Reinforcing Bottom Chord of a Flat, Metal-plated Floor Truss To Decrease Deflection

[DCEngr1]

Does anyone know of a known-successful method & procedure to add section to the bottom chord of a wood, metal-plated flat floor truss to decrease the deflection ?

 

[msquared48]

Can you post a diagram or picture of the truss?

Mike McCann
MMC Engineering

 

[DCEngr1]

Mike . . . attached.

Thank you.

Dana

 

[XR250]

I assume this is already installed. What is the actual issue and what is it supporting (other than just open floor loads)?

 

[JedClampett]

First, you need to temporarily shore the member to reduce the deflection. Otherwise, you're going to "lock in" the existng deflection.
But after that, it's a matter of increasing the Moment of Inertia. For a timber truss, the most efficient way to do this is to add steel plates to the top and bottom chords. Double the Moment of Inertia and you've cut the deflection in half. Just make sure the shear (that shear flow calculation that no one ever uses) is transferred from the timber to the steel along the chord.

 

[woodman88]

"known-successful method & procedure"
I doubt if there is one for this span and depth (24' x 18") at 24" oc. Maybe adding 2x12 x 24' DF-L SS to each face of each truss would help to decrease the deflection enough. At least the contractor would be able to attach the 2x12's in the field. Doubt if they can move all the pipes, etc. to place them though.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

At the center of the truss span, there is a box section. Could an additional beam be fished through there, or is this for Mechanical?

Mike McCann
MMC Engineering

 

[DCEngr1]

These trusses are already in place. My thought is to shore the floor structure to establish a camber, add sufficient section to the side(s) of the bottom chord, and the top chord if necessary - enough section to reduce the total deflection, under total dead and live load, to 1/240. As designed, the calculated deflection under full load is ~ 1/4 inch greater than allowable.

 

[SteelPE]

I don't think doubling the moment of inertia would exactly cut the deflection in half as shear deformation of the truss would also need to be considered. This is typically ignored in a standard deflection analysis of a solid beam as it tends to be minimal. I seem to remember using 85% of the calculated I as being a decent approximation for the deflection seen by a truss.

 

[EngineeringEric]

I have never done it so i am not sure on feasibility.... but you can shore and add plywood gussets to the length of the truss... this will give you a pretty good increase in Ixx and probably isn't too difficult to install without worrying of compromising the chords. Again, this is only an idea not sure if it is a good one