Columns in crawlspace 7

[DoubleStud]

Let's say you have series of dropped beams (3-1.75X11.875 LVL) and columns in a crawlspace of a residential construction. Let's say your columns in the crawlspace support 20K of point load(unfactored). I want to hear the consensus:
1. What would you use as column member?
2. How do you attach the column to the dropped beam?
3. How do you attach the column to the isolated spread footing?

I dont want to give too much info because I dont want to persuade you to go one way. I have a differing opinion from another engineer and want to see what other people are thinking.

Thanks!

 

[phamENG]

Much of it will be regional. Even terminology. In a crawl space, that will usually be a pier, not a column. Though if the grade drops away you could certainly end up with a column.

1. Here, that's going to be a CMU column all day. But my answer would be the same for 20k or 20lbs. Wood rots in our crawl spaces, so horizontal members go high enough to not rot and vertical members are masonry, concrete, or (most commonly) concrete masonry.

2. This is usually just a bearing condition unless there's uplift or the pier/column is part of the LFRS. If it's tall enough to be a column and not a pier, I like using the simpson cast in beam connectors (they're in the masonry chapter of the catalog).

3. Mortar unless the shear demand exceeds the allowable stress of the mortar, the column is reinforced, or there's uplift. In any of those cases, dowels.

 

[hokie66]

Reinforced masonry piers.

 

[jerseyshore]

CMU piers all day everyday around here. If it's in a flood zone or uplift is a concern then the footings will have dowels into the piers and using either the Simpson CCQ caps or a similar cast-in-place strap is most common for beam to pier connection.

 

[XR250]

Around here in retrofits it is either 6x6 PT or CMU. They usually attach the 6x6 with toenails or Simpson LPC6 cap and no base or a Simpson base. For 20k, it would be CMU.

 

[DoubleStud]

Interesting. Both of us were from the same firm. At the old firm we never use CMU for anything. So you guys added a fourth option. So how does the beam bear on this CMU pier? I assume you put treated wood in between the LVL beam and CMU laying flat (perpendicular to grain)? Isn’t the load pretty big for this wood shim/blocking?

 

[Eng16080]

Normally I would use a PT post with Simpson cap and base connectors, although this probably doesn't work with the high 20k load unless it's an 8x8. I'm not sure I've had this exact scenario before with such a high load supported in a crawl space. The CMU pier sounds good. Besides that, a concrete pier or steel column are other options.

 

[DoubleStud]

Eng16080, it is high because the loads are stacked on top of the support. So the load above is not on the span of the LVL beams. The LVL is supporting the floor just on that level.

 

[Flotsam7018]

Similarly, we generally avoid CMU whenever possible due to a lack of competent contractors in the area. If it's a fairly short column a no. 1 6x6PT post with caps and bases, otherwise I would look at a treated SCL or glulam column before jumping to a CIP pier.

 

[XR250]

I would avoid 6x6 as the bearing stress is over 600 psi. While still within limits, I see LVL's crussh at supports occasionally when the supporting pine did not (even perp to grain). Also, most elevated post bases are not rated for 20k unless grouted.

 

[SinStrucEng]

As others have said, I have used concrete piers in the past, with cast-in-place beam connectors. I don't spec CMU because we get a lot of shoddy installations and in the residential market, controlling the quality of work is difficult.

 

[Flotsam7018]

XR - interesting. I've seen plenty of crushing perpendicular to grain in sawn lumber beams at supports, but not in SCL. Were those cases in scenarios with excessive moisture?

 

[DoubleStud]

I am leaning toward 8x8 with simpson post base and post cap. For higher load (above simpson post base capacity) I will use HSS column with base plate and saddle on top.

 

[Brad805]

Unless there is other masonry required we would not spec masonry. I would look at a built up column if I could find a Simpson connections we could spec or use a steel column. We have a good supplier of teleposts for these loads built with varying head/base arrangements.

 

[DoubleStud]

We have a good supplier of teleposts for these loads built with varying head/base arrangements.

Do you mind sharing that product and the spec/capacity? It is not something that we typically specify at my old firm. But I don't mind using something new.

 

[XR250]

XR - interesting. I've seen plenty of crushing perpendicular to grain in sawn lumber beams at supports, but not in SCL. Were those cases in scenarios with excessive moisture?

Many were. No rot though. So maybe elevated humidity below 19% affects them.

 

[Brad805]

Doublestud, it is a small company in Nisku Alberta called Weisure They have made custom sizes for us. I would have thought there must be a large supplier in the US.

 

[phamENG]

So maybe elevated humidity below 19% affects them.

This. With the exception of Weyerhaeuser's treated PSLs and Pacific Wood Tech's treated LVLs, all SCL on the market that I'm aware of is dry use only.

 

[XR250]

Wouldn't dry use be categorized as below 19% moisture content?(I should have said MC in my previous post, not humidity). Basically typical crawlspace environment (hopefully)

 

[phamENG]

Reference design values reflect dry service conditions, where the moisture content in service is less than 16%, as in most covered structures.

So even where we might consider sawn lumber to be dry, SCL won't necessarily work.