Moveable Wall Deflection
It is a fact that in order for an operable wall to function properly (be relocatable) and acoustically (maintain a consistent perimeter acoustical seal), it must be installed within a parallelogram that is as dimensionally perfect as is possible. The question then becomes what project & product design considerations must be made prior to installation in order to handle incidences of on-site “deflection”. Moderco does not involve itself in the design or selection of any support structures but provides to the design team the estimated hanging weight of the partition and associated accessories.
Some facts and terms to consider:
Support Structure: Moveable Walls are typically suspended from steel beams that are designed to deflect from both live and dead loads.
- Moderco has several different choices of track, hangar rods and brackets that are used depending on the application. Track systems can also be suspended from other materials such as wooden beams or structural slabs
Dead Load: A permanent load that must be supported by the structure. ASCE 7-10 defines dead loads as consisting “of the weight of all materials of construction incorporated in the building”.
- Only the moveable wall track, stack track and accessories would be considered as being “Dead Loads”.
Live Load: ASCE 7-10 defines live loads as “ a load produced by the use and occupancy of the building that does not include construction or environmental loads such as wind and snow or dead loads”.
- Moveable walls are considered as being “Live Loads”.
Deflection: In structural engineering terms, deflection is defined as “the movement of a beam from its natural or original position as a result of applied forces or loads or from the weight of the body structure itself.”
- When a moveable wall is suspended from a steel beam, a load is applied and the beam will “want” to deflect because of this applied load. Minimizing this movement (deflection) is mandatory for the moveable wall to function properly and acoustically.
Positive Deflection: A load resulting in the structural beam bending upwards.
- This would be a highly unusual and rarely encountered situation. It would only occur if the moveable wall beam is interconnected to the building structure immediately above it and that building component is subjected to variable live loads that would cause it to move upwards when the loads are removed. For example it could be a floor that is designed to be loaded resulting in a downward deflection to the horizontal plane. When the load is removed the floor would return to its normal position and bringing the moveable wall with it in an upwards (positive) direction.
Negative Deflection: A load resulting in the structural beam bending downwards.
A downward movement of the moveable wall supporting steel beam resulting from the live load/weight of moveable wall panels.
Design Considerations to Prevent Deflection:
1. Decrease the load (a lighter moveable wall).
2. Shorten the span.
3. Stiffen the beam.
- Decreasing the weight of moveable wall panels will always result in a lower STC (acoustic) value. Shortening the span will obviously decrease the desired width of the wall. The practical solution is to “stiffen” the beam so that it will accommodate the weight and anticipated deflection.
Camber:
Cambering is taking a straight steel beam and converting it into one that has a formed vertical arc. It is typically used by designers to provide some artistic design in a structure. A cambered beam can also be used to negate the effect in a structure from unusual “Dead Loads”. There are some limitations ( as per AISC) on what beams can be cambered:
1. None less than 24’ in length
2. Beams in structures using moment frames which is virtually every steel structure.
3. Beams shallower than 14 inches.
4. Beams that have a web thickness ¼” or less.
- The above limitations eliminate the possibility of using a cambered beam to support a moveable wall. In addition because the wall track is installed before the panels and the track has to be installed perfectly level you cannot accomplish this using a cambered beam. If the track is installed level and then the panel live loads are applied, the wall will deflect downwards based on the amount of cambering, moving the track and attached ceiling with it and the wall will no longer be on a level plane.
ASTM E-557:
ASTM E 557 provides design details that should be considered when a moveable wall is used in a building. E-557 is included in virtually all specifications where moveable walls are required. Two important provisions for floor flatness and deflection are included:
- The floor immediately under the partition should not vary from a smooth level surface by more then +/- 1/8” in 12’ non accumulative (ASTM E- 557, 5.2.3).
- The weight of the operable partition, in addition to all dead loads, should be taken into considered when designing the supporting member. Deflection under maximum anticipated load should be no more than 1/8” in 12 ft. of opening width (ASTM E-557 5.4).
Moveable Wall base Drop Seals:
Drop seals are provided for 3 reasons: 1. Close the opening between the bottom of a panel and the finished floor. 2. Exert a measurable pressure on the finished floor in order to provide a positive acoustic seal. 3. Lock the panels into place. ASTM E-557, Section 5.4 says: ….”If greater deflection is anticipated …. an operable partition with bottom seals designed to accommodate the larger deflection should be specified.”
- We disagree with this statement. Drop seals should not be specified or relied upon to accommodate a “larger deflection”. As stated this is not their purpose. By doing so, excess physical strain on the system, unacceptable aesthetics as well as significantly reduced acoustic performance will result.
Summary
It is Moderco’s opinion and position that strictly adhering to the requirements of ASTM E-557 is the only practical way to eliminate deflections that might impair the desired operation of a moveable wall.
- Select a beam that will support the weight of the wall resulting in a deflection of no more than 1/8” in 12’ of opening width as per ASTM E-557 5.4.
- Ensure that the floor underneath the moveable wall does not vary by more than +/- 1/8” in 12’ non accumulative as per ASTM E-557 5.2.3.
- Contrary to ASTM E-557 5.4, do not rely on any moveable wall acoustic seals to compensate for any deflections encountered. That is not their purpose.
If you have any questions please call us at 1.800.363.3150
