be depending on the percentage of the structure to be renovated.
Have you been able to determine the relative exact construction materials and members as well as their mechanical properties?
In regards to what you are responsible to check. Imagine that there is a construction failure of the building after your design is incorporated (or even during its renovation). You must have performed a reasonable amount of investigation/evaluation etc to show that you did at least the minimum amount necessary (per the standards of care) to ensure that the existing structure, as well as the new, was adequate for the actual and anticipated loadings; not only as prescribed by your governing codes but also per your engineering judgement based on what you new or should have been able to know at the tine of your design.
A building built in the 60's is having an addition put on. It will require that overframing be put on the existing wood trusses. What exactly am I required to check before the overframing can be put on. I was going to verify the connection of the existing truss to the wall and if this does not hold to the current spec. then I was going to increase it. Thanks in advance.
------------------------------------------------------------
pmkPE (Structural) responds;
You need to check the resultant stresses from the increased dead load and potential new live loads. This can be very difficult without having the original design drawings to go by in regards to foundation capacities as well as members that are hidden from view. Also, their are no doubt existing stresses on your structure; they must be lkess than the allowable stresses for the existing materials resulting in a reserve capacity. Is the reserve capacity sufficient for the new and potentially increased loads. Also, do you local governing codes require you to bring the structure up to date - could be depending on the percentage of the structure to be renovated.
Have you been able to determine the relative exact construction materials and members as well as their mechanical properties?
In regards to what you are responsible to check. Imagine that there is a construction failure of the building after your design is incorporated (or even during its renovation). You must have performed a reasonable amount of investigation/evaluation etc to show that you did at least the minimum amount necessary (per the standards of care) to ensure that the existing structure, as well as the new, was adequate for the actual and anticipated loadings; not only as prescribed by your governing codes but also per your engineering judgement based on what you new or should have been able to know at the tine of your design.





RE: Valley frame load atop existing trusses
To actually solve the problem, the following steps might include:
Review the original truss engineering obtainable from
-Building department records
-Local truss fabricators
-Builder / owner records
-Fabricator+LoadSpec placard near bottom chord centerline
If you strike out on the above steps, and you probably will for older jobs, here are three approaches to solving the problem:
A.Determine and resolve the additional member stresses
1.Identify the metal plate supplier (a die-stamped mark on the plate)
2.Determine allowable MPC values (ICBO legacy report archives)
3.Analyze the truss using TPI (Truss Plate Institute) ANSI standards
4.Validate or modify the original trusses
B.Use a local truss fabricators review calculations
1.Create a truss calculation engineering request (Here's an example)
2.Locate a local truss fabricator (WTCA membership list)
3.Obtain fabricator truss calcs specific to your request
4.Certify the trusses will carry the additional load or;
5.Prepare truss modification (repair) engineering
C.Hire a truss engineering specialist to prepare an analysis and repair solution
This last approach is usually the quickest (typically one day)
RE: Valley frame load atop existing trusses