Another wood knee braced frame thread

[wayne_dwops]

I know you could knit a sweater with all the threads on this topic but I am still not satisfied with what I've found. I am hoping for a sanity check.

I have modeled an open (25'x15'x13'), wood framed, pitched ramada structure with a tile finish, king posts and knee braces in RISA-3D. As expected, gravity (D + RLL) is inducing intense axial forces through the knee braces and beams, far more than lateral. For discussion's sake, my model states I am throwing 4k axial compression through the brace, and 2.4k axial tension through the beam-column connection for a structure framed 8x lumber. A quick hand calc for lateral loads results in a worst case of about 0.5k per frame I am resisting under lateral load conditions.

Past forum posts suggest neglecting the effects of gravity on the knee braces. I consider this a viable solution to slimming down my connection design, in lieu of stiffening my beams or otherwise modifying the geometry/fixities of the structure. My problem is as follows: In my head, I can't resolve the behavior of a knee brace designed only for the effects of lateral load with how I expect the structure will behave once erected. Mechanics dictates that the brace will try to resist gravity load effects, attempting to approach my RISA-3D model results. Is there a way to design the connections so that the joint can accommodate the clear span gravity deflection of the beams, while still providing support for lateral load effects? A quick web search of these structures does not yield any overly complicated fastening details, rather they all look incredibly simple. What have you done in this situation? Am I way off base?

One immediate thought I have is to allow the dead load deflection to set in before installing the braces, but roof live load effects alone still outweigh the lateral load I am putting through this connection. It may be that I just need to beef up the beams to attract less load to the braces. I've attached an elevation of a knee brace below.


Thank you in advance.

 

[TRAK.Structural]

Typically I will neglect the knee braces when designing the beams for gravity loads as a conservative measure.............but like you, I have a hard time justifying neglecting the DL and Lr component of knee force that gets transferred to the columns/beams. If you are using the knee frames for deflection control and to distribute moments from lateral loads then I believe you are by default assuming the knee braces are engaged according to stiffness; the same assumption would have to follow through for gravity loads.

Specifying installation of the knees after the DL is applied helps in theory but I'm not sure how practical that is.

13 feet tall is not insignificant (imho), I think you would be justified in making changes to member sizes to accommodate the forces/connections from your analysis.

 

[jhnblgr]

How realistic is the base pin connection? Wouldn’t it more realistically need to be able to be fixed?

 

[Harbringer80]

How realistic is the base pin connection? Wouldn’t it more realistically need to be able to be fixed?

For a wood post, pinned is very realistic unless you are embedding the post in concrete. Why would it need to be fixed?

 

[Eng16080]

A quick hand calc for lateral loads results in a worst case of about 0.5k per frame I am resisting under lateral load conditions.

Can you clarify what this means? I assume you have two frame lines in each direction, meaning a total of 4 posts, 4 beams, and 8 knee braces. Is this correct? If so, does that mean a total of 1k lateral load on the structure, split between the 2 frame lines?

Roughly, how far do the knee braces come down from the beam line?

Is there a way to design the connections so that the joint can accommodate the clear span gravity deflection of the beams, while still providing support for lateral load effects?

Curious what other think about this, but I would consider either using a smaller (less stiff) knee brace and/or designing the connection to only resist the smaller loads due to lateral. In that case, I think it's reasonable that the connection will deform a little under the higher gravity load and due to this deformation, the load path will more closely resemble the clear span assumption. I think this should be ok as long as there's adequate ductility in the connection (which there probably is for a wood connection).

 

[jhnblgr]

For a wood post, pinned is very realistic unless you are embedding the post in concrete. Why would it need to be fixed?

Lateral loads and stability

 

[JoshPlumSE]

One thing I wonder about here is the difference between the reality of structure compared to the idealized elastic model.

In particular, there is an issue of how construction sequencing affects the behavior of the actual structure. I might choose to do the following:
a) Create a model with a fixed base and no knee braces. Apply all the dead loads of structure (or any other loads that will be added prior to the knee braces being added in.
b) Create a separate model with a pinned base and knee braces and apply all my other loads (lateral, live, et cetera).
c) Use the principal of superposition to combine the forces from the two models.... This will give you more realistic forces and moments. As long as my assumptions about construction sequencing are correct.

Now, if I am INCORRECT about construction sequencing, then I would accept the forces from your model for the design of all members. But, I would probably design the beams as if the knee braces weren't there. That's just my philosophy for beams. Design them as if removable supports are not present.

 

[gte447f]

Curious what other think about this, but I would consider either using a smaller (less stiff) knee brace and/or designing the connection to only resist the smaller loads due to lateral. In that case, I think it's reasonable that the connection will deform a little under the higher gravity load and due to this deformation, the load path will more closely resemble the clear span assumption. I think this should be ok as long as there's adequate ductility in the connection (which there probably is for a wood connection).

I don't think I agree with this idea, but maybe I don't understand. The gravity loads have significantly higher magnitude and are also theoretically less transient/more permanent, so if you design the brace connection for the lower forces due to lateral load, wouldn't the connections (or the braces) theoretically fail under gravity loads before any lateral load is applied?

 

[gte447f]

One thing I wonder about here is the difference between the reality of structure compared to the idealized elastic model.

In particular, there is an issue of how construction sequencing affects the behavior of the actual structure. I might choose to do the following:
a) Create a model with a fixed base and no knee braces. Apply all the dead loads of structure (or any other loads that will be added prior to the knee braces being added in.
b) Create a separate model with a pinned base and knee braces and apply all my other loads (lateral, live, et cetera).
c) Use the principal of superposition to combine the forces from the two models.... This will give you more realistic forces and moments. As long as my assumptions about construction sequencing are correct.

Now, if I am INCORRECT about construction sequencing, then I would accept the forces from your model for the design of all members. But, I would probably design the beams as if the knee braces weren't there. That's just my philosophy for beams. Design them as if removable supports are not present.

This makes sense for the dead loads, but not the roof live loads. Also it may be conservative for beam design, but I don't think that is an issue. The issue is that the magnitude of brace forces is significantly higher under the effect of gravity loads (even if its just roof live load) than for lateral loads.

To the OP, I say just design everything for the higher loads, unless it just cannot be accomplished.

 

[Harbringer80]

Lateral loads and stability

Draw a free body diagram, apply lateral loads and you will see this is stable. It's similar to a pinned base moment frame.

 

[ANE91]

Agree with releases and support idealizations. Disagree that neglecting gravity is the move.

Parametrize the brace locations but keep them at 45s.

Lateral: shifting braces towards posts generates very high axial brace forces and vice versa.

Gravity: opposite of lateral.

Sweet spot should be around quarter points on the beam.

As a last resort, revert to a shallower brace. Less-stiff braces wouldn’t help, I don’t think…you’d just get more deflection for the same reaction, right? (EDIT: only true for statically determinate...see below corrections.) Share of load is restricted to the constant-depth beam. (EDIT: for rigid supports...corrected to springs below.)

All of this is readily confirmed/disproven with a frame model. It’s been some time since I ran the numbers, so I’m likely misremembering something. Let us know what you find.

 

[KootK]

This, in addition to only being able to count on the knee braces in tension, is one of the main reasons that I'm a bit knee brace adverse.

Yeah, the sequencing argument has merit. That said, I find sequencing to be tough to control in wood construction. Often contractors are pretty adamant about using the sequence that saves them the most $$$ and keeps things moving as fast as possible.

I feel that the magic to making a go of knee braces is the combination of:

1) A stiff beam. A very stiff beam. And combine this with some flexy posts. Without this, you get the prying action shown below which can be a nightmare for the beam to post connection in wood. You are basically trying to create a moment frame that is stiffness skewed heavily towards the beams relative to the columns.

2) A connection arrangement that plausibly has a bit of give to it somewhere. With a stiff beam and the potential for 3/16" axial give at each end of the knee braces, things improve markedly.

 

[ANE91]

1) A stiff beam. A very stiff beam. And combine this with some flexy posts. Without this, you get the prying action shown below which can be a nightmare for the beam to post connection in wood. You are basically trying to create a moment frame that is stiffness skewed heavily towards the beams relative to the columns.

Maybe I'm missing the point, but I don't see how the beam stiffness is relevant at all. It's constant, so the load going to each support is unchanged with different beam sizes. I'd agree if I_x varied. See below...top models are 2x3 DF-N, bottom models are (6)-2x12. Prying is unavoidable (unless braces move in). Nothing really changes except for minor deep-beam stuff.

 

[KootK]

Maybe I'm missing the point, but I don't see how the beam stiffness is relevant at all.

I believe that you are missing the point. In all of your beam diagrams, you've treated the interior supports as rigid. They are not at all rigid. Rather, they are springs, the flexibility of which will be dominated by the flexural stiffness of the columns. As such, a high ratio of beam to column stiffness will tend to minimize the interior support stiffness and reduce those reactions and thus the prying effect.

In the extreme, consider columns of zero flexural stiffness. Now you have a simple span beam.

 

[GJoeRMK]

For discussion's sake, my model states I am throwing 4k axial compression through the brace, and 2.4k axial tension through the beam-column connection for a structure framed 8x lumber

Usually the transfer of forces in a elastic analysis is made by the following criteria:
- The stiffer the object the more force will attract to its element
So you are not crazy getting these results, but there is a catch:
When the structure get into an non linear domain, that knee braced connection is not expected to offer the required ductility

Is there a way to design the connections so that the joint can accommodate the clear span gravity deflection of the beams, while still providing support for lateral load effects?

I remember in RC structures we have a rigid zone between beams and columns, you may try to mimic that for a wood system

 

[ANE91]

I believe that you are missing the point.

I figured. Thanks for setting me straight. I'm all caught up, now.

 

[Eng16080]

I don't think I agree with this idea, but maybe I don't understand. The gravity loads have significantly higher magnitude and are also theoretically less transient/more permanent, so if you design the brace connection for the lower forces due to lateral load, wouldn't the connections (or the braces) theoretically fail under gravity loads before any lateral load is applied?

It was more a hypothesis than anything, but my thinking is that if the connection "failing" means that it deforms some small amount, then at that point the knee brace "support" suddenly becomes less stiff and more of the beam's load goes directly to the post. The thinking is that the deformation is small and not something catastrophic which causes the knee brace connections to lose all strength. Maybe this causes some slip in the connection, where under lateral load, the initial response will be a small movement in the frame before the brace connections engage.

Is this a good solution? I'm not sure. Maybe not. If the full 4k compression force doesn't cause failure in the other members, then I'd probably just design the brace for that load and be done with it. (Maybe that's easier said than done though.)