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Floor Truss Strongback

Floor Truss Strongback

Floor Truss Strongback

(OP)
When is it a REQUIREMENT to install strongbacks ? Most publications list it as a RECOMENDATION and not required.
When L/480 or L/600 is specified, does that require strongbacks ? Are the building codes specific on this ?
Thanks for any feedback.

 

RE: Floor Truss Strongback

The typical use of strongbacks is to mediate bounciness of floors by spreading an individual truss deflection over three to five trusses. This is a quality issue not a code issue, as such, they are not required by the IBC and IRC building codes.

Garth Dreger PE
AZ Phoenix area

RE: Floor Truss Strongback

Darth is correct - but anything over about 14' probably should have at least one.

RE: Floor Truss Strongback

(OP)
Should put it in, I agree. But how do you tell an inspector that it is not required, after the framer and builder refuse to install it.

RE: Floor Truss Strongback

Who sealed the plans??  Does the truss mfg show it.

IF they do - you probably don't have a snowball's chance in Hell changing their mind - unless the truss plate engineer gives you a new drawing.

Why not just do it??  It can't cost that much?? And it does serve a purpose.

RE: Floor Truss Strongback

If it is on the approved plans, then it is required.  As a subset of the administrative portion of the IBC, approved plans cannot be changed except by the design professional.  What is shown on the approved plans is therefore mandated by code.

RE: Floor Truss Strongback

In addition to load sharing to eliminate excessive differences in deflection of adjacent trusses, strongbacks are also used during erection to prevent roll over.

RE: Floor Truss Strongback

The framer and builder don't get to decide whether or not to install.  It is up to the engineer (either the EOR or the delegated truss engineer).  

RE: Floor Truss Strongback

I see no reason why you should tell an inspector that it is not required.
It's currently the builder's problem; let it remain so.

RE: Floor Truss Strongback

Review the expected truss deflections:
Where adjacent trusses can expect differential deflections, the need for strongbacks to reduce sheetrock cracking is justified. The need is greater for high climatic/humidity changes, large snow load variations, and/or different interior bearing conditions. The truss engineer can justify specifying strongbacks rather than being subject to future expense for callbacks to fix sheetrock problems.

RE: Floor Truss Strongback

I thought this was what bridging was for??????????   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Floor Truss Strongback

Strongbacks are not typically used as structural components.
They are typically not required for strength of the floor system.  Your floor will not collapse if you don't install them :)

Where strong-backs come into play is in the serviceability
aspect of design.  Codes usually require X-bridging or blocking in between joists at certain intervals to help resist vibration and create a "system action" for performance of the floor system.

Strongbacks are easier to install than X-bridging and as a result have become the predominate  vibration resisting mechanism for open web floor joists.


There is no set rule for strong back size or spacing.  Depending on the methodology used for calculating vibration characteristics of floors, engineers will use the stiffness of the strong backs in the calculations.  

Strongbacks perform better at mid span and areas of max. deflection.  The deeper the strongback the better it resists the effects of floor vibration.  

If you have forgotten to install, and the engineering drawings show them, contact the truss designer so that they can provide an alternate detail.  If you have short span engineered open web joists, (<10 or 12 ft) its not as critical to have strongbacks.

 

_________________
C

RE: Floor Truss Strongback

A detail or sketch would be nice and add to the understanding of the discussion...

RE: Floor Truss Strongback

Ahhh my friend the strongback.....As a former Truss-Man I will tell you that everyone has a different opinion. The design check program places them where the plate company wants them or the program realizes a CSI/design/code value could be altered for strength. My advice is to follow the
Truss Calc design as shown. Many times truss designers run that CSI up to high levels to be competitive so using the strongback is a must. Strongbacks are not x-bridging. Strongbacks are for the structural integrity of the (diaphragm) system. X-bridging is never called out on a truss diagram and is specified the SE. One time I had a contractor scream at me how the strongbacks are there to transfer lateral load....so he could ignore the x-bridging, well it was a $15,000 mistake when the sheetrock came back down to install it.  

RE: Floor Truss Strongback

jg is correct here.

Also if the drawing was sealed and the jobsite is in an area in which a seal is required, they are required if they are designed to be in there. You may be able to get another PE to seal the drawings without them "if" they pass all appropriate calculations.

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