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Fwd: Re: [Fwd: Tilt-up roof ledgers]

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In a message dated 97-03-19 11:45:05 EST, you write:

 I am working on a tilt-up building project and have had several
 discussions as to what is the current design thinking for roof and floor
 The buildings are one story multi tenent light industrial with
 diaphragms of approximately 70 ft. by 200 ft. and 22 ft. to top of
 walls.  "Very typical".  With small buildings like this I still prefer
 to use timber ledgers designed using hankinsons formula and seperate
 HD's for wall ties.  I have also been using 300 plf. wall tie back force
 for several years and am considering increasing this to 500 plf.
 I have had several engineers tell me that I should be using steel angle
 ledgers because it avoids the cross grain bending problem and the use of
 hankinsons formula.  I have concerns about the use of ledgers unless
 prying is considered at the ledger with regards to the angle or channel
 legder thickness and also the tension loads on the bolts.  Seems that
 the use of HD's would still be a good idea but this defeats the use of
 the steel ledger.
 Am I off in left field with an old way of thinking (An old xxxx that
 need to move forward), or are timber ledgers with HD's still considered
 acceptable.  I have used steel ledgers on large warehouse buildings
 where the spans were large.
 Would appreciate some other oppinions.
 Thanks in advance.
 Jim Dane, P.E.


I would be comfortable with still using the wood ledgers with pairs of
holdowns for the wall ties at the roof, but might consider using steel at the
floor depending on the live load requirements.  By using the steel channel or
ledger,  I would agree you still need to check for prying action on the bolt
due to the vertical gravity load and the lateral load pulling on the
horizontal leg of the angle or top flange of the channel.  If you check
combined shear and tension the bolts obviously have less capacity for
transfering shear.  I wonder  how many engineers check for prying action on
the angle installed using a single horizontal row of bolts vs just assuming
the connection is in pure shear due to vertical loads. 

The other question is installation, is the contractor going to make a
templete to match the anchor bolt pattern for shop drilling (most likely not)
or start burning holes in the web in the field (with weld washers ?) to erect
the ledger (or possibly use epoxy anchors for erection through the shop
drilled holes).  It is much easier to use a wood ledger for matching the
embedded anchor bolt pattern and keeps the carpenter happy and reduces costs
on the job (less steel) and can be reliable if you design for the proper
out-of-plane wall forces.

How do you make the connection of the wall ties from the purlin to the
channel?, are this bolted straps each side of the purlin welded to the
vertical leg of  the angle or the web of the channel?.  I would think this is
more expensive then using the holdowns which I guess could be used instead of
the steel straps.  I also would think you may still need one bolt through the
angle ledger within 6" each side of the purlin to help spread the load into
the wall and minmize out-of-plane bending of the ledger.  You still need a
wood nailer on top of the channel for the plywood attachment.  Depending on
where the nails are placed relative to the nailer bolts, you can still have
cross grain tension on the nailer (nails located between nailer edge away
from the wall face and nailer bolts) as the wall trys to pull away so you
have not eliminated cross grain tension by using steel.

As Brian Cain mentioned, take a look at the SEAOC Blue Book,  I think you
will be designing for higher out-of-plane forces then 300 plf depending on
how thick your wall is.     

My two cents worth.

Michael Cochran
Brain L. Cochran Associates
Los Angeles, CA.

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Richard Lewis, P.E.
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