strengthening 1

[FloydLloyd]

when modifying a structural element results in increased load, does it make sense to design for added load or for the whole load?

for example, a truss bottom chord (2x8, SP#1, metal plate connection, at heel)
existing bottom chord tension 8.25 kips
new loading 9.9 kips

If I add a side plate, and just design the new member and connection for 1.74 kips, is that sufficient? Is there enough yield, or strain, to share load before the original member fails?

 

[KootK]

For brittle systems, such as wood in tension, consideration needs to be given to the relative stiffness of the parts and how load will share amongst them. Your instincts are leading you in the right direction on that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[FloydLloyd]

Thanks. I once had a colleague say he'd "rather be lucky than right!".

Might be some information in the Wood Handbook, USDA, or other sources investigating tensile strains in wood. Also, must consider joint slip data from TPI, SBCA. Joint slip might be the deciding factor in the load sharing question.

Anybody have a usable tensile strain value for southern pine?

 

[KootK]

All said and done, designing reinforcement for 100% of the load being dealt with within that reinforcement may be less costly than the engineering exercise of trying to nail down the load sharing.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kipfoot]

I have typically assumed that combined cross section shares the new load. So, the original piece may have some existing stress that is increased by its portion of the added stress. The new piece, then, would not carry all the additional load.

Anybody have a usable tensile strain value for southern pine?

link to southernpine.com

 

[dik]

By changing the load regime with an existing structure, just a caution... you may be changing the manner of loading slightly from what was existing and you may end up with some unintentional 'cracking' or movement.

Dik

 

[SAIL3]

I agree with KootK in designing the new reinforcement for the full load......in this case it would be difficult to determine the relative stiffness of each component...

 

[briancpotter]

Is the side plate the same material as the original chord, or is one steel and the other wood? If it's the latter, I'd be concerned about the new reinforcement taking the additional load plus some of the previous design load (I'm assuming the 8.25 kips is the design load and is not actually being experienced by the chord).

 

[dhengr]

FloydLloydWright:
The problem with these kinds of reinforcements or repairs is that the existing loaded joint or member must move/slip/strain some more (beyond its existing state) before the new member starts to come into play. And, during that stage of the loading process, you may be seriously overloading/overstressing the existing components. The best thing to do is to make some effort to unload that members or joints by taking the existing load off of them. Then apply the reinforcement and remove the shoring, and you can start to consider their sharing of the load in accordance with their strength and stiffness.

 

[OldBldgGuy]

dhengr nailed exactly what everyone seems to forget. No reinforcing or new structure gets loaded until the existing one reacts beyond what was intended. If you don't install the new members with the load removed from the existing so they can share when you put it back, you have to assume the full load for your new design and assume it doesn't get applied until the existing fails. Unfortunately that usually means unacceptable effects on the existing finishes, if not worse.