Wood Roof - Thrust 9

[Buleeek]

Hi everyone,

I'm analyzing the TFEC Bulletin No. 2018-12 "Behavior of statically determinate and indeterminate rafters". If I use a collar tie (or strut) system (with no horizontal reaction at the bottom of the rafter)can I assume there will actually be NO horizontal force at the wall top plate? I know that collar ties should not be used to carry the thrust, but struts yes (located higher than top plate level in this case).

I am not sure what situation occurs in a real life. I can't assume that the rafter has no thrust at the bottom, since it is resting on the wall plate and is always notched/nailed/connected.

What do you usually do in such situation? That assumption determines a wall design and how much thrust it will carry.

Thanks,

 

[r13]

I am not a wood designer, but bothered by the implication of the roller support in the model. Though the stud walls are considered not capable of provide lateral restraint, they are physically connected to the roof truss, and the connection will feel horizontal force (shear) while there is relative movement. In real world of design, this small displacement of the joint could have been ignored by experience, and may have been taken care by other method/mechanism, but the force is there. I think adding an axial spring to close the gaps maybe is desirable.

 

[jayrod12]

retired,

the wood walls will rotate outwards to accommodate the amount of movement required for the roller support assumption to be true. It may be both walls moving half the amount required, or one wall moving all of the amount required. But that's the mechanism we wood designers use to validate our design assumptions.

If, for some reason, the walls are prevented from moving at the top the required amount for the roller assumption to be valid, then the horizontal thrust forces must be designed for.

Estimating the stiffness of the spring would be borderline impossible, and quite frankly a waste of research time and money considering the assumptions we have always used are working fantastically.

 

[r13]

jayrod12,

I acknowledge the claim, that the "deflection is small", since it was made through the accumulated experience on designs with adequate member stiffness (rafters, ties). But from practical point of view, I think the tie should be placed as low as possible, and the walls maybe are better off if connected close to end of the tie, rather than connected at the tip of the rafter.

I won't argue the necessity of adding spring support, but the evaluation on the spring is not that troublesome by using "close the gap" method, just a little tedious to add another iteration in the analysis.

 

[jayrod12]

Agreed on tie location. the lower the better or else you put a lot of bending into the tail of the rafter. Ideally the tie would be the ceiling joists, and therefore would meet your recommendation of wall connection close to the tie connection. However it is not always possible and is not always required to be that way.

I'm not sure I follow what you are indicating regarding the spring support and "closing the gap". I am of the opinion I can put whatever I'd like into a model, but whether that translates appropriately to real life is a different argument. I don't see how to get an accurate spring constant for the as constructed condition is possible. And therefore introducing it into the model is essentially useless as the reality will not reflect the modelling.

 

[BAretired]

retired13,

To assume pin and roller supports is conservative. If the wall resists horizontal movement, the stresses in the rafter and rafter tie are reduced, so the design is safe.

More likely, the roof deck will tend to span the length of the roof as a rigid diaphragm, providing a force at points A and E in the direction of the roof slope. This action could change the force in the rafter tie from tension to compression, sometimes causing the rafter tie to fail by pushing through the roof deck as reported by dik.

Stress reversal of rafter ties was mentioned earlier in this thread and is recognized by engineers, but diaphragm action requires proper nailing and continuous chord members which must be carefully inspected if used to support gravity loads. In addition, future changes or additions made by the owner could render diaphragm action unreliable, so for these reasons, it is not normal practice to consider it.

BA

 

[r13]

Use the unit/dummy load method. The deflection due to gravity load is known, therefore the two values can be proportioned to get the additional force in the truss due to the "closing force". Learnt during college days, hope my memory is not mistaken.

By the way, what is a ring beam, does it play any role in preventing lateral movement of the roof truss?

 

[r13]

BA,

Yes, I can see the benefit of the roof sheathing that forms diaphragm. Isn't that been treated as deep beam?

 

[BAretired]

The unit/dummy load method is a method of solving indeterminate structures. It could be used to solve this structure if A and E are pinned and there is also a rafter tie, making it indeterminate to the first degree. Essentially, the technique is to make the structure determinate by removing a force, then adding a unit force in the same direction as the force removed. Then solve for the force required to provide consistent deformations.

There is no ring beam in this thread. The only ring beam I can think of is the beam around the springing line of a spherical or conical shell.

BA

 

[r13]

My question was what is the purpose and function of ring beam. Does it has any significance in resisting lateral load. If it is not required at all time, why?

 

[BAretired]

Yes, I can see the benefit of the roof sheathing that forms diaphragm. Isn't that been treated as deep beam?

It would be treated as a deep beam, and is often relied upon by framers, but for reasons given earlier, usually not by engineers.

BA

 

[BAretired]

My question was what is the purpose and function of ring beam. Does it has any significance in resisting lateral load. If it is not required at all time, why?

Ring beams are usually reinforced concrete members intended to resist lateral load. They may be used at the top of a masonry wall, but usually not on top of a wood stud wall. I would guess that the main reason they are not used is cost, but there a host of other reasons why they are impractical on a wood framed building.



BA

 

[r13]

BA,

Thanks.

 

[jimstructures]

Retired13,

A ring beam can be used at the top of the wall to resist the tendency of hip roof rafters (either hip, common or jack) to kick out where they rest on the top of the walls. The ring beam is usually a steel beam with the strong axis oriented horizontally and with fixed moment resisting connections at the corners. These work when a rafter tie is not used so the owner gets a cathedral roof. They also usually object to column supports under the ridge board/beam.

These are not very common as they are expensive and difficult to detail/construct.

Jim

 

[r13]

james,

Thanks. Getting better picture now.