High Strength Bolts used in Wood Connections 4

[Splitrings]

It appears to me that it is acceptable to use Grade 1,2,5 & 8 SAE J429 bolts in wood connections, according to NDS '05. Appendix I.4 addresses this topic. Is this correct? I have attached a CADD detail of a connection I have designed. This connection is used to connect the hoop truss of a fabric covered structure to a 10"x10" SYP #1 post. The connection yield mode that governs is IIIs, which is fastener yielding in the side member. My side member is the steel bracket. If I use a Fyb=140 ksi for a Grade 8 bolt this connection is adequate with two 5/8" bolts. I have shown three in my detail, as I orginally designed it with Grade 1 bolts. The bracket comes from the hoop structure manufacturer with only two 11/16" holes. The landowner would like to only use the two holes and not add a third. Any thoughts?

 

[Lion06]

If it works with two, I see no compelling reason to require three.

 

[msquared48]

I agree with SEIT.

I also trust that the single bolt or shaft going thru the metal brackets in double shear is adequate?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Splitrings]

I guess I am looking for concurrence that the use of grade 8 bolts is acceptable in wood connections. Has anyone used them in the past?

I have quickly checked the bolt to bracket connection. It is a single shear connection. It appears adequate. I am only designing the foundation up to the post bracket connection. The manufacturer is providing the stamped design for the structure itself.

 

[woodman88]

Splitrings, My NDS (the 2001) Section 11.1.2 state that "...apply to bolts meeting requirements of ANSI/ASME Standard B12.2.1". I think the 2005 NDS also states this, but please check it. If so, you just need to check the above standard for your bolts.

Garth Dreger PE
AZ Phoenix area

 

[cntw1953]

I don't design wood structure, just to provide a little thinking here.

For steel connection, in general one can substitute lower grade bolts with higher grade with fewer bolts as long as stresses on bolt and bolt holes (ahear & bearing) are within allowable, and there is adequate area to develop strength required to prevent web tear-out (block shear). Maybe there are similar concerns in wood.

 

[MiketheEngineer]

Grade 8 bolts can be rather expensive... But if that's what it takes - then use them.

 

[csd72]

I find it hard to believe that the timber is stronger than the grade 5 bolt.

 

[dhengr]

Splitrings:

I would be surprised if the NDS prevented you from using high strength bolts in a wood connection. But, I would also question your design if they were needed. Without having done the calcs. you did and without knowing more about the connection parts and loads, there seems to be something out of wack here. I assume the small dia. bolt is yielding in bending. The length of the bolt cantilever is killing you. Bolt bending and wood bearing/crushing will lead to a sloppy connection after the first loading. Has the hoop truss manuf’r. ever used his brackets with embedded SYP posts before? How are you holding your posts down against these forces, embedded in what? Watch a form of block shear and bolt end distance and spacing if bolts are stacked in a vert. line. Why not redrill his two holes to the dia. you need? Why not drill larger holes in the 10x10 for a 9 - 9.25" long pieces of pipe with an I.D. of .625'+, improves bearing area on wood and the combined steel pieces bolt & pipe make the wood crushing problem go away, thus canti. length approaches zero, and you have single shear involving all steel parts. Drill a 1/4" plate to match their bracket, bend a 1" bottom leg on it (an angle), drill this for a some flush head screws and let this into the 10x10, and you have transferred the shear load through the 1" leg. Or, buy 11" grade 8 bolts and watch the connection move up and down each time its loaded.

Good luck,

 

[dhengr]

You didn’t say pressure treated 10x10 SYP, but if that’s what you meant, check the treatment used as it relates to corrosion of hardware, and plan and spec. accordingly. This large a post is probably treated with the older copper arsenate chemicals which were a little less corrosive. But you should check this. Maybe you will be in even bigger trouble, needing all stainless steel hardware. A minute ago, I thought I was just trying to save you the trouble of finding 11" grade 8 bolts.

 

[Splitrings]

A little more follow up info. I have attached the connection calculation capacity from the AWC website. As you see the calculator uses grade 1 bolts, Fyb = 45,000psi. The capacity of 1 - 5/8" bolt is 1445#. I need 4300# of capacity. As you can see failure modes IIIs and IV are controlling. These as fastener yielding in the steel bracket (IIIs) and fastener yielding in side and main members (IV).

Posts are treated with CCA, not the new ACQ. Our agencies standards require stainless steel be used with ACQ. We can use hot dipped galvanized hardware with CCA treated lumber. I have had no trouble getting grade 8 bolts in long lengths. I used 15" ones on a project last year. Our project is not going to require a huge number of bolts, so cost will not be significant.

I believe the standard NDS refers to in '05 for bolts is AMSE/ANSI B18.2. I don't have a copy and know it will take me weeks to get it. I'll have to order it.

 

[hokie66]

I think dhengr and csd72 are correct. The bolt is yielding in bending because the timber is crushing, increasing the lever arm. A higher strength bolt is not going to solve that.

 

[zdude]

The NDS tables are nice for most quick calcs, but nothing beats programming a spreadsheet to do these connection calcs. The strength of your bolt factors into the K3 calculation, and does have an effect on the strength of the connection. A check of my spreadsheet shows that a 90ksi Fy bolt will still have the same failure mode (3s) but the connection strength is 1900 lbs.

 

[dhengr]

Zdude:

It is not surprising that the stronger bolt will lead to a stronger connection and the same mode of failure AND more movement in the connection at design load. The higher Fy bolt material, given same bolt dia., and the bolt essentially acting as a cantilever beam with a point load from the 1/4" bracket pl.; the higher Fy allows a greater tip load and greater tip deflection before the bolt starts to yield. Except, the bolt is supported along its length by the wood bearing/crushing strength, akin to a beam on closely spaced spring supports of varying stiffness, and a lack of real end fixity. And, with the stronger (higher Fy) bolt, connection equilibrium will be reached with greater tip load (from the bracket), greater bolt tip deflection, and more joint movement. In effect, the various yield modes are an effort to represent these kinds of conditions.

Off the top of my head I don’t know, but does 90ksi = Fy for a grade 8 bolt? If it isn’t too much work, what with your spreadsheet and all, could you post all six of yield mode values, for 5/8" dia. and Fy = 45 & 90ksi. Could you do the same thing, for all six modes, for 3/4" dia. and Fy = 45 & 90ksi? This might be a bit of a learning moment for all of us (maybe just me), to start to get a feel for how dia. or Fy changes affect this problem. I don’t mean for you to ignore your wife &/or your girlfriend for the whole weekend to do this good deed. I don’t want to get you in trouble with both/either of them. Maybe I already have, and that I should have just been, ‘signif. others however many,’ including a pet if you wish.

In my old age, things sure seem to be getting complicated. Following all the code steps these days to design a single bolted connection, long hand, can take darn near a long as we used to spend on major parts of the whole building. We are really getting to complicated, having to include too much minutiae in every calc. and are really not producing better building for all the extra work. I wonder if we are saving the clients any money, saving materials, are designing significantly safer structures. I know we aren’t getting paid enough for the extra work and effort.

Thanks in advance for your effort.

 

[Splitrings]

Thanks Zdude. I too, have a spreadsheet that calculates the 5 failure modes. I also compare it to the calculator on the AWC website for conformation. For those of you that really want to sink your teeth into this, Technical Report #12 on the AWC website goes into considerable detail on these connections.

http://www.awc.org/pdf/tr12.pdf

 

[zdude]

Splitrings,

I do the exact thing that you do. I usually will check the NDS tables to verify my spreadsheet. You never know if a formula I programmed will cause an error.


dhengr,

You know, I really like knowing the mode of failure for numerous reasons, not the least of which is the ductile vs. brittle.


Here are the failure modes and strengths I calculate for the 5/8" bolt (in lbs with Cd=1.15)
Option 1, Fy 45,000
Mode 1s- 105515
Mode 1m- 3905
Mode 2 - 4740
Mode 3m- 5430
Mode 3s- 1445
Mode 4 - 1840

Option 2 Fy 90,000
Mode 1s- 105515
Mode 1m- 3905
Mode 2 - 4740
Mode 3m- 5520
Mode 3s- 1920
Mode 4 - 2600

Here are the failure modes and strengths I calculate for the 3/4" bolt (in lbs with Cd=1.15)
Option 1, Fy 45,000
Mode 1s- 12615
Mode 1m- 4685
Mode 2 - 5690
Mode 3m- 6565
Mode 3s- 2000
Mode 4 - 2655

Option 2 Fy 90,000
Mode 1s- 12615
Mode 1m- 4685
Mode 2 - 5690
Mode 3m- 6725
Mode 3s- 2715
Mode 4 - 3755

 

[cntw1953]

min Fy (gr 8) = 130 ksi, see chart thru link provided below.

 

[Splitrings]

I think Zdude may have been referring to Grade 5 bolts. They have a Fy of 92,000 psi.

Does anyone have ANSI/ASME Standard B18.2.1? This is the standard that NDS '05 requires bolts to meet. I am curious if it lists Grade 1,2,5 & 8 SAE bolts.

 

[Billybaru]

Splitrings,

ASME B18.2.1 is a dimensional standard for a variety of hex and square bolts, cap screws, lag screws, etc. It does not contain any specific mechanical properties for the bolts. The grades 1, 2, 5 & 8 bolt material and mechanical properties are covered in specification SAE J429, which is available at

http://www.sae.org.

 

[Splitrings]

Thanks for the info Billybaru.