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Rafter ties (metal straps) hogheavenfarm 04-02-2008
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Posted by Matt W. Barrow on April 3, 2008, 3:14 am

> wrote:
>>
>>
>> > The smaller dormer could easily experience more turbulence and be
>> > subjected to greater forces, and since code requires that certain
>> > areas (end zones) be designed with a greater wind load for a given
>> > wind speed, you're right in not skimping on the dormer.
>>
>> Pardon,
>>
>> How could a smaller dormer experience MORE turbulence?
>
> More turbulence than a roof without a dormer?

Okay...maybe I'm missing something here: the point was not a dormer versus
NONE, it was a smaller one versus a BIGGER ONE.


> Easy. You're putting
> an object in the way of the air stream => turbulence and quite
> possibly increased localized wind speed.

Okay...you've totally lost me.



Posted by hogheavenfarm on April 3, 2008, 6:33 am
I was thinking more of uplift which is generated by the square footage
of the roof area (and the pitch). A large uninterrupted area would
actually have a greater uplift than if a dormer or two created some
turbulence, which by its nature, disrupts the uplift forces. The
dormers are also at right angles to the wind direction (facing the
wind) so the uplift forces don't act on the dormer roof, only the main
roof. The gable end of the dormer would be subject to a direct wind
load, but not an uplift force.
Remember also, that our dormer units are completely preassembeld, so
they are not part of the roof structure, (no studs continue through to
the roof space to the floor). If a wind was to completely sever a
dormer from the roof, no structural component of the roof system would
be compromised.
Back to the original question - I was asked whether the square
footage of the dormer roof creates enough uplift to justify full 'wind
treatment' . Not being an engineer, I can only surmise that a smaller
footprint creates less uplift. At what point are all the bells and
whistles necessary?

Posted by RicodJour on April 3, 2008, 9:40 am
> I was thinking more of uplift which is generated by the square footage
> of the roof area (and the pitch). A large uninterrupted area would
> actually have a greater uplift than if a dormer or two created some
> turbulence, which by its nature, disrupts the uplift forces. The
> dormers are also at right angles to the wind direction (facing the
> wind) so the uplift forces don't act on the dormer roof, only the main
> roof. The gable end of the dormer would be subject to a direct wind
> load, but not an uplift force.
> Remember also, that our dormer units are completely preassembeld, so
> they are not part of the roof structure, (no studs continue through to
> the roof space to the floor). If a wind was to completely sever a
> dormer from the roof, no structural component of the roof system would
> be compromised.
> Back to the original question - I was asked whether the square
> footage of the dormer roof creates enough uplift to justify full 'wind
> treatment' . Not being an engineer, I can only surmise that a smaller
> footprint creates less uplift. At what point are all the bells and
> whistles necessary?

Code does not differentiate so you are required to conform to the roof
assembly requirements on all parts.

R

Posted by RicodJour on April 3, 2008, 9:48 am
wrote:
>
>
> > wrote:
>
> >> > The smaller dormer could easily experience more turbulence and be
> >> > subjected to greater forces, and since code requires that certain
> >> > areas (end zones) be designed with a greater wind load for a given
> >> > wind speed, you're right in not skimping on the dormer.
>
> >> Pardon,
>
> >> How could a smaller dormer experience MORE turbulence?
>
> > More turbulence than a roof without a dormer?
>
> Okay...maybe I'm missing something here: the point was not a dormer versus
> NONE, it was a smaller one versus a BIGGER ONE.

Roofs are designed with loads that are attributed to zones. Eaves,
edges and ridges are subjected to greater stress and the design loads
and nailing pattern requirements are in acknowledgment of the
increased stress on those areas. Since a smaller dormer is
essentially all end zone the requirements are actually more stringent
than in the middle of the roof field or on a larger dormer.

> > Easy. You're putting
> > an object in the way of the air stream => turbulence and quite
> > possibly increased localized wind speed.
>
> Okay...you've totally lost me.

A certain volume of air is moving at a certain speed across a roof,
that creates a certain pressure and a certain load. When the wind
encounters an obstacle it doesn't just stop or change direction with
no change in pressure. Whether you approach it as a Venturi effect,
an airfoil or simply compressing the air, the net effect is the same -
the wind pressure/load is increased locally.

R

Posted by hogheavenfarm on April 3, 2008, 10:39 am
Ok, thanks all, that makes some sense to me . Just wondering what the
rest of the world did.

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