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DESIGN CONSIDERATIONS
Thermal Expansion and Contraction
Due to thermal expansion and contraction, Plexiglas® sheet is subject to greater dimensional change than other materials with which it is used in construction. This dimensional change difference is shown in Table 8.
Rigidity
Plexiglas® sheet is not as rigid as many other materials used in building, although it is more rigid than most other thermoplastic sheet materials. Lower rigidity can cause the material to deflect under load, and as a result, foreshorten. Rigidity expressed in terms of modulus is shown in Table 8.
Table 8: Thermal Expansion/Contraction and Rigidity Comparison
of Plexiglas® sheet vs. Other Building Materials |
|
Material |
Expansion Coefficients
("/"/°F) |
Typical Modulus
(psi) |
|
Plexiglas® Sheet
Aluminum
Steel
Plate Glass |
0.0000410
0.0000129
0.0000063
0.0000050 |
450,000
10,000,000
30,000,000
10,000,000 |
Temperature/Humidity Bowing
Different temperature and/or humidity conditions on the inner and outer surfaces of Plexiglas sheet may cause the sheet to bow somewhat in the direction of the higher temperature and/or humidity. However, this type of bowing is reversible, and the sheet will return to its original flatness when the temperature and humidity differentials are equalized.
Bowing does not affect visibility through flat transparent Plexiglas® sheet, but may cause distorted reflection. In translucent or opaque panels where visibility through the material is not required, surface textures or formed designs will help disguise specular reflection distortions.
Service Temperature
The allowable continuous service temperature ranges for Plexiglas® G sheet (180°F to 200°F) and Plexiglas® MC sheet (170°F to 190°F) are sufficiently high for exterior applications and fluorescent lighting.
Installation
To accommodate the greater thermal movement and deflection foreshortening, Plexiglas® sheet will perform best when these recommendations are followed:
- Plexiglas® sheets or panels should be installed in a channel frame engaging all edges of the material so that the material is free to expand and contract without restraint.
- The channel frame or rabbet depth should be sufficient to allow for thermal contraction and foreshortening of the Plexiglas® sheet, without withdrawal of the edges from the frame. Channel depth specifications are given in theglazing sections of this brochure
- Through-bolting or use of other inflexible fastenings that do not provide for expansion and contraction may cause failure of the installation.
- Before installation in the channel frame, Plexiglas® sheet should be cut sufficiently shorter than the channel frame dimensions to allow for thermal expansion.
- Sealant compounds and tapes should be types that are sufficiently extensible to accommodate thermal expansion and contraction of the Plexiglas® sheet, and that adhere to both the Plexiglas® sheet and the frame. Sealants and tapes should be tested for chemical compatibility with Plexiglas® sheet.
- Forming Plexiglas® sheet will increase its rigidity if the shape is properly designed. Whenever practical, formed Plexiglas® sheet panels should be specified for large unsupported areas where wind or snow loads are involved. Trade associations, such as the American Architectural Manufacturers Association (AAMA) in Chicago, have developed design recommendations for formed shapes, such as free-blown skylights.
- If forming Plexiglas® sheet is not practical, increasing the thickness of a flat Plexiglas® sheet will increase rigidity. If flat sheet is used in overhead glazing, it is to be installed on a slope greater than 10° from the horizontal, and engaged along all four edges.
Design Stress
IMPORTANT -- Stresses considerably below ASTM tensile and flexural values will produce a light surface check known as crazing. To avoid stress crazing, design limits for continuously imposed loads should not exceed 1500 psi for Plexiglas® G sheet and 750 psi for Plexiglas® MC sheet at or below room temperature.
Stresses of slightly greater magnitude but of short duration may not cause crazing. Optimum properties can beobtained by sanding cut edges as noted under the Plexiglas® glazing preparation section.
Custom design using Plexiglas® for load bearing should only be done by a design professional familiar with plastics.
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