Truss deflection

[hoshang]

Hi all,
I'm designing a steel structure (Pratt type) with the following information:
Span: 21m
Bay spacing: 3.7m
DL (excluding self-weight): 1kN/m^2
LL: 1kN/m^2
Wind speed: 100 km/h
Upper chord: HSRE 6x5x0.375
Lower chord: HSRE 6x5x0.375
Post: HSSQ 5x5x0.5
Diagonal: HSSQ 5x5x0.5
Strut: HSSQ 5x5x0.5
Purlin: HSSQ 4x4x0.25
Ridge beam: HSSQ 4x4x0.25
When calculating it, I'm getting deflection of 52.8mm. When comparing with L/400, it's getting acceptable. The siding will be glass. I'm not aware if this deflection is permissible for such a siding.

 

[le99]

You have to get the installation/deflection tolerances from the siding company.

 

[XR250]

You might want to verify that your chord sizes are readily available.

 

[JLNJ]

You didn't provide much information as to the specifics of your installation.

2" sounds like a lot for glass.

 

[rb1957]

is that displacement over the entire span or one bay ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[hoshang]

2" sounds like a lot for glass.

Thanks. But how much deformation is not a lot?

is that displacement over the entire span or one bay ?

I don't know what do you mean.

 

[BAretired]

That is reasonable for a truss spanning an open area, but far too much for glass. You need to break the wall up into smaller sections. Window suppliers can handle 10mm maximum deflection of the structure above.

BA

 

[KootK]

It will be your absolute deflection that governs your design. I try to keep it in the 3/8" to 3/4" range unless I know that it's a system that's specifically designed to accommodate more than that.

 

[jayrod12]

I've always been told to never exceed 3/4" when supporting glass, or having a beam above a glass wall with a slip joint as that is the most that their connections can tolerate.

 

[hoshang]

Window suppliers can handle 10mm maximum deflection of the structure above.

Where does this 10mm come?

I try to keep it in the 3/8" to 3/4" range unless I know that it's a system that's specifically designed to accommodate more than that.

I'm not aware where does this 3/8" to 3/4" come. Can you provide a reference?

I've always been told to never exceed 3/4" when supporting glass, or having a beam above a glass wall with a slip joint as that is the most that their connections can tolerate.

Can you elaborate on this and provide a reference?

 

[jjl317]

I don't think you will find a reference, as the amount of movement that a glazing system can accommodate can very greatly. Generally, systems with more smaller lites of glass and more joints can safely accommodate more movement than larger lites with fewer joints. And certain system types (i.e. unitized curtain wall) can generally accommodate more movements that others (point supported glass).

In my mind, I can think of two options - like le99 said, reach out to the company providing the system, if the project is that far along. Or, if early in the design, you could just clearly identify how much movement the glazing system needs to take, and let the glazing system designer resolve. I can remember several jobs where our work on the design of the glazing system was influenced by the design movement of the superstructure.

 

[HouseBoy]

Seems like the allowance for deflection will be highly dependent on the size of the window sections and how they are "held in place".

What is the size (and configuration) of any given piece of glass?

A sketch (even rough) would be helpful