Aluminum Bracket Thermal Expansion 3

[Adamian]

I am designing a sign consisting of a 20'x8'x1/8" aluminum sheet and 1"x1"x1/8" angle attached to an existing steel lattice-like Architectural fixture with screws every 12". The lattice is made of 1"x1" steel tubes. The thermal expansion coefficient for aluminum is 12.3 10^-6, and steel 4.7. The difference is 7.6(in/in F).
Differential expansion is (7.6^10-6 in/in F)x(20ft)x(12 in/ft)x(212F-32F)=0.328in.
Is this a large enough expansion to be included in the design? For example I was thinking of making the predrilled holes in the aluminum angel elongated to allow movement.
Is there a Code requirement for that?
Thank you!

 

[Ron]

Two things..yes, that's enough to include in the design. If you don't you'll eventually fatigue your fasteners. Secondly, you can use slotted holes for the attachment and remember to use stainless steel fasteners to mitigate the corrosion issue between steel and aluminum.

Even though the corrosion charts show that aluminum will corrode when connected to steel, the initial corrosion of the aluminum will passivate the surface and the corrosion will reverse...the steel will corrode more severely than the aluminum.

 

[Adamian]

Thank you Ron!
I will specify slotted holes to allow 0.17in movement each way. Can you also help to pick the right screws?
Thank you!

 

[desertfox]

Hi Adamian

Some additional points, firstly from this link I get the coeefficients of expansion to be:-

12.3 and 7.3 (in/in F) for aluminium and steel respectively.

http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

When you bolt through to connect the alumium and steel together what measures have you taken to allow the steel and aluminium to move relative to one another directly under the bolt head.
Basically what I am saying is if the joint is clamped to tight then putting slotted holes in the aluminium might not work.
Finally your differential assumes that both materials heat to the same temperature what happens if they don't.

desertfox

desertfox

 

[Ron]

Adamian...DF brings up some good points. If the area coverage of the aluminum is much greater than the exposed steel area, your differential movement might be quite a bit more than you have calculated.

Next, make the slots larger than you need...to accommodate at least some inaccuracies in the assumptions.

Next, you still need to isolate the aluminum from the steel. When one is in contact with the other, with frictional sliding motion between them, corrosion will occur without regard for the type of fastener.

As for the fastener, check ASCE 8-02 for fastener strengths based on various stainless steel alloys.

 

[Adamian]

Thank you all for valuable comments!
I was using the same source for thermal coefficients, and (my mistake) 4.7 is for steatite one row above steel.

The expansion to be counted is now 0.5in if aluminum goes from 0 to 100 degrees Celsius, and steel maintains its temperature.

I am also going to redo the sign attachment using Unistrut. This will reduce drastically the steel to aluminum contact surface. I will use plastic washers too. And the existing support structure will sustain minimal damage if less intrusive elements such as steel straps and clamps are used in this design. I will post the detail when ready.
Thank you Ron and DF!

 

[desertfox]

Hi Adamian

Your welcome.

An additional comment for the bolted joint, if it were me I would calculate the expansion of the thickness of the materials being clamped by the bolt and then use some belville washers to allow differential expansion whilst still maintaining some bolt preload.

desertfox

 

[bimr]

What you are describing sounds like an architectural panel. Have you reviewed your design with one of the firms that make these panels? It would seem like your sign is an application for one of the composite aluminum panels.

See the product details on:

http://www.bamcoinc.org/

 

[bimr]

http://www.graphic-al.com/aluminum-composite-material/graphic-al-or.html

http://www.alcoa.com/aap/north_america/catalog/pdf/specifications/Reynobond_FabricationGuide.pdf