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Re:3-hinged arch w/ NO tension ties - how?

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John,

Where in Georgia is this project located?  In fact, for a large portion of the state, seismic loading is a major issue under the new building code.

However, for the reactions you indicate, the foundation does not seem outrageous.  The thrust load is very large.  Most geothech's in Georgia allow for a friction factor of 0.35.  With the combined vertical reaction and weight of the foundation, I get a friction force of 50 kips or so.  This is less than the applied thrust of 60.9 kips.  Therefore, the engineer must have counted on passive pressure to resisit the sliding.  Now, with a footing thickness of 8 feet, the passive pressure should be substantial.

You are right that tension ties would be the most efficient solution (although it would take about 4 - 1 inch diameter A36 tie rods to resist this much thrust).  However, given the design limitations of no tie rods, this foundation design seems reasonable.

Adam Vakiener, P.E.

From: "John H. Cato, jr., R.A." <jcato(--nospam--at)accessatc.net>
To: <seaint(--nospam--at)seaint.org>
Subject: 3-hinged arch w/ NO tension ties - how?

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Hi folks,

I'm an architect in GA who's been subscribed to this list since earlier =
this year.  I've been enjoying the wide discussions since that time - =
structures is something I've always enjoyed.  Finally making my first =
post here...

I've got a scenario that I would like to get a broader perspective on.  =
I was involved in some early preliminary (architectural) studies for the =
entry pavilion (a 40'x80' add-on to the end) of an 'ag arena' - used for =
showing horses, cattle, etc.  The main arena space is 200' (clear span) =
x 300' long - standard pre-engineered building frame - 16' eave height, =
2/12 pitch and 25' bays.  Basically an open structure with 6' wall =
panels down from the eaves - leaving the bottom 10' open.  End =
conditions are closed on the entry pavilion end and enclosed gable end =
on the opposite end (down to 10' above ground).  Remainder of the lower =
spaces is open all around.  The frames are your classic three-hinged =
arch - executed in steel.  There are no real seismic issues here in this =
area.

The situation is the foundation conditions.  From discussions with the =
contractor/erector, due to the desire for allowing maximum flexibility =
(mud-bogging, tractor pulls, etc - in addition to the above-mentioned =
livestock events) there was to NOT have any tension ties across the =
bottom (what I am most familiar with).  I have not talked with the =
individual who engineered the foundations (don't even know who he is) - =
but the solution used was a massive block of reinforced concrete at each =
arch bearing -- 6'w x 8'd x 14'l equating to around 100kips (150 pcf =
concrete) of 'gravitational mass'.  From what I've seen, there's nothing =
beyond the standard anchor bolt scheme connecting the frames to this =
'footing'.

The erection drawings that I've seen (the erector let me look this over) =
show the following reactions (worst case kips) at a 'typical frame':

               horiz      vert
D+L         60.9      43.0
D+W1->  -27.6     -20.0

(hope this lines up -- 3 columns, last two are numbers)

My question is this:

Is this approach the most rational? I hate to question whomever came up =
with this solution - I just haven't quite seen this approach used =
before.  To be sure, 100kips of 'dead load' can resist quite a lot (I'm =
sure any uplift could easily be handled here).  It's more the horizontal =
loads that I would like some enlightenment on.  My general solution is =
to go the 'classic' route of putting tension ties between the opposite =
sides of the frames -- buried deep enough to not interfere with any =
perceived uses (in the interior spaces).  Corrosion issues would =
necessitate encasement in concrete (or galvanizing or running thru PVC =
pipe, etc) -- but that is neither here nor there -- this approach was =
not the one taken.

I'm thinking also of these cantilevered rain shelters over the gas pumps =
seen quite a lot around here at the convenience stores -- some real =
decent wind loads (and imposed moments) would occur during high winds -- =
and I think I've seen some pretty decent 'blocks of concrete' for their =
foundation.

I'm not looking for a solution (per se) here -- just some opinions on =
the *concept* chosen and what others might make more (or less) sense.  =
One thing that occurred to me was some pilings (or, technically, I guess =
this is a 'batter pile' and/or 'deadman') sunk at each foundation to =
further tie things deeper into solid bearing (obviously here, loading =
laterally).  I've not done a great deal of research here (but still =
digging) - recently acquired earlier this year Robert Brown's "Practical =
Foundation Engineering" 2nd ed and only done some cursory looking there =
- but it appears he states that piles only have limited ability on =
lateral loads (and very little data on that) (pg 4.108).

Again, please understand -- I am not and was not involved in any of the =
engineering on this -- my only responsibilities were some architectural =
design work on the ancilliary entry pavilion that connected to this =
arena. I'm only attempting to gain insight into the conceptual approach =
to this problem and its solution (since said solution is outside my area =
of experience).  Any references to any literature that substantiates =
this would be appreciated (in addition to the expertise here).

What would you do?

Thanks in advance.

-- john.


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