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wood screws

wood screws

wood screws

I'm an intermediate engineer who has worked almost exclusively with steel and concrete.  I am now working on the design of a curtain wall/window system where the clips that connect the vertical mullions to the structure are fastened to wood blocking with wood screws.  I found the allowable withdrawl and shear values of the screws and looked over the NDS manual so I think I'm headed in the right direction but I have 3 items that I'm concerned about.
  The first is that I have looked at all the factors and they should be 1.0.....except I'm not sure about load duration (Cd)I want this to be 1.0 also but I'm not convinced this is correct due to the fact that my load is essentially from wind(ASCE7-98).
    Secondly because my load is due to wind am I allowed a 33% increase in stresses?
    Third..am I correct in using an interaction equation with actual withdrawl & shear values divided by thier respective allowables if this is less than or equal to 1.0 fastener is ok?
   Thanks in advance for any help...if there is a good reference for these questions let me know

RE: wood screws

I typically use a safety factor of at least 3 and up to 5 for fasteners in direct tension that are not "through-bolts", which would include screws, wedge anchors, epoxy anchors and the like.

In wood, you have to be concerned about several things.  First, wood will shrink in a controlled environment and expand when wet.  These actions change the fastener engagement characteristics.  Secondly, a curtain wall system is subjected to significant temperature variations and the associated thermal movement tend to work pretty hard on the fasteners.  Third, if you are not using stainless steel fasteners, you can count on some corrosion which initially helps pullout values, but if there is loss of cross section can cause fastener failure.

If you are connecting an aluminum system into the wood, you should certainly use stainless steel fasteners, otherwise the dissimilar metals in contact will cause the steel fasteners to corrode and fail.

Under ASCE 7-98, the fasteners should be designed on their tributary area for "components and cladding".  For curtain walls, the tributary area of the fastener might be fairly large.  As for the 33% overstress...only valid if you are using allowable stress design throughout and ASCE 7-98 tends to be a bit unclear on this point.  Theoretically and in past codes, that approach should be fine.  I usually use it, unless there is some compelling reason to be a bit more conservative.

RE: wood screws

thanks for your input Ron..I did use the C$C values for computing my wind pressures but your comment on tributary area is causing me to rethink the values I had used for my external pressure coefficient.  I had chosen a value from the table to determine the loading on the mullion then simply designed my connection to the resulting reaction...since my tributary area to the connection is different then I would have to recalc the wind load with a different ext coeff...
  I don't believe I'll have an issue with thermal movements since the clips are not fastened to the mullions they are just sliped in and are not fastened to the mullion.
 I hate the fact that I need a saftey factor on the screws but is does seem appropriate.  I know when I design hilti expansion anchors they say to decrease the published values of allowable tension by I think it was 35%....3 to 5 seems a little high I will get some input from the engineers in my office as well.
   what do you think about useing an interaction equation
(ft/Ft)+(fv/Fv)=1.0 for the combined loading?

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