So far the consensus
is that reinforcing steel is not required when the steel column extends
through the concrete foundation on a post hole or flag-pole type foundation.
This is consistent with the way I’ve designed in the past and I will add this
to my comments. I might consider adding a cage just for additional protection
but my thoughts are that these poles are taking a penalty for being
cantilevered columns. The penalty is intended to compensate for the columns
flexibility, however, the stiffness of the columns are much less than that of
a traditional shear panel with the possible exception of a Hardy Panel where
the R value is assumed to be 4.4 for a narrow panel and 5.5 for a traditional
Hardy Frame. I’ve designed the columns for deflection and the deflection is in
an elevated shear value of 690-pounds where all other shearwalls would take
690-lbs x (2.5/5.5) or 276-pounds – more than half of the load. I think that
the design is overly conservative to begin with as the size of the steel is
increased to keep the deflection within the allowable 0.005H or in this case
0.005(144-inches) or ¾-inches.
I’m upset about
current code requirements and penalization for cantilevered columns based on
the City of Los Angeles
Emergency measures following
the Northridge Earthquake. The R value was reduced to 2.2 for cantilevered
columns even though they were designed for deflection because of the unknown
factors at the time. The Seismology committee arbitrarily adopted an overly
conservative R value and then recommended later that the shear in the
direction of force where the columns occurred could be reduced to penalize
only the line of shear where the columns occur. Thanks a bunch!!! There was no
real measure of the flexibility of the columns when they were designed to
deflection and not simply to allowable bending stresses as had been the
methods of prior codes.
Furthermore, I was
upset when the engineering community started to come around on these columns
after it was determined that the damage to the Northridge Meadows apartments
(where the subterranean parking collapsed the first floor cripple walls and
soft story) because the soft story was not supported on cantilevered columns
but columns that were pinned at each end making the soft story hazardous.
Sill, Professor SK Ghosh decided that by maintaining an R of 2.2 on columns it
would force the engineer to design properly. Who the hell has the right to
watch over my shoulder and decide if I am designing competently or not? It is
the client that pays for this and if the penalties are extensive, it creates a
stigma that follows the engineering community by substantiating the
non-professional’s claims that we are over-designing our structures at the
expense of the public.
If this seems like an
angry post, in some ways it is. However, when is the structural engineering
community going to open its eyes to the fact that there is competition in this
area of engineering coming from Architects willing to wet-seal plans and who
don’t need to take responsibility until the damage is done if it happens
within a ten year period of legal responsibility. This type of arbitrary
design penalties are hurting our industry and we should be lobbying to stop
this – if only for the sake of the 90% of all buildings that are designed and
constructed that happen to be for residential use.
Sorry – not go back
Dennis S. Wish,
I take full
responsibility for my opinions.
Sent: Wednesday, November
19, 2003 1:42
Subject: RE: Flag-Pole Foundations for
I don't use
rebar if the pole extends through the concrete. Consider the concrete to
be an unreinforced thrust block.
weight of adjacent concrete acting in friction to see if will resist the pole
reaction at the surface.
Sent: Wednesday, November 19, 2003 1:03
Subject: Flag-Pole Foundations for
I have two Flag-pole foundations
designed for the front of a custom home where the forces in each column
(lateral force) based on an R of 2.2 is 690-pounds placed at 12’-0” above
the top of the foundation. The entry is flatwork which is monolithically
placed and is, in my opinion, sufficient to restrain the top of the column.
A 18-inch square foundation pier extends up from the flatwork six inches and
the foundation is 4’-0” below soil and 30” square. The Column is a TS
6x6x5/16” square tube has approximately 6-inches of flatwork on the two
exposed edges of the flatwork (outside edges) and is continuous between
The plan check corrections asked
for two things:
1.The minimum reinforcement for
the concrete foundation
2.The clearance surrounding the
columns to show adequate restraint of the flatwork. It is assumed that the
area in front of the columns will also be done in either brick/tile or
concrete sidewalks with an area abutting the entry flatwork.
Unless the lateral loads were
large, I have not in the past provided reinforcement with this much cover
around the steel columns. The columns extend through the foundation and are
secured to a 12” thick erection pad. The column should be sufficient to
handle the moment in the base, and the concrete transfer the bending moment
below grade without the need of a cage or vertical rebar. The passive soil
pressure should also be sufficient due to the analysis to resist overturning
of the columns with such low lateral loading. The erection pad is more than
sufficient to support the entry roof dead and live loads.
Am I missing something here? Is
there a minimum steel requirement for a flagpole with small lateral loads?
Where would I find the information in the 97 UBC that might reference
ACI-318 on this issue?
I have always used flatwork as a
means to offer restraint against rotation of small lateral load columns.
What, if any, is the minimum edge requirements to the edge of the foundation
especially since the piers extend above a post hole foundation that is
30-inches square and 48” deep?
Dennis S. Wish