Operable moveable walls are designed to create flexible spaces. To divide space, provide acoustic control within the created space, be aesthetically pleasing, easy to relocate etc. However, deflection – the bending of the overhead support structure that supports the moveable panel systems – can disrupt these desired characteristics that will lead to performance issues, unnecessary and costly repairs, and result in safety concerns. To address deflection, you must understand what it is, the causes, the prevention, and considerations at the “design” stage to mitigate and hopefully fully limit any deflection that might occur.
What is deflection? When considering the deflection of moveable operable wall systems, deflection refers to the vertical movement or displacement of the overhead moveable wall supporting structure – typically a steel beam – under the weight of the operable wall panels and other loads.
Types of Deflection and Types of Loads
There are two types of Deflection:
- Positive Deflection: A load resulting in the steel beam bending “upwards” – highly unusual.
- Negative Deflection: A load resulting in the structural beam bending downwards (common due to the weight of the moveable wall panel system).
There are two types of “Loads”:
- Live Loads: A load that can vary in magnitude and position over time. For example, panels and other dynamic components.
- Dead Loads: A constant static load that does not change. The supporting structure, the track and support systems and any other permanently attached components are examples of “dead loads.” The panels when they are in the storage area. Some designers, to simplify calculations, will consider the panels as being a constant quasi-dead load. This is possible if the intent is to leave the panels for an extended period (such as a gymnasium divider) without being moved.
Accepted Deflection
Depends on who you speak with – the operable wall manufacturer or the structural designer – and the height and weight of the wall and desired acoustic performance and aesthetics. Use ASTM -557 as your guide.
Negative effects of Deflection
Structural deflection can cause gaps to form between the wall panels and the floor, affecting the walls performance in terms of sound isolation, appearance, aesthetics, panel movement and light control.
ASTM E-557
Is a crucial standard that outlines standards and methods to ensure the optimal performance of moveable wall systems including installation, deflection, and the construction of the opening in which the operable wall will be installed. The maximum deviation in floor level will be +/- 1/8” in 12’. Re overhead structural support, it specifies a maximum deflection of 1/8” in 12’ non-accumulative for the supporting beam. Adhering to these maximum deflection criteria (every moveable wall specification mentions ASTM E-557 and the need to adhere to its standards but rarely are they followed) will ensure that the wall will function as desired. ASTM E-557 must be considered at the design stage and implemented during construction and by doing so the negative effects of any deflection will be negated. The Structural Engineer and the Architect should monitor adherence to ASTM E-557.
Causes of deflection
Very simply, deflection is caused by gravity.
- Structural Loading: Excessive weight of the panels on a structural system not designed for zero deflection. Zero deflection should be the goal.
- Material properties: Consider the total weight of the wall system, determine the materials from which it is manufactured (aluminum is lighter than steel for example) and see if you can achieve the same field results with a lighter material.
Preventing deflection (structural design, material selection, track system design, installation, and alignment) at the design stage should be the goal.
The role of the panel base seals in compensating for deflection
The panel base seals can in a limited way assist in mitigating the effects of building deflection. However, the primary purpose of panel base seals is threefold:
- When activated the base seals will compress against the floor locking the panels together thus preventing movement.
- The seals provide an acoustic barrier preventing sound transference.
- They finish off the base of the panels contributing to the aesthetics.
However the primary role of the base seals is to compensate for floor variations within ASTM E-557 limits.
On an extremely limited basis the base seals, rigid along their entire length, may compensate on a minimal basis for some deflection.
- They are designed to compress and expand, absorbing vertical and horizontal movements that occurs due to building deflection.
- By having the built-in capability of moving vertically they can help prevent gaps from forming between the wall and the floor.
But base seals should not be relied upon to contribute in a significant way when dealing with building deflection.
General design considerations
What factors significantly influence deflection in movable wall systems?
- Panel weight and component weights: it is important at the design stage that the structural engineer acquires from the manufacturer upon which the project specification is based the entire weight of all products that will be installed. In addition to the panels this would include tracks, support systems, and any unusual accessories such as pass doors and glazing that will be incorporated into the system. Do not rely just on “panel weight.”
- The span length: The distance between supports. Longer spans deflect more. If you wish to limit deflection, consider introducing intermediate support points or using deeper beams.
- Other factors to consider are material properties (steel is stiffer than wood or aluminum), load distribution across the beam, construction tolerances, the inclusion of stiffeners and sway bracing, the selection and use of the correct wall seal systems, overdesign for stiffness, optimize the beam section, determine the deflection limits based on acoustics and aesthetics, the lightest panels possible while still achieving the desired acoustic results etc.
- The support material and its shape that will be used. For example, steel is stiffer than wood or aluminum.
Base Gap calculation
When a load (the operable wall) is applied to the structural support the wall will want to deflect downwards (a negative deflection) because of the applied load. Conversely if the floor above the structural member is designed to support unusually high loads, when the loads are removed, it is possible that the floor and wall support beam will deflect “upwards” – a positive deflection. This is very unusual. If a negative deflection occurs of say ½” in the center of the run, then the panels will stay tight together at the top, but a gap will occur at the base of the wall. Called the “ice cream cone” effect. How much of a gap? To calculate the gap in the center of the run, the simple formula is GAP = (Total Deflection/Panel Height) * Panel Width. All in inches for consistency. Examples at various heights with a deflection of ½”will be:
- 10’ high. Gap will be 0.2”
- 20’ high. Gap will be 0.1”
- 30’ high. Gap will be 0.067”
- 40’ high. Gap will be 0.05”
To confirm these basic calculations, consult a structural engineer. Regardless, the gaps are significant and will negatively affect the acoustic performance of the wall system as well as the aesthetics.
Can anything be done in the field to correct an undesirable deflection without modifying the primary structural element?
The short answers are “No” or “Maybe.” Therefore, you must compensate for the resulting gap in some other way. Some suggestions – mostly unacceptable – are:
- There are fasteners available that can be connected to both the track and the support beam that allow for some adjustment based on slotted holes which will provide a degree of flexibility. Excessive adjustment, however, may compromise the walls’ stability.
- You could add some shims (Wood, Metal etc.) at the base of the panels to account for the gap.
- Fill the gap between panels with some flexible sealants.
But the bottom line is that it is crucial that you consult with a structural engineer to assess the specific situation and have him/her recommend the most appropriate solution. Maintain the wall to prevent deflection (regular inspections, cleaning and lubrication, panel adjustments, addressing deflections immediately, operational guidelines etc.) as much as possible. Get after it when you first notice any deflection.
Conclusion
In conclusion addressing deflection in movable walls requires a multi-faceted approach including careful design (this is the real key) of the support beam based on zero deflection, material selection, proper installation, and ongoing maintenance. By understanding the factors contributing to deflection and the workable solutions owners and occupants can ensure the long-term performance, safety, and acoustic integrity of the movable wall systems.
Collaborate with a structural engineer and the desired wall manufacturer.
