Story Drift resistance by post caps only? 1

[Cayroo]

Does anyone know how to calculate the story drift on a building grid line whose only lateral force resisting members are post caps, such as Simpson PC or EPC caps, and the grid line does not contain any other type of shear wall?

 

[whyun]

This will depend on the bottom restraint. Is the column cantilevering out of some fixity? If pinned at the base, the structure is unstable. Unless you can justify partial fixity at the post cap and account for the beam such that frame action can take place. I don't have confidence in this approach...

is the column HSS or pipe? or is it wood?

 

[Cayroo]

I am asking about wood posts with a Simpson PC or EPC, rated for a lateral load of between 925 to 1610 pounds in the direction of the beam, with a Simpson column base. Would the base connection preclude the use of the column cap to transfer shear down to the foundation?

 

[whyun]

The lateral force you describe indicate the amount of force that the Simson connector can transfer to the column. Now, the load is in the column. The stability of the column (bending stress and deflection at the tip), now, will be a function of the support condition at the base. If it is pinned (as in any type of post base connectors provided by Simpson), it cannot resist any bending moment present in the column. it is a mechanism. Post must be "fixed" to ensure structural stability.

If you have a bunch of these columns in a row, it will form a flimsy frame. Although the PC's have nominal moment resisting capacities, it is negligible, thus I wouldn't be able to rely upon a frame action.

Then again, I am an engineer from California. Serious seismic zone.

Any other engineers have any thoughts on this topic?

 

[DaveAtkins]

I agree with whyun. As far as I know, none of the Simpson connectors are meant to resist moment, just shear or axial forces. I don't think the Post Caps have enough rigidity to create a rigid frame. DaveAtkins

 

[ronster]

If you go with a CC column cap with CB base that are 7 gage and secured with 5/8" diameter bolts then you can develop some moment capacity at the top and base of your column. D ivide your lateral load at each column by 2 to determine your top and bottom moments and see if it works.

 

[uleaum]

I would like to know where whyun and ronster are getting the values for moment capacity of Simpson connectors. I have considered using an extended steel gusset plate (1/4" material,not gage) fixed with nails to create a moment connection in wood, but I know of no generally accepted practice that would create a moment connection in wood from linear members.

A wood frame might be built, but my experiences have shown that the limited capacity of connections prevents even the smallest loads from being resisted. I tried to do it with an open gazebo and couldn't make it work. Depending on your seismic zone and the geometry of your structure, you might make it work.

One option that I think might work is to try creating a moment connection at the top of the column from plywood, and leave the base pinned. Edge nailing might spread the load out versus concetrated connections from lags or bolts as found in a frame. You would have to use something like the elastic vector method to analyize the connection. Keep in mind that you can pre-drill nail holes to get a tighter spacing if required.

Drift should control anyway. You should be able to model this in software for drift as a frame with a diaphragm. I have never done it but I think most good analysis programs do it. I don't exactly know how you should account for nail slippage in this case but I'm sure it can be done. I don't know where to begin on diaphragm strength unless your software does that (any ideas).

If you don't have software for deflection analysis it's time to learn matrix analysis!

If someone knows a better way to analyse this type of connection let me know.

 

[ronster]

Simpson does not publish allowable moment capacities for these connections. I use the the Simpson CC post cap and CB post base, which are 7 gage (3/16&quot steel with 5/8" diameter bolted connections (no nails) at porches, decks, and gazebos where lateral loads are small, deflection requirements are not a main concern (no windows or doors) and you have no walls to use as shear walls.
Moment capacities can be determined the same as if you were adding steel gusset plates. Moment=Force (all bolt shear or plate in tension) x distance between bolts.

 

[boo1]

Consider using a steel pipe with bolted end plate.

 

[uleaum]

In the conditions ronster listed the Simpson CC could be acceptable depending on the geometry and analysis of method for the decks. If the deck is attached on two sides it's OK. If the deck is attached only at one side the deck sheathing (if planks are used) needs to be framed at a 45-degree angle to qualify for rotational analysis of a flexible diaphragm. (UBC/IBC code assumed)

In the condition of the original question where no other lateral resisting elements exist the CC cap with bolts is unacceptable. The tolerance for wood bolt holes is 1/16". The tolerance for bolt holes in the connector is 1/8" oversized. Your worst case slip is going to be 1/4" (one bolt high, one low). The greatest distance between bolts in the listed CC series is 8" (Simpson CC68,(7.5^2+3^)^0.5). This slip yields a story drift ratio of sin(arctan(0.25/8))=0.0312. This value exceed the maximum seismic drift ratio of 0.025 for low rise wood structure (UBC 1630.10.2) before you even apply a load.

While there is no code required minimum for drift from wind (UBC/IBC), the Structural Engineering handbook recommends a maximum of 0.0035 and an average of .0025(3rd ed p23-29). Basically an order of magnitude difference.

The reason that wood connections can't resist moment becomes easier to see when you realize that bolted connections(and other connection types un wood) resemble a shear tab in steel (Bolts penetrating the perpendicular to the web) and do not transfer moment. Even if the material was steel in this situation it wouldn't resist moment.

With these kinds of deflection you have to examine P-delta forces, but even without that your bolts should be failing. I recently tried to put a steel truss in small garage to resist wind (I ended up having to put a moment resisting frame in-extended end plate bolted connection). With a strut attached 18" from the column-beam connection. I ended up with about 7000 lbs of shear at each connection point. Now think about moving the connection points to 8" and doing that in wood.

I mistakenly called the bolted wood system I was referring to in the previous post a "wood frame" for purposes of illustration. In reality it would be a wood truss (pinned connections). All wood trusses (roofs, floors, etc.) are designed with pinned connections. The gusset plates applied do not transfer moments. The "extended gusset plates" I was referring to above would continue down the chords and struts several feet to minimize the maximum slip angle and prevent rotation.

Sorry for the length of this post, but I wanted to stress very important point. Unless you do extensive design and calculations, wood members do not transfer moment at connections.

 

[ronster]

To clarify things:

Maximum bolt holes in connectors are 11/16" for 5/8" diameter bolts. This allows for a maximum combined slip of 1/8" not 1/4". A more likely combined slip would be 1/16" and that is assuming the shear force in the bolt is large enough to overcome the friction force.

I use these to justify resisting wind loads in decks not in buildings. There are no windows, doors, or finish elements to worry about racking.

7 gage material is 3/16" thick. Plenty thick to handle the wind forces being applied to a deck.

For example, 500 pounds is realitic wind force applied to a girder line in a deck. Assume a 3 post frame and you have 250# shear at the middle column and 125# at the end columns. Assuming a fixed base and cap then the moment at the middle column is 250#x4' (half the story height)=1,000#'. Divide that by 8" bolt spacing and 2 bolts and you get 750# shear on the bolts. A 5/8" carriage bolt in double shear with steel side plates can take about 1500# to 4000# depending on direction of grain and size of post.

I respectfully disagree with you that wood members with bolted steel connections can not develop moment. You just have to have an understanding of the application that you are using them in.