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Re: Retrofit Wall Anchorage

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Section 1633.2.8.1 is the overriding requirement in your case "design of
out-of-plane anchorage to "Flexibale diaphragms"" and therefore, Rp = 3.0
and ap=1.5 is applicable.  Also note various factors given in 1633.
thru 5.

"Mike O'Brien" <mikeo(--nospam--at)> on 08/03/99 02:07:31 PM

Please respond to seaint(--nospam--at)

 To:      seaint(--nospam--at)                                   
 cc:      (bcc: Niaz A. Nazir/EQE)                            
 Subject: Retrofit Wall Anchorage                             

I am retrofitting the wall anchorage on an existing single-story concrete
tilt-up building with a wood panelized roof in Orange County.  I plan on
using 3/4" diameter epoxy anchor bolts into the 6" thick concrete walls for
the HDs or Zone4 CTs.  Thru bolts are not possible due to architectural
considerations.  The code appears to be unclear on what force these anchor
bolts need to be designed for.

Formula 32-2 of the '97 UBC is the applicable equation, but the proper
for the Rp factor is ambiguous.  On page 2-18 of the code in the right
column about 3/4 of the way down the page, the paragraph reads "Rp is the
Component Response Modification Factor that shall be taken from Table 16-O,
except that Rp for anchorages shall equal 1.5 for shallow expansion anchor
bolts, shallow chemical anchors or shallow cast-in-place anchors.  Shallow
anchors are those with an embedment length-to-diameter ratio of less that
When anchorage is constructed of nonductile materials, or by the use of
adhesive, Rp shall equal 1.0"

This brings up the question of what is the difference between a chemical
anchor and an adhesive.  This is unclear.  What would a Hilti epoxy be
considered?  This determines if the Rp should be 1.0 or 1.5, a sizeable

To build further on this ambiguity, the 
Æ97 UBC section 1633.2.8.1
ôOut-of-plane wall anchorage to flexible diaphragms
ö note #1 reads 
Elements of the wall anchorage system shall be designed for the forces
specified in Section 1632 where Rp=3.0 and ap=1.5.
ö  This very clearly
states that the Rp should be 3.0 for wall anchorage and notes no

To sum it up, it is not clear what Rp should be used in calculating the
design wall anchorage forces for a retrofit of an existing building.  The
contenders are the following:

Rp=3.0 per 1633.2.8.1 for wall anchorage
Rp=1.5 for shallow chemical anchors
Rp=1.0 for anchorage by adhesive

For new construction, Rp=3.0 will typically govern.  I don
Æt see the
necessity to double or triple this wall anchorage force from (service)
to 1.26W or even 1.88W for a Ca of 0.44.  The 
Æ94 UBC wall anchorage force
was 0.45W so using a Rp of 1.0 would represent more than a quadrupling of
the wall anchorage force for retrofitting tilt-ups.  This is too extreme
even for an engineer who considers himself conservative.

If the wall was not so thin and a longer embedment was possible, the issue
of whether the Rp should be 3.0 or 1.0 would still remain.

Mike O