MOVABLE HATCH
ME 493: FINAL REPORT – YEAR 2015
JOHN ROBEY, ENIS INAN, ZACHARY WERBER, CODY HANNAN, & DAVID PRIOR
PORTLAND STATE UNIVERSITY
ADVISOR & SPONSOR – Zdenek Zumr
Table of Contents
i.
Executive Summary.................................................................................... 2
I.
Introduction and Background ..................................................................... 2
II.
Mission Statement ...................................................................................... 3
III.
Main Design Requirements ........................................................................ 4
IV.
Conceptual Design Evaluation Summary .................................................. 5
V.
Final Design................................................................................................ 7
VI.
Design Evaluation .................................................................................... 12
VII. Conclusions and Recommendations ......................................................... 18
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i. Executive Summary
The present project consisted of designing a movable hatch that would split two floors of
a residential dwelling to rent out to future tenants. There were six key PDS requirements that
governed the design. 1) The hatch must conform to the relevant, 2014 Oregon Building Codes. 2)
A minimum range of motion of 0 to 90 degrees must be provided, where the angle is measured
with respect to the horizontal. 3) The actuation must require no more than 50 lbf. of activation if
it is mechanical, or be simple to operate if electrical. 4) The hatch must be capable of remaining
locked in either the 0 or 90 degree position. 5) No significant structural modifications are
allowed to the residential dwelling. 6) Installation must be simple enough and fully utilize,
within reason, the manufacturing facilities at PSU.
There were three components to the design: the hinge, platform and actuation. A
modified European hinge that provided a controlled and smooth circular rotation was used as the
hinging mechanism. The platform consisted of ¾” thick plywood as its main material, 1” 16
gauge square steel tubing as the frame, a polyurethane chamfer on the side to minimize wear and
complemented with 3/8” thick hardwood flooring attached on top. Three supporting ledges
across the perimeter of the platform bear the static loading. Two linear 9” actuators providing
dynamic and static forces of 200 and 400 lb., respectively, with a stroke speed of 0.3 in./s were
used to provide the actuation.
The final design will require three hinges in its operation. An FEA model was
constructed to evaluate the structural strength of the platform, while a scaled and representative
test stand was built to verify the other 5 key PDS requirements. All key PDS requirements were
met. As an estimate, the movable hatch would cost around $1075 to fabricate, which is within
the project budget.
I. Introduction and Background
Residential homes often have a trapdoor in the first floor that provides easy access to the
basement or crawlspace and conceals it when not in use. In some cases, a trapdoor is also used to
separate one floor of the house from the other. The latter is the focus of the present project and
hence for the duration of this report, a trapdoor is synonymous with movable hatch (MH). In
general, MH design seeks to optimize mobility and aesthetics. Mobility-wise, it is expected to
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move in a 0 - 90 degree (deg.) range of motion (ROM), specifically from a horizontal (“closed”,
0) to a vertical (“open”, 90) position. The ROM is provided via either mechanical or electrical
actuation. Aesthetically, any mechanical and electrical components of the MH such as hinges,
actuators, etc. are to be as well concealed from plain sight as is feasible. Figure 1 is a
representative case of a typical trapdoor that satisfies all of the aforementioned requirements.
Figure 1. One of many MH designs. Note the following features: the MH’s appearance matches the rest of the environment, and
that the supporting hinges are concealed from residential view when it is not in use. Also, the minimum 0 to 90 deg. ROM is
provided, with the present MH currently in the 90 deg. position.
II. Mission Statement
The intent of the present project is to design a MH that conceals the bottom floor of the
residential setup depicted in Fig. 2 so that the owner of the house can separately rent out both
floors to different tenants, with the MH being the boundary between them as well as an
additional 48 square feet of space to the second floor when in the closed position.
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Figure 2: Target location where the MH will be inserted. Dimensions of the spacing are 11.75’ +- 1/32” long, and 3.92’ wide.
The homeowner will remove the half wall shown on the right in Fig. 2 and the MH will
be installed in its place. It is to start at the top of the stairwell and extend all the way to the wall
above the entrance door. Further, the MH will provide a ROM of 0 - 90 deg. where 0 deg. is
defined as ending with the top step of the stairwell and 90 deg. being when it is parallel to the
right wall. Finally, it is to provide sufficient structural support both as a substitute for the original
wall when open, and as a part of the floor when closed.
III. Main Design Requirements
These are the most important criteria, refer to the PDS report for the full list of
specifications.
1. The MH must adhere to Oregon Building Codes
a. Maintain a minimum live load of 100 psf. and its own weight
b. Deflection must not exceed 0.13” (L/360, where L is the smaller
dimension of the hatch).
2. The MH must have a minimum ROM of 0 to 90 deg.
3. If the ROM is provided by mechanical actuation then it must have a maximum
actuation force of 50 lbf. Otherwise, any electrical actuation must have a simple control scheme
that does not require specialized programming knowledge to implement or repair.
4. Any mode of failure on the MH must occur while it is mechanically locked at either 0
or 90 deg.
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5. For installation and structural purposes, no significant modifications to the structure of
the house are allowed.
6. Assembly and installation of the MH must be simple enough to require only basic
carpentry and building skills, and should strive to utilize as much of the PSU facilities as is
feasible.
IV. Conceptual Design Evaluation Summary
The overall MH design process was split into three parts: the hinge, platform, and
electrical actuation. A summary of the alternative conceptual designs, their evaluation, as well as
the justification and strengths of the final design for each is presented in Sections IVa, IVb and
IVc of this report, respectively. Out of all three components, the hinge design had the most
alternatives since it was the most significant and hence has a separate document accompanying
this report that summarizes all of them, see “Conceptual Design Evaluation – Hinge.” In general,
the final design for each component was selected based upon how well they aesthetically blended
with the household interior as well as their ease of fabrication, with an emphasis placed on
minimizing the involvement of third parties during the latter.
IVa. Designing the hinge
Several factors governed the hinge design. The first was how smoothly it provided the 0 90 deg. ROM, where a “smooth” rotation was one that resulted in a near-perfect, quarter circular
arc. The second factor was the hinge’s risk of failure both during the MH’s rotation and when it
is stationary in either the 0 or 90 deg. position. Finally, the last factor was how much of the
hinge’s underlying mechanism would be in plain sight if it were manufactured and whether or
not this would significantly affect the household aesthetics.
There were three conceptual hinge designs, A, B, and C considered for the present
project, with Design C chosen as the final solution. While all were able to provide the necessary
ROM smoothly, Design A posed a risk of failure when the platform was in the 0 deg. position.
Specifically, it required the insertion of a thin material in the domain of the hinges that would
result in a significant amount of localized stress in its vicinity when the MH was loaded to
building code specifications, and hence increase the likelihood of failure. Conversely Design B,
although simple to manufacture, required a notch underneath the platform surface that would
cause difficulties with installation. Although more complex in its assembly than Designs A and
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B, Design C did not have their shortcomings and was also easily manufactured using PSU
facilities. Further, it provided the smoothest ROM of the 3.
IVb. Designing the platform
Initially the platform was to be made from lightweight aluminum honeycomb due to its
high specific strength, but this idea was abandoned as the costs would be well over the $1000
budget. Instead, plywood was selected as the main material because it is cheap, easy to obtain,
and is commonly used in home renovation. After extensive discussion with the sponsor, the team
later decided to add steel tubing along the platform bottom because not only did it provide
additional structural strength, but it also facilitated fabrication and installation by allowing one to
weld or bolt attachment points for the hinges and actuators. The first platform design used
circular steel tubing distributed in an “X” shape. This idea was discarded, however, for two
reasons. One, shear forces along the platform in the plane of the tubing were negligible. Two, the
distribution of the tubing did not coincide with the joists inside the right wall of the house in Fig.
2, with this latter fact being important because the actuators and hinges were going to be
mounted along these studs and on the steel tubing. Hence, a new design was proposed using
square steel tubing distributed uniformly across the platform. Finally tiny trims, or “ledges,”
were added along each edge of the platform to act as supports when it is in the 0 deg. position.
IVc. Designing the actuation
Initially, the actuation was going to utilize a counterweight, gear and pulley system
concealed inside the right wall of Fig. 2 that, through a low-powered motor, was going to
provide the power required for the 0 - 90 deg. ROM. It was an optimal solution in that it not only
provided the necessary ROM, but it also preserved the household aesthetics as all of the
actuation mechanisms would have been hidden from plain sight. However, Requirement 5 in
Section III prevented further pursuit of this solution because installing the counterweight, gear,
and pulleys required significant structural modification of the right wall. Hence the next
alternative was to use electrical actuation. To be consistent with Requirement 3 in Section III, the
team decided to use simple, linear actuators. Although this solution would expose the underlying
machinery governing the MH rotation, it was the only option that provided enough power to
rotate the platform smoothly while also maintaining the required operational simplicity.
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V. Final Design
The final design satisfies nearly all of the PDS requirements, see Section VI for more
details. One important issue, though, is that the underlying hinging and actuation mechanisms
will be exposed in plain sight so it is not the most aesthetically pleasing design. However this
disadvantage is significantly reduced by using plywood as the main platform material because, as
the ensuing figures will show, its color contrasts well with the steel. Further, the design is
relatively simple to manufacture as most of the parts can be made using PSU facilities.
Section V is divided into four parts, with Va, Vb, Vc discussing the final hinge, platform,
and actuation designs, respectively and Vd discussing the overall assembly.
Va. Hinge
The final hinge design is below.
Figure 3: Clockwise from left: a) Stationary hinge, b) Hinge in mid-motion
In essence, Fig. 3 is a simplified European hinge that uses links AB, CD, and arm FG to
provide the 0 - 90 deg. ROM. Note that the above configuration of AB and CD constrains the
platform to a smooth, controlled and predictable quarter-circular arc while FG constrains it to
only one type of circular motion (i.e. it prevents the MH from wobbling); this is illustrated in
Fig. 3b. Finally, link DE serves as the support.
A deadbolt locking mechanism is also incorporated into the design in Fig. 3, shown in
detail in Fig. 4.
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Figure 4: Deadbolt lock that keeps the MH locked from motion at the 90 deg. position. MH is currently in the 90 deg. position.
The mechanism works as follows. Slider BC is kept in place at locations B and C when
the MH is either in the 0 deg. position or when it is about to begin its 90 – 0 deg. rotation. Once
it is at the 90 deg. position, slider BC is moved from points B and C to points A and B, where the
flap at A prevents the MH from falling down due to self-weight. This keeps it locked in position
at any mode of failure in the 90 deg. position; for the 0 deg. position, the hatch is kept in place
via supporting ledges installed along the perimeter of the left, central and right walls of the
residential dwelling in Fig. 2. Refer to Section Vb for more details on the supporting ledges.
Vb. Platform
Figures 5 and 6 show frontal and interior views of the final platform design, which is 11.4
ft. long and 3.9 ft. wide.
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Figure 5: Frontal view of the platform design.
Figure 6: Interior view of the final platform design.
The platform design uses ¾” thick plywood.as the primary material accompanied by a
3/8” thick hardwood flooring cover shown in Fig. 5, with the latter used for aesthetics reasons.
On the platform bottom, a frame comprising of 1” square 16-gauge steel tubing, point A in Fig.
6, is used to provide additional structural strength when the MH is in the 0 deg. position, and to
also serve as attachment points for the hinges, linear actuators and deadbolt locks shown at
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points B, C and D, respectively. The distribution of the tubing matches that of the floor joists
behind the right wall in Fig. 2, allowing a one to one installation for the actuation and hinges.
Connecting each piece of tubing at E in Fig. 6 are gusset plates, used to minimize the likelihood
of bending and shear failure during static loading of the MH; these were deemed necessary by a
preliminary FE analysis. Finally an angled polyurethane chamfer, represented as the plane FF in
Fig. 6, is installed at the end of the platform. This is to minimize wear on the MH resulting from
sliding along the floor surface at the initial portion of the 0 - 90 deg. rotation. The platform has
only a 1/10” gap, well within the allowable ¾” specified in the PDS report.
To support the loading requirements outlined in Requirement 1 of Section III, the
platform will rest on three supporting ledges each 2 inches thick, with these ledges shown and
labeled below in Fig. 7.
Figure 7: Configuration of the supporting ledges AB, BC and CD for the platform to rest on in the 0 deg. position. These will bear
the loading outlined in Requirement 1 of Section III. AB is 9.43 feet long; BC is 3.75 feet, and CD is 3.38 feet.
Aside from being the primary supports, ledges AB, BC and CD will also serve to keep
the MH locked in the 0 deg. position, complimenting the deadbolt locking scheme of Fig. 4.
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Vc. Actuation
Figure 8 shows the two linear actuators that will be used to provide the necessary ROM
for the MH.
Figure 8: Clockwise from left – a) Stationary Actuators, b) Actuators in mid-motion
The actuation will be provided by two 9” stroke linear actuators from Firgelli
Automation, each providing a dynamic and static force of 200 and 400 lb., respectively, with a
stroke speed of 0.3 in./s. These loading requirements were computed using a factor of safety
(FOS) of 1.5x the total weight of the platform and the hinges. To operate the actuator, all the user
has to do is press a dual-throw rocker switch. Both actuators will then extend if the motion is
from 0 – 90 deg. or shrink if it’s the opposite 90 – 0 deg. Once either the 0 or 90 deg. extremes
are reached, they will then shut off via their internal limit switches, triggered at maximum (90
deg.) or minimum (0 deg.) stroke length. A 12 V battery will provide the necessary electrical
power. Fabrication-wise, the actuators came with pre-designed switches, brackets, and pins for
mounting provided by the manufacturer. The only component that was designed by the team is a
mounting bracket to use for the studs in the receiving wall, shown as point C in Fig. 6.
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Vd. Overall Assembly
An extensive internal search of the house in Fig. 2 was conducted to obtain accurate
dimensions; refer to the attached document “Internal-External Search” for more details. Using
these dimensions, a 3-D model of the entire house was generated and the MH was superimposed
into it. The result is shown below in Fig. 9:
Figure 9: Final 3D model superimposing the MH with the house space in Fig. 2.
VI. Design Evaluation
Evaluation of the final MH design consisted of two things: an FE analysis on the platform
in Fig. 6 to verify that it can withstand the static loading specified by Requirement 1 in Section
III, and to build and use a scaled, representative test stand to verify that the actuators and hinges
provide the necessary ROM. Section VI will be comprised of three parts VIa, VIb, and VIc each
discussing the FE analysis, the test stand and how the present design meets or fails to meet the
remaining PDS criteria (i.e. the ones not mentioned in Section III), respectively.
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VIa. FE Analysis
A 3-D FEA model of the platform in Fig. 4 was generated with the following
assumptions for simplicity and safety to verify that Requirements 1a and 1b in Section III were
met. 1) The platform was assumed to be comprised of only the ¾” thick plywood and 16 gauge,
1” square steel tubing, both assumed isotropic and elastic. A modulus of elasticity and Poisson’s
ratio of 30(106) psi and 0.28 were used for the steel, respectively, while these values were
1.1(106) and 0.3 for the plywood. 2) The total loading was assumed to be uniformly distributed
along the platform surface, with the total loading being 9000 lbf. that incorporated an FOS of 2
on the total contribution of the platform assembly’s weight along with the 100 psf. pressure force
specified in Requirement 1a of Section III. A fine mesh of 10-node quadratic tetrahedron
elements were made. Results show a deflection of 0.012”, which is two order of magnitudes
lower than the maximum of 0.127 in. with negligible stress magnitudes along the platform.
Figure 10 shows the FEA model with the loading and boundary conditions, while Fig. 11 details
the results of the analysis.
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Figure 10: From top to bottom – a) Distribution of the BCs used for the model. Pinned conditions were assumed for the gussets
and supporting ledges. b) Loading on the platform. A uniformly distributed load was used with a total magnitude of 9000 lbs.
Figure 11: Deformation of the MH from the resulting loading, dimensions are in inches. Note the maximum deflection of 0.012”.
Maximum stress magnitudes were negligible, and hence are not included here.
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VIb. Test Stand
To ensure that Requirements 2-5 of Section III were met and that the MH is simple to
install and manufacture, a representative, scaled test stand was built using a setup that exactly
models the residential dwelling in Fig. 2 by maintaining the same distribution of the floor joists.
The scaled MH dimensions were 45” x 60”. Figure 12 depicts the entire test stand assembly.
Figure 12: Scaled test stand assembly. Note that one hinge was used, and that one supporting ledge was installed opposite the
MH.
Several observations were made from the test stand. 1) Although the correct, smooth
circular rotation was provided for both the 0 – 90 deg. and 90 – 0 deg. rotation, there was a slight
irregularity during the motion that required the user to support the MH on one side while it was
rotating. However, this was due to the fact that the two 9” actuators used in Fig. 12 were not
identical and so it is a manufacturer, not design, error and hence outside the scope of the project.
2) The hinge requires a very precise alignment for it to provide the ROM, which introduces a
slight difficulty in the MH’s installation. 3) The hinges and actuators bared a greater portion of
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the static loading on the MH in the 0 deg. position than initially thought. However, the test stand
did not contain the entire perimeter of supporting ledges outlined in Fig. 7, meaning the
implications of Observation 3 are questionable and should not immediately be assumed to hold
true for the full-scale MH.
Aside from the slight irregularity outlined in Observation 1, the motion of the test stand
did confirm that Requirements 2 and 3 of Section III were met. Requirement 4 of Section III
was confirmed as both the deadbolt lock and supporting ledges were effective in keeping the MH
stable at the 90 and 0 deg. positions, respectively, with the latter verified by having the team
stand and jump on the MH while it was in the 0 deg. position and noting that the MH only
slightly budged under the impact loading. Requirements 5 and 6 were met during the installation
of the test stand. Note from Fig. 12 that no significant structural modification was made to the
scaled floor joists, and that the entire process of installing the MH, actuators, hinges and
supporting ledge required about a day’s work while fabricating the hinges and ledges took about
another day. Note: fabrication of the test stand itself, which took considerably longer, is not
included, as the residential dwelling already has the floor joists and other relevant components
installed.
VIc. Evaluation of Remaining PDS
One of the “Performance” requirements in the PDS document specified that the MH
would, when in the 90 deg. position, act as a support in place of the original wall in Fig. 2 for the
residents when they descend the staircase. This requirement was met via the deadbolt locking
mechanism, shown in Fig. 12. The “Environment” (A), “Quantity” (B), “Maintenance” (C),
“Safety” (D), “Manufacturing Facilities” (E), “Company Constraints & Procedures” (F),
“Documentation” (G), “Life cycle” (H) and “Applicable Codes and Standards” (J) requirements
were also met, see the attached “Product Design Specifications” for more details. For A, the MH
would have carpet placed above the hardwood flooring that matches the color of the carpet
throughout the interior of the household in Fig. 2. For B, the final design uses three hinges which
is well below the maximum eight. For C, the introduction of the polyurethane chamfer in Fig. 6
prevents the likelihood of wear on the MH while the smooth rotation of the hinge in Fig. 3 is a
valid substitute for lubrication. For D, the relevant FOSs were 1.5 for the actuators and 2.0 for
the FEA analysis, well within the minimum of 1.5. For E, the only component of the MH that
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requires a third party manufacturer is the platform itself which is well within the budget of the
project, refer to the discussion on F below for more details. Everything else such as the hinges,
actuators, mounts, etc., can be made using PSU facilities.
For F, the cost of the MH is split into three components: the cost of the actuators, hinges,
and platform. Disregarding the initially purchased 24” actuator (see the “Internal/External
Search” document for more details), the total cost of the actuators was $330. The total cost of
building two hinges was $130, multiplying by 1.5 for the third hinge brings this up to $195. See
the attached receipts detailing the costs of the test stand in Fig. 12 for more details. Finally,
manufacturing the full-scale platform itself would cost around $550, obtained as an average of
the four quotes provided for the sponsor in the compilation binder and taking into account the
polyurethane chamfer. This yields a total budget of $1075. Considering that some conservative
extrapolations were made from the cost of the test stand in Fig. 12, it is safe to assume that the
actual, final budget would be within the uncertainty of the $1000 limit.
For G, a final compilation binder containing the reports, an installation manual, all
relevant CAD and assembly drawings, and necessary receipts and quotas will be submitted to the
customer along with a flash drive containing a digital copy of these files. For H, the main
materials used were plywood, hardwood flooring, steel and polyurethane. These materials are all
recyclable. J was met through the satisfaction of Requirement I in Section III. Finally the last
requirement, “Size and Shape,” was also met as the dimensions of the hatch (11.4 ft. x 3.9 ft.)
matched, within uncertainty, the dimensions of the spacing (11.8 ft. x 3.92 ft.).
Despite most of the PDS requirements being met, there were some that were not. These
were “Testing” and “Weight.” The former requirement was not met because the team did not
have enough time to thoroughly test all of the loading conditions outlined in Requirement I of
Section III. The latter requirement could not be met because the loading and aesthetic
requirements outlined in that same requirement necessitated a compromise between strength and
durability – the platform needed to be reinforced heavily with plywood and steel tubing, but the
greater reinforcement also increased its weight.
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VII. Conclusions and Recommendations
To conclude, the final design for the MH meets all of the key PDS requirements. Further,
it is simple to manufacture and all of the materials are readily available in the local area. We do
recommend, however, that the customer get a third opinion on the present MH design before
fabricating and installing it as not all of the testing could be conducted within the six month time
frame of the project.
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