Wood Column Eccentricity

[DonPhillips]

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?

Don Phillips

http://worthingtonengineering.com

 

[271828]

As far as I know, one just has to use judgment for the situation at hand. In the past, I've (probably conservatively) assumed half the column for connections framing into the side. For bearing connections, I looked at the bearing stress as probably being triangular, which threw some eccentricity in there. Not sure if this is really necessary but it seemed reasonable at the time.

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.

 

[LGARG]

Hi guys,

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.

 

[DonPhillips]

Very helpful. I'll research the 2003 IBC to see if there is a similar requirement there, which is the code in Ohio.

Don Phillips

http://worthingtonengineering.com

 

[LGARG]

Don i think that the IBC2006 use the same limits for the story drift, the only difference that i found is that IBC calculates the maximum inelastic displacement as the elastic displacement* a factor CD(usually 4).


Lgrossi
AMP CONSULTING

 

[RARWOOD]

95% of the design work I do, is in wood. To the best of my knowledge there is no provision in the N.D.S. that requires wood columns to be designed assuming a minimum eccentricity, as was required with concrete sometime in the past. 271828 is correct it is a matter of engineering judgement.

 

[UcfSE]

While it apparently is not specifically required, assuming some minimum eccentricity seems a very prudent decision, unless you are confident that the contractor can install the column within an rch of perfect.

 

[Taro]

LGROSSI, you said "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)."

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.

 

[LGARG]

TARO my english is not so good. I apologize for that.I´m not a plan checker I´m a designer. I´ve been working on two big residentials houses(20000 sq ft) and in each one i´ve received plan check corrections saying that i have to calculate interior steel columns that don´t act as lateral resistant elements with these provisions.
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

 

[Taro]

LGARG, No need to apologize. Your english is as good as or better than many of the native english speakers that post in these forums.

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.

 

[Taro]

Don, I forgot to mention that the minimum eccentricity does not necessarily apply to steel either (as mentioned in the original post). I usually design steel columns for the connection eccentricity, but Socrates Ioannides has published a couple papers that say a typical sheartab connection provides rotational restraint to the column that compensates for the eccentrically applied load in the same manner as I described for wood columns.

 

[DonPhillips]

Again, thanks for all of the responses. I could not find anything in 2003 IBC but did find something in ASCE 7-05:

"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

 

[Taro]

Don, 6.5.12.3 does not pertain to eccentric axial loads on columns. It is meant to apply a torsional wind load to the building to account for non-uniform and non-orthogonal wind pressures. This is analogous to the seismic accidental torsion provisions of 9.5.5.5.2.

 

[DonPhillips]

I see that now. I think I will tell the plans examiner that allowing for an eccentricity is probably good practice but nothing in the code would allow him to enforce that.

Don Phillips

http://worthingtonengineering.com