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Re: URM - Ledger Connection

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Thanks for the response Ara. I spoke with Ken Kayastha in Long Beach this
morning and he confirms what you reported. The only catch here is that we
removed a mezzanine in the retrofit stage of this project. The URM wall,
which is over 120 feet long, is 100 percent solid at the line of the ledger.
The actual shear resulting from the mezzanine is less than 80 plf along the
wall. Because we are replaceing what was originally there, the plan checker
is willing to accept the ledger connection so long as we follow the
guideline for maximum shear values derived from the original 1985 UBC Table
24E (which I believed was used to determine the shear anchor value in the
RGA and UCBC). This, of course, calculates to 133% of the table value of 750
lbs per anchor or (as noted in UCBC Appendix 1 Chapter 1) 1000 lbs per shear
connection.
Reducing the anchorage spacing and deepening the ledger to allow for
appropriate staggered bolts with proper edge clearances of 4 bolt diameters
means that I will be using a ledger 4x10 rather than the 4x6 originally
calculated.

There will also be an awing of conventional wood framing added to the
structure at the entry. The city is accepting this since the combined dead
and live load (gravity) with a 4' tributary area is less than 160 plf along
the ledger. This should not impact the wall at all.

I do admit to not feeling comfortable with ledger connections in URM walls,
however, I will be using intermediate stud partitions that separate the
bathroom areas as temporary supports for the ledgers. Therefore if the wall
should give way, the ledger reactions will transfer to the ends of the
partitions sufficiently to get people away from the area.

I would need something much stronger to change the owners mind about
exposing the wall. The least I can do is resonable protect the public who
will use the building by the provisions in the code and by what the building
offial is willing to accept.

Any additional comments would be appreciated.

Dennis
-----Original Message-----
From: Amaloyan <Amaloyan(--nospam--at)aol.com>
To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
Date: Thursday, December 18, 1997 12:08 AM
Subject: Re: URM - Ledger Connection


>Dear Dennis:
>
>If I were you I would  (NOT) add any more vertical loads to the URM
>walls. If you remember City of Los Angeles does not allow additional
>loads to be imposed on the existing system. If that was not the case
>you would see new additions on top of existing URM walls.
>
>I would support the entire mezzanine independently for vertical and
>lateral loads. By connecting the walls for out of plane loads to the
>mezzanine,
>you are also introducing in plane shear loads into the wall.
>The 22 1/2 anchor bolts do transfer shear and tension by virtue of
>their geometry and use of epoxy.
>
>The answer to your question regarding actual shear capacity of a 3/4"
threaded
>rod imbedded 8" into the URM wall is:
>Depending which epoxy system you use Covert or Hilti or Epcon you will get
>4000 to 5000 lbs shear capacity. The normal safety factor
>is (5) for connections at ultimate capacity or (3) for values achieved when
>bolts
>have deflected at 1/8".
>
>You can test the specific product (min 5 bolts per wall) and use the test
>results.
>The building official in your jurisdiction may give you a modification
based
>on
>the test results. The other item that may be a factor is the bearing
capacity
>of a
>URM assembly. If I remember correctly 100 psi was the magic number.
>
>100 psi was used to get the prescriptive sizes like 6x6 plates for the
tension
>bolts
>and 4x4 angles 12" long for transfer of wall tension loads into the
diaphragm.
>
>The bottom line is do not connect the new mezzanine to the URM walls and
>convince
>the owner to have an independent structural system to be built. Tell the
owner
>when
>an earthquake hits to run and hide under the mezzanine, since the only
thing
>left standing after a long and hard shaking would be  the new structure
>;0)
>
>Happy Holidays
>
>Ara Maloyan P.E.
>
>
>