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RE: Flag-Pole Foundations for light framing

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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 to work.

 

Dennis S. Wish, PE

I take full responsibility for my opinions.

 

-----Original Message-----
From: Al Grathwol [mailto:AGrathwol(--nospam--at)BoyleEngineering.com]
Sent:
Wednesday, November 19, 2003 1:42 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Flag-Pole Foundations for light framing

 

I don't use rebar if the pole extends through the concrete.  Consider the concrete to be an unreinforced thrust block.

Check the weight of adjacent concrete acting in friction to see if will resist the pole reaction at the surface.

ag

-----Original Message-----
From: Dennis Wish [mailto:dennis.wish(--nospam--at)verizon.net]
Sent: Wednesday, November 19, 2003 1:03 PM
To: light_framing(--nospam--at)structuralist.net; seaint(--nospam--at)seaint.org
Subject: Flag-Pole Foundations for light framing

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 columns.

 

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?

 

TIA

 

Dennis S. Wish P.E.