FLITCH BEAM BOLTING - SIZE AND SPACING 1

[PT99]

In designing flitch beam bolting, for size and spacing, do you analyze shear flow for steel to lumber, or just divide the total shear stress for each bolt equally.
Is the NDS value for the bolt to lumber controlling (not much, just 580 lbs when parallel to grain.)?

And for spacing, is there also a controlling value for unbraced steel to consider in a drop header?

 

[KootK]

In designing flitch beam bolting, for size and spacing, do you analyze shear flow for steel to lumber, or just divide the total shear stress for each bolt equally.

1) For flitch beams, the center of gravity of the plate is usually coincident with the center of gravity of the lumber, or very close to it. When that is the case, there really is no appreciable shear flow and you only need to supply bolts to transfer the transverse, applied loads from the point of application of the loads to the various plies in proportion to the EI values of the those plies.

Is the NDS value for the bolt to lumber controlling (not much, just 580 lbs when parallel to grain.)?

2) Because of #1, you'll be concerned with bolt capacity perpendicular to grain.

And for spacing, is there also a controlling value for unbraced steel to consider in a drop header?

3) I don't know how much attention this gets in practice but, I agree, it ought to be a consideration. And I've seen detailing where the bolts are spaced more tightly on the compression edge than the tension edge which, to me, suggests a concern for plate buckling. In the past, I've just considered little strips of plate running from bolt to bolt in the compression zone as little plate "columns". Then I'll compare that to some of the tabulated stuff that I've seen out in the wild (shared below).

Another question becomes what you're unbraced length ought to be for lateral torsional buckling. And, frankly, I don't know how to answer that question rigorously so I simply do not use flitch beams unless compression edge bracing is copious and convincing.

Some resources that you may find helpful:

Gang Nail Manual: Link

NAHB Manual: Link

StructureMag Article: Link







HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[Agent666]

To add to Kootk's #1, you also need to consider long and short term E for the timber to bound the issue. In the short term timber is stiffer, so higher load percentage carried by the timber, over time creep effects shed a proportion of the load to the steel, lessening the load in the timber. This results in larger overall deflections. It's conservative to just check the plate alone for deflections though which makes things pretty simple if you aren't shapening your pencil on the calculations.

For global lateral buckling, I've just estimated the behaviour as being equivalent to a solid timber member of the same overall dimensions. I suspect this is overly simplifying things though because of the much larger load carrying capacity of the flitch plate and hence amplifying any out of plane buckling forces compared with equivalent timber member. But I guess its better than nothing (which people sometimes do).

 

[hokie66]

In addition to the bolts to take the load from the timber into the steel, don't forget near the ends to provide additional bolts to take the load back into the wood if the wood is delivering the reaction in bearing. This is emphasized in the Jim DeStephano article which KootK linked above.