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Lateral capacity of wood column
3

Lateral capacity of wood column

Lateral capacity of wood column

(OP)
Say I have a wood column (load bearing primary compression member) in my extremely old house, assumed fixed end connections.

I want to load the column laterally to tie something down (treat as point load directly in the center). How can I figure out what the capacity of this wood column is and what the maximum load it could handle be in that direction? I don't have any experience in wood design and I'm a student.


Two major questions:

1. Is my setup for the statics of the column appropriate? As in - treat the compression member, laterally loaded with a point load in the middle, as a beam with a point load in the center with pinned supports?
2. Assuming above is correct, how do I go about assuming material properties of a wood "beam" and evaluating its capacity to hold something laterally?

Thanks

RE: Lateral capacity of wood column

A few thoughts:
- If this is an actual condition you are trying to analyze (IE not a theoretical problem you are doing for school), please consult a licensed professional engineer to evaluate this column. You don't want to load up an existing column and potentially cause a failure.
- Wood posts are typically assumed to have pinned end conditions. There simply isn't a good way to develop fixity in wood members at connections, in most typical conditions.
- You need to consider the combined effects of gravity and lateral stresses on the columns simultaneously.
- You will need to evaluate the connections at the ends of the columns. Columns are not always designed to carry lateral forces, so while there may be some nominal connection provided at the ends (something like toe-nails into the floor plate) it likely was not designed to carry much load and may need to be reinforced.
- There are several textbooks available that discuss the design of wood members. I would buy one if this topic is not being adequately explained in your coursework. You'll find the allowable stresses for wood members are listed in the National Design Standard Supplement provisions if you are in the US.

Good luck!

RE: Lateral capacity of wood column

(OP)
Alright - thanks, @curveb!

RE: Lateral capacity of wood column

Hello. I think I have an answer for you!

To model the wood column as a beam with a point load in the center, with pinned supports is reasonable, assuming that the column is straight and has no significant eccentricity or other imperfections. In this case, the column can be analyzed as an Euler column. However, it's important to note that there may be other factors to consider, such as the condition and quality of the wood, the size and shape of the column, and the configuration of the supports.
To evaluate the capacity of the wood column, you'll need to determine its material properties, including its modulus of elasticity (E) and its allowable compressive stress (Fallow). These values will depend on the species of wood and the grade of the lumber. You can find this information online or in wood books.
Once you have determined the material properties, you can calculate the maximum allowable compressive load for the wood column using the Euler column formula:

Pcr = (π² * E * I) / L²

Where:
Pcr = critical load or buckling load
E = modulus of elasticity
I = moment of inertia
L = length of the column

For a column with pinned ends, the moment of inertia is:

I = (1/12) * b * h³

Where:
b = width of the column
h = height of the column

Assuming you know the dimensions of the column, you can calculate the critical load. Then, you can use a factor of safety to determine the maximum allowable compressive load for the column. A typical factor of safety for wood columns is 2 to 3.

Note that this calculation assumes that the load is applied perfectly at the center of the column, and that the column is perfectly straight and uniform. Any imperfections in the column or eccentricity in the load may affect the capacity of the column. Therefore, it's always best to be on the side of caution and choose a lower allowable compressive load to ensure the safety of the structure.

I hope this is useful for your work, good luck with it!

RE: Lateral capacity of wood column

U.S. design code covers a very similar situation as you've described here, using a combined bending/compression equation to evaluated the capacity of the column. This information can be found and designed for, for free from the American Wood Council (AWC) National Design Specification (NDS) (see this link: https://awc.org/pdf-viewer/?idp=1414&idf=15). Refer to equation 15.4-1.

In order to establish the actual design properties for the wood column, you'll need to identify the wood species, and the visual grade (these should be stamped on the beam somewhere, but depending on age of beam, may be faded, but still visible). Once the wood species and grade is identified, you need to refer to the NDS Supplement, depending on species and column size, see tables 4A, 4B, & 4C of the NDS Supplement: https://awc.org/pdf-viewer/?idp=4126&idf=4.

As discussed by CURVEB, if this is a real situation, you need to consult with a licensed professional engineer who is experienced with wood design. Wood design is a bit of a unique animal and has some areas where it is easy to make a mistake with calculating, so I'd recommend working with someone who has experience dealing with the nuance of wood design.

RE: Lateral capacity of wood column

Keep in mind that you should assume some level of eccentricity, even if the member is loaded primarily in compression. Developing a concentric load onto a wood column is quite difficult to achieve, partially due to the fact that you have to load column perfectly at the center, but also because the wood itself is generally anisotropic for design of stiffness/strength. Therefore, the column may tend to deflect one direction more than the other (this is why Emin is much less than the design Modulus of Elasticity) for wood products, to try and account for. However, I would suggest designing all axial loads with some level of eccentricity unless you have a compelling reason not to.

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