Lag Screw specs

[glenn kangiser]

http://www.kineticsnoise.com/SeismicBook/pdfs/a7/a7.3%20-%20seismic%20design%20data%20for%20lag%20screws.pdf

[jfmann]

Posting this table is grossly misleading without clear explanation of the conditions. Within the other pages not posted (see the link provided) are key notes that explain; (1) A a "steel side plate" is being used, and (2) A load duration factor of 1.6 is used for seismic loading.

Also.......no basis (such as building code or reference code) is listed for capacity values. Considering that the source is apparently a Canadian company, the source may be a Canadian code..........which could be fine, although it should be clearly noted as the source. The use of the 1.6 load duration factor indicates that the US standard (National Design Specification) may also be the basis.

For wood-to-wood connections..........that are NOT resiting seismic or wind forces...........design capacity is going to be MUCH less than shown in the table. For floor loads......or deck loads..........the load duration factor will be only 1.0 so that capacity is then only 62-percent of the listed capacity. Capacity will be reduced even further considering the difference between wood-wood and steel-wood connection.

I will try to remember to add another posting here for actual design capacity for selected lag screws based on NDS code provisions for wood-wood and general floor / deck loading. 

For roof snow load.......load duration factor is 1.15.

[glenn kangiser]

Thanks for the added important information.  Sounds like you know what you are talking about.

[Don_P]

The connections calculator at the link below is developed from the NDS tables and includes most approved "dowel type" fasteners.
http://www.awc.org/calculators/index.html

I just noticed the span calc is now available at the itunes store, will wonders never cease  

Welcome to the forum jfmann, it sounds as though your expertise would be a good addition here... SE?

I don't know that load duration has been explained in the past. I'll take a stab and hope to be corrected where I fail, I'm just a carpenter. Wood is capable of considerable "overload" for short periods of time. A load that would cause failure over a long period of time can be absorbed for short periods with no harm to the wood. a load duration factor of 1.0 is the baseline, it is a load that the wood could safely bear for a period of 10 years without injury to the wood or any change in its recovered shape when the load is removed. A snow load that is only on the roof for a couple of months can be 15% higher than that 10 year baseline load and entirely recover its undeflected shape when the snow melts. The framing can take a 10 minute round of wind or shaking that is 60% higher than that 10 year load without injury, and finally it can take an impact that is roughly double the 10 year load without damage.

Basically, as relates to this, if we use a fastener based on its short term strength for a long term load, it won't be a good thing.

Ahh, I think it is dawning on me how you got here. Good paper!  

[jfmann]

Design shear capacity of some typical lag screws for wood-wood connections are listed below (per NDS-2005), based on the following conditions;

1-Full penetration of the thread (see further discussion of this essential requirement)
2- Connecting 1-1/2 inch thick "side" member to another ("main") member.......with both members being the same wood species
3- Shear force is "perpendicular to grain" of both members being connected. This condition occurs when a ledger board is connected to a rim board / edge joist.........as for typical support of an exterior deck at back wall of house. The ledger board is considered the "side" member......while the rim board / edge joist is considered the "main" member (that takes teh screw thread). Greater capacity is available if at least one of the members is loaded "parallel to grain". Of course the grain of wood runs in the long direction for standard sawn lumber boards.

For the given conditions.......baseline ("Reference") shear capacity depends on size (diameter) of lag screw and wood species (density of wood). The baseline / reference capacity can be increased if a Load Duration Factor might be applicable (see prior posting by Don P......and further discussion below).

Lag Screw Diameter             Reference Lateral (Shear) Capacity

3/8-inch                             110 pounds (SPF) to 140 pounds (Southern Pine)
1/2-inch                             170 pounds (SPF) to 230 pounds (Southern Pine)   
5/8-inch                             240 pounds (SPF) to 310 pounds (Southern Pine)

If side and main members are different species..........shear capacity will be less than the value for the case of both members being the species with higher density. This is when calculations based on code provisions are necessary.

It is essential to understand that design capacity is dependent on adequate penetration into the main member. I have been involved in evaluation of very unfortunate personal injury cases caused entirely by lack of adequate penetration for lag screws installed to "secure" guard railings along wood elevated wood decks. This major problem is also a primary cause of deck collapses...........that are all-too-often considered "accidents".

For full design shear capacity...........penetration must be at least 8 times the screw diameter (8D). For a 3/8-inch lag screw........full penetration is therefore 3 inches. For a 5/8 inch screw........full penetration is a relatively large 5 inches.

For lesser penetration........down to an absolute minimum of 4D.......reduced design capacity available. When using standard tables in NDS-2005, reduced capacity is calculated as the ratio...........(Actual Penetration / 8D) times the reference capacity (from table). When using code provisions (Dowel Equations).........the reduced penetration automatically results in reduced capacity.........although you must be sure that the minimum 4D is being provided.

Proper edge distance is also essential. For the ledger board condition, center of lag screw must be at least 4D from the "loaded edge".........which is the bottom edge of the ledger board. For a 1/2-inch lag screw........minimum edge distance to loaded edge is then 2 inches.

As for Load Duration Factor (LDF)...........it is important to realize (and very often overlooked) that the typical time (duration) used to determine an appropriate LDF is CUMULATIVE during the entire expected life of the building. This means that you do not get to repeat applications of the design loading many times. The time for each application of design loading must be ADDED to the time for all previous applications to obtain the total time used for determination of the LDF.

Also........the LDF is only one of several "Adjustment Factors" that might be applicable for wood design in general.......including connections.........especially for exterior conditions. For connections see Table 10.3.1 in NDS-2005. For connections, other adjustment factors include "Wet Service Factor" and "Group Action Factor".

[jfmann]

Correction to last post.

For the rim board / edge joist......."loaded edge" is bottom edge. However, for the ledger board.........loaded edge is the TOP edge.

The loaded edge is the edge that force from the lag screw (or any other connector) is pushing towards. Load from deck joists pushes downward on the ledger board. This downward force is resisted by upward force provided by the lag screws. For the ledger board.........upward force from lag screws is pushing towards top edge of the board.

For the rim board........the lag screw is pushing downwards. Therefore, the loaded edge is the bottom edge.

[jfmann]

For further discussion of deck attachment to back wall of house.......see the following on my company web site;

http://www.structural101.com/Support-For-Exterior-Deck.html

I am a structural engineer with 33 years total experience in structural engineering........currently licensed in New Jersey, Pennsylvania, Maryland & South Carolina.

[glenn kangiser]

Thanks for the good information, jfmann.  Very generous of you to take time to post that for us.

I browsed your site and articles.  Very well written and easily understood.  A wealth of information that we don't normally get for free.  Thanks again.