Minimum number of bolts

[71corvette]

Many design codes and specifications (e.g. AASHTO & AISC) require that bolted connections for structurally significant components contain a minimum of two bolts, regardless of the strength required.

Today I was working with the NDS Timber design spec and noticed that this requirement doesn't seem to be included. Did I miss it, or is one connector all that is required for bolted/lagged timber connections?

Thanks!

 

[IFRs]

I recently became involved in the 2005 NDS code and I did not find a minimum number of bolts required. Note that there are 4 modes of connection failure (Appendix I). I programmed them all so I could duplicate their connection load tables (Table 11G in particular) and then make adjustments for our specific details. I never knew there was so much to wood design!! I prefer redundancy in my connections because 28 years of experience has shown me that the connections are where the failures tend to occur and are the details most overlooked by designers.

 

[archeng59]

I like the two bolt minimum regardless of material.

 

[whyun]

Poke one hole on a piece of paper with a pencil and spin the paper.

Try the same with two pencil holes.

I always liked this analogy with bolts.

 

[IFRs]

Sometimes you want the paper to spin, for example, to have a true pinned connection.

 

[Dinosaur]

I am not an authority on wood design.

I can see situations that could arise where one bolt has way more capacity than the connection requires. Off the cuff (remember I am not an authority on wood design), I would probably be OK with a single bolt if the calculations show I have more than twice the capacity I need. This philosophy is consistent with ACI that requires you to provide stirrups when the shear demand exceeds half the capacity of the concrete alone.

 

[RARWOOD]

I do a lot of wood design and the best advice I have ever recieved, was to provided resistance for the known forces. What the engineer mean't who gave me this advice, was that you shouldn't add fasteners or increase member sizes unless you knew why you were doing it.

Applying this to the design of wood connections the rule of thumb I follow is the fewer the number of fastners the better. Don't use eight bolts if only four will carry the load.

Because of wood shrinkage and distribution of load in connections the use of more fasteners then you need often times produces poorer results. To get a feel for this look at the geometry factor and the reduction required for eight bolts or eight shear plates in a row.

In practice I seen connections that by leaving out every other bolt, you could increase the calculated compression capacity.

Returning to your orginal question, I am not aware of any code provision that requires the use of a minimum of two bolts. In practice I see a lot of single bolt connections.

Personally I like to use a minimum of two bolts for several reasons. As stated above for the prevention of rotation and for redundancy. I also think it is good detailing practice .

 

[SlideRuleEra]

Here is a link to a Master's Thesis on "Experimental Investigation of Group Action Factor for Bolted Wood Connections" (free .pdf download)

http://scholar.lib.vt.edu/theses/available/etd-01022002-094311/

Chapter 5 has a good summary with some interesting conclusions. If I read it correctly, the "optimum" number of bolts per connection (for the loading tested) may be three.

My own experiences indicate that a reasonable compromise, in many cases, is one bolt (sized to take the loads) AND a small number of appropriately sized nails for partial redundancy and to improve joint rigidity.

http://www.SlideRuleEra.net

 

[Lutfi]

I use two bolts to eliminate single point failure mode regardless of how small the load is.



Regards,
Lutfi

 

[Structr]

I would never use just one bolt for a gravity load-carrying connection, but for a beam seat on masonry, for example, where the bolt only provides a "lateral tie", such as the Simpson GLB beam seat, one bolt would be feasible.