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Factor of safety for field tested connection.

Discussion in 'Residential Structural Codes' started by BayPointArchitect, Mar 9, 2018.

  1. BayPointArchitect

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    Hypothetically speaking, if a joist hanger was fabricated and tested and failed at 2,000 lbs of force, is it then acceptable to say that the custom joist hanger has a design value of 1,000 lbs?

    Thanks

    ICC Certified Plan Reviewer
    NFPA Certified Fire Plan Examiner
     
  2. Mark K

    Mark K Platinum Member

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    Suggest that this is not so simple.

    Be careful that you do not reject a joist hanger that is totally compliant. The installed capacity may have a smaller effective factor of safety because some of that factor of safety was to accommodate less than perfect installation.

    In general the design standards often produce an effective factor of safety less than 2.0. Note the load factors used are different for live loads and dead loads.
     
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  3. cda

    cda Sawhorse

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    Call the manufacture and ask??
     
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  4. Phil

    Phil Sawhorse

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    Probably better to use a safety factor of 2.5. See 2018 IBC Section 1709.3. Might be worth seeing if there are any ICC Acceptance criteria for joist hangers.
     
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  5. Mark K

    Mark K Platinum Member

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    Maybe I am missing something but the original question was about field testing which implies it is already installed.

    Section 1709 is about preconstruction load testing not testing of in place construction. It is also intended to apply to products that are not provided for in the code. The last I knew joist hangers were a commonly used product.

    Not clear why the testing is needed.

    If there is a chance of failure under the loading protocol it may be better to just replace the hanger. If the connection fails under load you not only fail the connection but you also will likely cause damage to some of the nearby construction thus resulting in the need to do even more work.
     
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  6. Phil

    Phil Sawhorse

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    Mark K,
    If needed, 1708 is the section for in-situ testing. If it is already installed and tested to failure, then, it is broken or badly distorted and no longer has any structural capacity. That is why I assumed pre-construction testing was more appropriate.

    The question is about a custom joist hanger. There is no code section on joist hangers qualifications. ICC probably has an Acceptance Criteria if you care to spend a couple hundred dollars for more information. The AC probably has a lower safety factor after destroying more specimens in a lab.
     
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  7. cda

    cda Sawhorse

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    Oh

    Was it tested at 1000??
     
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  8. steveray

    steveray Sawhorse

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    Does it have an engineers stamp on it? For construction and installation? Play through....
     
  9. BayPointArchitect

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    These custom joist hangers will be massed produced with plenty of extras. The idea is to slip beams over the top of the existing ceiling joists where the joists are currently supported by bearing walls. The LVL beams will be supported at the ends with wood columns down to foundation. The joist hangers simply grab onto the ceiling joists from above before the supporting bearing walls are removed from below. Simpson Strong Tie does not produce a "perfect" solution for this. Thus the need for a "custom" joist hanger that is similar but not the same as what is readily available from the Simpson catalogue.

    So when asking the question, "is that custom joist hanger adequate"? We can assemble together a pair of LVL's, joist hanger, and one 2 x 8 (ceiling joist), then superimpose a load of two and one-half times the (D+L) load for twenty-four hours.
     
  10. Mark K

    Mark K Platinum Member

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    Imposing a factor of safety of 2.5 is inconsistent with what the code requires in other similar situations and in my opinion is more conservative than warranted.

    This reminds me of a situation I faced many years ago. I specified a simpson strap but the plan checker objected because it was not on an approved list of products. I then modified the drawings to show a steel strap of the same dimensions as the Simpson strap with the same nailing but with the Simpson name removed from the drawing. I calculated the strength of the steel strap using code provisions. I then calculated the capacity of the nails using the code provisions for nail capacity. Then I submitted this and the owner got a permit.

    There was no reason why I had to go through these additional steps.

    Prior to requiring this testing program I suggest that the engineer review the connection and justify the capacity based on his/her calculations.
     
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  11. tmurray

    tmurray Sawhorse

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    Hanger manufacturers have more conservative load reductions than the %50 reduction stated from what they have told me.
     
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  12. BayPointArchitect

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    Thank you Mark. I understand how a calculated (non-destructive) approach makes sense. But I am just curious enough to compare calculated expectations with actual load failure. We are wanting absolute certainty that a 40% design load is acceptable by some code reference.

    The thing that got me started in this direction is that Simpson makes reference to each #10 wood screw as having a shear capacity of 180 lbs. But then they have a high concentration of screws in a very small area. So never mind the calculations. This just does not seem right to me because the wood gets too butchered with screws.

    Thanks to TMurray also. Knowing that hanger manufacturers subscribe to the same principal is helpful.
     
  13. Mark K

    Mark K Platinum Member

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    The calculation proposal respects the code and is code legal.

    What I am hearing is that because you are uncomfortable with what you see you are unwilling to accept a code compliant design. What is the technical basis for your concern? What is the code basis for rejecting calculations?

    My impression is that the nail spacing on custom hanger is the same or greater than that on the standard hanger. You appear to be willing to accept this spacing for a standard joist hanger but not for a custom joist hanger. This does not appear to make sense.

    Have you checked the code provisions regarding spacing of fasteners?

    It would not surprise me to find that the joist hanger capacity is compatible with the code values for the fasteners.

    Capacity of fasteners in wood is highly variable but this variation has been taken into consideration when establishing the code values. Because of this variation in the fastener to wood capacity using one or two tests to establish the capacity of the fastener to wood failure mode can be meaningless. If you are testing for fastener to wood capacity you would need many more tests. This can be avoided by accepting the code values for this failure mode.
     

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