Wood Column Eccentricity
Wood Column Eccentricity
(OP)
I was asked a question from a residential plans examiner about point loads on a wood, built up column. He was telling me when he analyses columns, he assumes an 1" eccentricity but he hears from engineers that only pertains to steel.
In reviewing NDS, you do need to account for eccentricity, but I typically assume axial loads act through the center of the column. I could not find in NDS any reference to assumed eccentricty due to placement errors.
Anyone have any opinions?
In reviewing NDS, you do need to account for eccentricity, but I typically assume axial loads act through the center of the column. I could not find in NDS any reference to assumed eccentricty due to placement errors.
Anyone have any opinions?
Don Phillips
http://worthingtonengineering.com






RE: Wood Column Eccentricity
I don't know about a 1" eccentricity for steel either, BTW.
I know something like that used to exist for concrete, but doesn't anymore, and that it's taken into account with the 0.8 max phiPn.
RE: Wood Column Eccentricity
I do a lot of plan check corrections with this eccentricity that you said. For interior steel column that don´t act as lateral resistant element , i ussualy add an excentricity equal to 0.0025H (H=top plate height) or the maximum inelastic displacement (delta m, see Ubc section 16.33)(the greater of this two values). I think that you could calculate the story drift and add this displacement as an eccentricity.
I hope taht this could be usefull to you.
RE: Wood Column Eccentricity
Don Phillips
http://worthingtonengineering.com
RE: Wood Column Eccentricity
Lgrossi
AMP CONSULTING
RE: Wood Column Eccentricity
RE: Wood Column Eccentricity
RE: Wood Column Eccentricity
Are you saying that you are the plan checker and require the designer to use this eccentricity? Or are you the designer who is being required to use this eccentricity? Either way, it is probably not enforceable.
RE: Wood Column Eccentricity
Regarding adittions and small houses, i´ve never received a plan check correction with these provisions, but i always use a 0.5 in eccentricity to prevent any plan check correction.
Once aggain i apologize for my english, i´m traying to improve it.
Lgrossi
RE: Wood Column Eccentricity
The plan checkers are incorrect if they say you are required by code to design gravity columns for an eccentricity equal to the maximum story drift. A perfectly acceptable procedure is to design the leaning columns for axial load only and to design the lateral-force-resisting system for the increased load due to P-Delta effects. Of course, it may be more expedient to just comply with their wishes than fight the issue.
Don, I do not know of any code requirement to design wood columns for a minimum eccentricity. Some engineers recommend using 1" or so, but it just depends on your engineering judgment. You could argue that if there is some rotational restraint provided by connection hardware, then the effective length of the member is less than the idealized pin-pin model and this would compensate for potential eccentricity.
RE: Wood Column Eccentricity
RE: Wood Column Eccentricity
"6.5.12.3 Design Wind Load Cases. The MWFRS of buildings
of all heights, whose wind loads have been determined under the provisions of Sections 6.5.12.2.1 and 6.5.12.2.3, shall be designed for the wind load cases as defined in Fig. 6-9. The eccentricity e for rigid structures shall be measured from the geometric center of the building face and shall be considered for each principal axis (eX , eY ). The eccentricity e for flexible structures shall be
determined from the following equation and shall be considered for each principal axis (eX , eY ):
{equation and variables ommitted}
The sign of the eccentricity e shall be plus or minus, whichever causes the more severe load effect.
EXCEPTION: One-story buildings with h less than or equal to 30 ft, buildings two stories or less framed with light-frame construction, and buildings two stories or less designed with flexible diaphragms need only be designed for Load Case 1 and Load Case 3 in Fig. 6-9."
It would appear the intent is to account for eccentric column loads due to story drift, as mentioned above. However, from a practical perspective, I doubt a column will see full loading from dead and live loads at the design wind load. At least I found some reference that supports what the plans examiner was doing, although 1" seems arbitrary unless you calculate the eccentricities.
Any further comment before I get back to the plans examiner?
Don Phillips
http://worthingtonengineering.com
RE: Wood Column Eccentricity
RE: Wood Column Eccentricity
Don Phillips
http://worthingtonengineering.com