Sill plate anchor requirement 1

[FSS]

Have a basic question regarding wood sill plates for which I can't seem to track down an exact source. According to NDS Table 8.5.3 the minimum edge distance for a bolt loaded perpendicular to grain is 4D for loaded edge and 1.5D for unloaded edge. For a wall subjected to wind pressures (positive and negative directions), this 4D requirement technically prevents the use of 1/2" diameter bolts in a 2x4 sill plate, since half of 3.5 inches is less than 4D distance. Of course, 1/2" and even 5/8" bolts are commonly used as anchor bolts. Can anyone help clear up the apparent conflict?

 

[gatorpe]

I can't help clear up the question. But I am investing this question on my own, because of construction tolerances. It seems as though inspectors allow 1/2" to 3/4" of plate overhang without consulting the engineer or design professional. This puts me in a position where I have to consider the edge distance to be as low as 1" (considering 1 3/4" edge distance requirement for a drilled and epoxied anchor in conscrete).

Anyway, if you've gotten further feedback, I'd love to hear it.

 

[akastud]

I don't have my code open right now, but I believe that you can have reduced distances with reduced values. We do not let our architects use 2x4 walls on the exterior for this reason (and we are in a 70 mph region), but they usually work for interior walls and partitions with the reduced interior wind load.

 

[ERV]

Depending on your jurisdiction (building code), IBC 2000, Section 2308.6 or UBC 97, Section 1806.6, both have similar requirements with a minimum 1/2" anchor bolt spaced 6' O.C., no edge distance requirement (loaded or unloaded) except ends (max. 12" and min. 7D). I looked all through my 2001 NDS and could not find the 4D reference to loaded edge and 1.5D to unloaded edge for SILL PLATE bolts.

 

[akastud]

ERV,

I thought the 4D and 1.5D refered to all bolts in wood.

 

[DaveAtkins]

What I do in this situation is assume that the bolt is whatever diameter meets the 4d edge distance requirement (and no, edge distance cannot be reduced). Therefore, for a 2X4 sill plate, assume the bolt is 3/8" diameter, even if 1/2" or 5/8" is used. But be careful with this -- don't let a contractor install an expansion bolt so close to a concrete edge that it will spall the concrete.

DaveAtkins

 

[ERV]

akastud,

The NDS requirement of 4D and 1.5D is for bolting two members, main and side member (SEE Table 11.5.1A, NDS 2001 Edition, read all the print). IT DOES NOT REFER TO SILL PLATES BOLTED TO CONCRETE. If this were so than SIMPSON could not market their Strongwall panels, for they use 2- 7/8" and 4-5/8" bolts for a 2'-8" panel centered to the bottom plate for 2x4 walls.
What may I ask is the out of plane load at the foundation level?

 

[akastud]

ERV,

It appears I am just out of date, I have the 97 NDS and it specifically says that edge distance, end distance and spacing shall apply to bolts installed with concrete or steel as the "main or side member"
I believe that the loading that FSS was talking about is the wind load on the wall.

 

[AlohaBob]

Section 8.5.4 NDS gives Full value for 4D and Minimum 1/2 Full value for 2D and interpolate your actual. 1/2" in 1-3/4 edged distance = 3.5D or 75% full value.

For tall walls, or high thrust due to wind or other pressure, it can be a creative excercise to design the connection. Sill bolting may not provide enough in many cases.

 

[DaveAtkins]

AlohaBob,
Section 8.5.4 refers to END distance, not EDGE distance.

DaveAtkins

 

[AlohaBob]

The Values that I refer to are perpedicular to grain loading edge distance. I think the length of the plate provides the orientation for end distance typically.

 

[ERV]

The NDS is produced and published by the American Forest & Paper Association.

If one wishes to obtain the proper reference for Sill or Bottom Plate Bolting requirements then it would be necessary to obtain the WFCM also produced and published by the American Forest & Paper Association.

Please see Tables 3.2 through Table 3.3A, 2001 WFCM, (depending on wind or seismic). This should clear up any misconceptions and the tables provide excellent reference and design values. Again, no edge distance requirement. And as in my earlier post max. 12" from and ends and min 7D.

Anyway, anyone involved in design of wood structures should have this manual if nothing else.

 

[AlohaBob]

The load capacity of a wood sill or any bolted connection is less in cross grain than parallel to grain. Minimum edge distance seeks to prevent splitting the plate. That is the failure mode.

The end and edge full value minimum distances are the same 4D for compression but 7D end distance for tension. See section 8.5.3 and 8.5.4.

The NDS in it's entirety is adapted into the IBC in section 2306.1.

 

[umgrego2]

I think we're losing sight of the big picture. The 4d requirement that is being referred to is the loaded edge distance. So this value would only be of concern on the windward sill plate. But the large majority of the wind load gets transferred to the shear walls, either directly or through the roof diaphragm. In the shear walls, you only need to be concerned about the regular edge distance which is 1.5d.

I don't agree that sill plates have some sort of special requirements for edge distances. These values are irrespective of the main member's material because we're concerned with local failure in the wood itself.

As for the reduction of strength method, referred to by AlohaBob, that only applies for loaded end distance, which is for loads parallel to the grain.

I would suggest modelling the structure as if the windward sill plate offers no resistance. Transfer the loads to the shear walls and make sure that they are strong enough.

Cheers,

Mike

 

[AlohaBob]

Umgrego2

The shearwalls will carry three loads.

A shear load. This will be resisted at the plate by parallel to grain design bolt values. The edge is not loaded so the minimum 1.5D applies.

An out of plane load. Wind, seismic, or other. The shearwall being wood resists this load typically like a simple beam. The upper half load transfering into a diaphram at a floor, the lower through cross grain bearing on a bolted sill plate. Component wind loads have higher suction load than inward pressure. The shearwalls main function is a shearwall, but it still sees normal wall loads. This is where the issue of edge distance comes. I don't typically worry about it, the area that concerns me in this design limit is basement wall connections to floor diaphrams. It's too often neglected. The loads are higher, the plates splitting is a problem.

And Vertical.
Plate bolts don't typically address this. You use a holdown. I think Simpson got so popular because they designed their hardware with the 7D stud or post end distance and bolt spacing requirements incorporated right into the self jigging prefab hardware for tension. They meet code requirements and provide tested design values.

 

[ERV]

Anchor bolts are assumed centered to the sill plate. The commentary to the WFCM gives precise calculations for design (pages 62 thru 69, 2001 WFCM Commentary. The reference in the NDS is NOT the specification for anchor bolts and sill plates. Concrete to wood connection in the NDS may be used for ledgers and other such components.

Check out the WFCM:

http://www.awc.org/Standards/wfcm.html

Merry Christmas!

 

[gatorpe]

Section 8.5.3.1 (page 111) of the 1997 NDS Commentary provides guidance that allows edge distances as little as “2D” from the loaded edge when loaded perpendicular to grain, provided that we account for reduced capacity of the bolt. The way I understand it, we can reduce the capacity of the bolt in proportion to the reduced edge distance ratio.

This probably introduces new challenges, such as decreased anchor spacing or increased anchor diameter, but at least we have the means to comply with the current design standards and allow for construction tolerances on an exterior 2x4 wall sill plate.