Thumb rules for sizing timber columns ? 2

[FE_struct1]

Hello all,

I'm about to start work on a timber structure and - having never done timber before - am curious as to what rules of thumb one should use when doing initial sizing of timber columns. For concrete I've previously used 0.4f'c as a prelim sizing tool for axial capacity but have failed to find anything similar for timber.

I do appreciate that any rules - if they exist - would be different for glulam, CLT etc. Thanks in advance !

 

[1503-44]

I estimate col size using
L/Dmin >= 50
Correction Sorry. I wrote that wrong, but did the example calc correctly.
Dmin >= L/50

Fc = 500,000/(L/Dmin)^2
L = Effective buckling mode length
Dmin = width of column (in the plane of buckling)

Dmin for 10ft height pinned end column = 10ft x 12in/ft / 50 = 120/50 = 2.4in min
Try 3x3"
L/D = 120in/ 3in =40

Area = 9in2

Load= 5000lb
Sc = 5000/9 = 555psi

Fc = 500,000/(L/Dmin)^2
Fc = 500,000/40^2
Fc = 312 psi

Sc > Fc N.G.

Try 4×4"

 

[FE_struct1]

Thanks for the reply 1503-44! One quick question:

Dmin >= L/50

I'm guessing this valid for one-two storey columns only ? If yes, what would be the rule for say 5 or 6 storeys for a typical residential/office space ?

 

[phamENG]

That's a rule for all wood columns (at least under the requirements of the NDS in the States). You must brace the column at intervals not exceeding 50 x dimension of the parallel face.

Example:
A 2x4 stud has to be braced about its weak axis every 50*1.5in = 75inches and its strong axis every 50*3.5in = 175in.

So if you have a timber column that is 8" square, it needs to be braced laterally every 50x8in=400inches (33.33ft) in both directions, regardless of the amount of load. Possibly (and likely) less than that to prevent buckling depending on the magnitude of the loading.

If you're looking for rules of thumb, limiting the length of a compression member to 50x its minimum face (Dmin as 1503-44 put it) is a good one to use for preliminary sizing regardless of the size of the building.
Don't listen to me for rules of thumb. Jayrod's got it below.

 

[jayrod12]

I would say as a rule of thumb dmin>L/35. At 50 you're likely not to get it to work except for the lightest of loads.

L is the unbraced length so that would be per floor height. 5 or 6 storeys would be a different animal because bearing capacity governs much of the designs with that kind of load.

 

[NorthCivil]

Design enough timber columns and you will eventually have memorized the capacities of various built up stud packs / columns depending on unbraced length.

so get designing, my friend.

only rule of thumb really to start is the column should be wide enough to catch the entire beam, most of the time

 

[1503-44]

Yes, 50 is a little slim, but the number was easier to remember.

Any height of stories, as long as you brace.
But with increasing loads from each floor, you will eventually reach the compressive limit f'c of the first floor column, as that one carries the load of all floors above.

 

[Celt83]

Make yourself some nomographs: Link

- Check compression perpendicular to grain in wall bottom plates (typically controls immediately for 2x6 SPF No.2 lumber)
- 5 PSF Lateral Live load consideration (there have been a handful of discussions about this on here)
- Minimum Eccentricity ( This has a significant impact on the capacity curves, but in platform frame construction there is some engineering judgement on how much to consider)

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

As NorthCivil said, once you have designed a couple, you will get a feel for it. No need for "rules of thumb"