Wood Shoring Design Questions 2

[bookowski]

Have some questions regarding design of wood shoring. Particular situation is a wood framed building (joists into masonry walls) that was vacant for 30yrs and is in very bad shape. 100% of the joists are being replaced in sections. The contractor wants to provide temporary stud walls to shore the joists during this work.

A few questions for this situation and in general:

- Studs will be unsheathed. If I provide a row of mid-height blocking along the wall and diagonal nailed 2x braced at each end of the wall can I consider all studs braced at mid-height in the weak direction?

- In reality the shoring walls will see little load, we are not jacking anything up. They are there in case one or more sections of floor fails during the course of work. Are there any references/guides as to a design approach. I've started off by assuming the shoring walls are the only supports, i.e. all joists at all levels come out of their pockets while sustaining a construction load at each floor (5 floors by the way). That obviously works, but is it too conservative?

- The plan is to reframe from top down in sections once shoring is in. This means that as soon as the work starts the shoring will be removed top down as they go and see less and less load. Duration factor for something like this - 1.15?

Just wondering if I'm being too conservative - it's a load of material for something that will start coming down almost as soon as it is complete. Any good shoring design books out there?

 

[KootK]

- mid height bracing assumption AOK.

- temporary structures by Ratay is a good reference.

- could even go 1.25 if you can preclude material hoarding long term. I like 1.15 though.

- sounds very conservative. could you shore only the floor beneath the one being worked on? Like reshoring in concrete? Regardless, I wouldn't load any one wall with more than two floors I don't think.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

The diagonal bracing needs to be design for 2% to 4% of the compression loads of all the studs being braced by it. So one, two, etc as it calculate out.

I would wedge the top of the shoring to the joists for full contact.

The duration factor applies to new lumber only. If they may use recycled lumber I would use a 0.9 factor.

Not having done this before reading a good reference would be required IMHO.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Archie264]

Woodman, I know you know far more about wood than me but do you have a reference for where it says that the load duration factors apply only to new lumber?

Bookowski, subject to Woodman's correction the load duration factor for construction loads is 1.25, as Kootk noted. Also, I know from previous posts that you have extensive experience with townhouse renovations so at some point trust your instincts borne from that experience.

 

[KootK]

The duration factor applies to new lumber only. If they may use recycled lumber I would use a 0.9 factor.

What's the logic with that woodman? It takes 10 years of cumulative loading near max stress to put you in 0.9 territory, right?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[bookowski]

Thanks for the input.

Kootk - In this case I wouldn't only do partial shoring, if it was in better shape I may. I didn't elaborate on how bad these joists are. This looks like something from an end of the world scenario. I think I may revise my loading though to assume that at most 2 floors are supported at any time as you said. Thanks for the ratay reference, looks like cheap copies are available used - will pick it up.

Never heard of the 0.9 Cd for recycled lumber but this will all be new so not relevant this time.

Archie - Yep it's a townhouse job. They are tough ones because you see that in practice almost everything stands up, but on paper nothing should. Always a game of finding the middle road with these.

 

[Archie264]

>>>They are tough ones because you see that in practice almost everything stands up, but on paper nothing should. Always a game of finding the middle road with these.<<<

Boy, you hit the nail on the head with that one. I've always thought that internal non-bearing walls add some combination of stiffness, shear wall or diaphragm action that goes unaccounted for. Either that or wallpaper has more structural capacity than we realize.

 

[woodman88]

So how do you determain what loads were applied to the old lumber?


Load Duration
Load duration measures wood’s ability to resist stresses when
those loads apply over time. The duration of load, or the time
during which a load acts continuously or intermittently on a
wood member, is an important factor in determining the total
load that the member can safely carry.
Wood can carry substantially greater loads for short
durations than for long periods. In other words, wood is able
to resist higher stresses when the load applies for a shorter
time—a feature that enhances its performance in seismic and
high wind zones. This also becomes a factor when building
designers must calculate stresses such as snow or construction
loads.

From the attacked link

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

Useful article woodman88 -- thanks. For me, it comes down to that ten years cumulative service business. Wood purchased new and used over and over by the contractor certainly wouldn't get there. It would need to be something like wood:

1) Installed in a near dead load only application for near to ten ten years.
2) Salvaged by the contractor for use as shoring.
3) Used as shoring in a near dead load only application.

It's possible but rather unlikely I would think.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

KootK

Maybe I am just unlucky to know of two conditions where it caused a failure.

Understand that if a piece of lumber is loaded to maximum beading stress for 6 days at a 1.25 duration factor you have culminated 6/7 it's allowable loaded time.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

You're definitely pushing my needle in the conservative direction woodman.

My general design philosophy is to put all of my conservatism into my load choices and the whittle everything else down to bare bones. More uniform factor of safety that way. I make some exceptions but not many.

What kind of testing had to be done to establish that your failures were due to load duration issues as opposed to just run of the mill overload/abuse?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

Testing LOL
Almost all lumber failures are from overload/abuse.

One situation was that a novice built some concrete forms, to be reused. Since the concrete would not apply load to the forms for more that seven days per use he used the 1.25 factor. After the fourth time, or so, that they used the forms they started breaking.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

The reason that I semi-facetiously brought up testing us that, without it, I don't see how you could conclusively attribute the failures that you've witnessed to load duration issues rather than, say, overload/abuse which we agree is the source of most failures.

The formwork failure example that you cited doesn't indicate a problem with recycled lumber. Rather, the designer applied the load duration factor incorrectly, failing to realize that it's based on cumulative loading.

The mechanical behaviour of lumber is the story of imperfections. Knots and fracture mechanics. Every extra time lumber is handled, there's a risk that it will be handled roughly (abused). And every time that lumber is handled roughly, there's a chance that it will fracture. In this sense, perhaps strength reduction for reused lumber is warranted. But it would have nothing to do with load duration.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[woodman88]

KootK

I am sorry but how is a "every time" not cumulative and "lumber is handled roughly" not a load being applied to the lumber.

I ask because what you wrote appears to be what I said. But you want to create an additional factor rather than using the one that exist "load duration factor".

Overall as I see it, we know alot about new lumber and how it can be used. But I have seen every little on old lumber and how to reuse it.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

Woodman:

I am sorry but how is a "every time" not cumulative and "lumber is handled roughly" not a load being applied to the lumber.

By repeated rough handling, I was referring to things like being thrown in and out of a truck and perhaps being run over by a Genie lift. These incidents would be almost instantaneous and would contribute next to nothing to the "cumulative" ten year bucket. More importantly, handling "loads" are different in nature and would compromise the lumber by creating and exacerbating fracture initiating flaws.

But you want to create an additional factor rather than using the one that exist "load duration factor"....Overall as I see it, we know alot about new lumber and how it can be used. But I have seen every little on old lumber and how to reuse it.

As I said above, I could get behind a "Reused Lumber Factor" to account for the kind of non-load duration issues mentioned above. But to bury such a factor as a fudge within the load duration factor framework strikes me as confusing and unnecessary.

This discussion has poked some holes in my understanding of the load duration factor. I used to think it a very simple thing. I may need to start a thread of my own.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dhengr]

Bookowski:
Why not shore from the bottom up and reuse most of the material as you work your way up in the bldg? If it is reasonable to assume that everything above isn’t going to come down on your head while you are working on the first fl., but you don’t want it falling in as you disassemble it, just shore below to the bsmt. slab. The reshore people can be working half the bldg. length ahead of the disassembly people; and then they come back just behind the disassembly and new floor installation people and move the shoring up on top of the new floor system. This is finally a floor system that you have some faith in, and you really only need one set of shoring material. I don’t mean to neglect duration of loading and reuse damage to materials, but my goodness, you are supporting DL’s and some minimal construction loads, which you have some control over, and preventing them from falling through to the fl. below. Using 2x4 studs and blocking at mid height for bracing is like three drunk sailors leaning on each other for support, until one of them moves a bit, and they all fall down (buckle). Some time ago JAE (as I recall) showed a picture of some studs buckling fairly dramatically under these very conditions. You might be better off using 4x4 or 6x6x8' (or some such) sill and top plates and a couple adjustable pipe columns, in multiples, for your shoring system. It takes some planning, coordination and maybe some min. bracing, but you only have to carry one set of shoring system up to the last floor level while you rebuild the roof. Also, remember you don’t want to leave too many levels or to long a wall section unsupported since those old masonry bearing walls depend on the floor system/diaphragm for their lateral support. Also, you should probably fire-cut the new joists and rap them in some sort of peal and stick flashing material (or 30# felt) to prevent direct contact btwn. the wood and the masonry.