38 × 8 ft, supported by Earl
Iteration 02 · November 14, 2026. The floor extends over the front corner posts. Two added shores share girder load with the Thomas body structure.
Working assumption. Both rear corners transfer load through the chassis drop-frame into the rails, with the same modeled support restraint as the front corners. The new shores land on structural body members whose stiffness and capacity remain unmeasured.
Load sharing with the Thomas
Engineering findings ↓- Peak member stress
- 232 MPa
- Peak deck drop
- 1.04 in
- Peak post compression
- 3,629 lbf
- Peak mount uplift
- 13 lbf
- Peak force into one shore
- 2,674 lbf
- Peak shore compression
- 0.535 in
Calculating frame and plywood load…
Stress governs at Transverse deck bearer 1. Dots mark existing posts; diamonds mark the two new shores.
Each readout is its maximum across the sampled placements.
The combined prop and bus load path is assumed to compress 1 inch per 5,000 lbf.
Peak uplift at the Rear passenger sill seat.
67% of assumed first yield. Below assumed yield in this static model. Support stiffness and strength still require measurement and design checks.
The two shores carry compression only. Stiffness is an explicit trial input for the entire landing-to-chassis load path. Tube walls are effective section thicknesses. Static stress is compared with assumed 345 MPa steel yield. All crowd counts use 225 lb per person.
Yes, shoring can substantially reduce girder bending
At 20 people on the new 38 × 8 ft deck, adding a shore below the midpoint of each edge girder reduces the ⅛-inch-wall trial from 449 to 232 MPa and from 6.04 to 1.04 in of peak deck drop. This comparison assumes an effective stiffness of 5,000 lbf/in at each shore. The highest member stress shifts to the forward transverse bearer. The force routed into one Thomas landing reaches 2,674 lbf.
The assumed support stiffness is a design variable. That 2,674 lbf compresses the modeled shore-plus-bus path by 0.535 in. The actual roof bow, side post, floor/body mounts and chassis path must carry that concentrated load and supply the assumed stiffness. The steel member stress comparison covers the added roof assembly. Capacity of the two Thomas landing points remains undetermined.
20-person comparison on the same 38 ft geometry
| Wall | 38 ft · no shores | 38 ft · 5,000 lbf/in shores | Peak force into one shore |
|---|---|---|---|
| ⅛ in | 449 MPa / 6.04 in | 232 MPa / 1.04 in | 2,674 lbf |
| 3/16 in | 333 MPa / 4.47 in | 172 MPa / 0.82 in | 2,643 lbf |
| ¼ in | 277 MPa / 3.71 in | 143 MPa / 0.72 in | 2,627 lbf |
Stress and drop are independently maximized over sampled crowd positions. Their maxima may occur in different cases. The paired shores together take up to 3,785 lbf in the ⅛-inch trial, while the existing chassis supports carry the remaining gravity load in each case.
12, 15 and 20 people with the trial shores
The following rows use 5,000 lbf/in at each shore and the same three effective tube walls. Each cell shows peak normal stress followed by peak deck drop.
| Dancers | ⅛-inch wall | 3/16-inch wall | ¼-inch wall |
|---|---|---|---|
| 12 | 165 MPa / 0.85 in | 124 MPa / 0.66 in | 104 MPa / 0.58 in |
| 15 | 195 MPa / 0.97 in | 145 MPa / 0.75 in | 121 MPa / 0.66 in |
| 20 | 232 MPa / 1.04 in | 172 MPa / 0.82 in | 143 MPa / 0.72 in |
How much stiffness must the Thomas supply?
This sensitivity uses 20 people and ⅛-inch walls. A stiffer landing reduces its vertical movement and changes where the frame bends. The reference trial of 5,000 lbf/in is an assumed value selected to expose this tradeoff.
| Stiffness of each shore | Peak stress | Peak deck drop | Peak shore force |
|---|---|---|---|
| 0 lbf/in | 449 MPa | 6.04 in | 0 lbf |
| 1,000 lbf/in | 252 MPa | 1.97 in | 1,943 lbf |
| 5,000 lbf/in | 232 MPa | 1.04 in | 2,674 lbf |
| 20,000 lbf/in | 227 MPa | 0.90 in | 2,886 lbf |
| Rigid vertical limit | 225 MPa | 0.85 in | 2,966 lbf |
Even the rigid-bearing limit reaches 225 MPa and 0.85 in of deck movement. Other spans, joists and transverse bearers continue to flex. Use the solved reactions to design the props, bearing plates, joints and body reinforcement, then replace the trial stiffness with a justified assembly value. The model credits vertical support at the shores. Sideways bracing and moment restraint at those landings require their own details.
Why the footprint changed to 38 × 8 ft
Owner-approved change · November 14, 2026. The previous 36 ft floor began about 20½ in behind the front corner-post centerlines. Extending the deck 2 ft forward puts those centerlines about 3.46 in inside the new front edge. The longer girders and floor can bear over the front portal. The extension also gives a whole-foot stock-planning dimension.
The new analytical floor runs from x = −6.357811 m to x = 5.224589 m. Its width is 2.4384 m and its modeled top surface is at z = 4.0204 m. The 13-joist, four-X-bay roof grid is redistributed over the 38 ft length. The four interior posts and front/rear portals use their original coordinates. The earlier short front return members are removed from the analytical schedule because the extended edge girders now occupy that load path. New shore locations are x = −0.261812 m, y = ±1.1692 m, at the midpoint of the 21.09 ft interior span.
Frame and plywood dead load becomes about 2,825 lb at ⅛ in, 3,808 lb at 3/16 in and 4,742 lb at ¼ in. These are increases of about 46, 50 and 54 lb over the original report. Shore tubes, landing plates and reinforcement have yet to be selected and their weight is excluded. A fabrication takeoff must include those parts.
The rear drop-frame assumption
Both rear corners participate in transferring roof load through the Thomas rear chassis drop-frame to the main rails. The model assigns the rear portal and transverse sill the same sections and fixed inboard rail-seat restraints as their front counterparts. Four outboard corner posts feed the two portal sills, which transfer to four assumed rail seats; the four interior uprights have four additional rail seats. Loads at each seat are calculated from stiffness and crowd position.
The prior report’s “rear portal engaged” option already used that idealization. Iteration 02 makes it the explicit baseline at both ends. The improvement shown above comes principally from the two additional intermediate shores. No new drop-frame stiffness, section size or local capacity was measured.
The native HDX literature describes extended roof bows, body mounting clips and a rear drop-frame. It supports investigating those load paths. It supplies no allowable concentrated load for our proposed landings, and its 2020 specifications require verification against Earl’s 2007 build. Thomas HDX construction brochure.
Landing and connection work still required
Locate structural roof-bow/side-post intersections at each proposed shore position, or design a load-spreading rail between verified structural members. Trace the force through the body, its mountings and the chassis. The model uses no roof-skin bearing capacity. Check local crippling, tube-wall buckling, shear, fasteners, welds, fatigue and body/chassis relative movement. Verify front and rear sill attachment geometry, rear drop-frame condition, engine access, and any hold-down reactions.
With the 5,000 lbf/in trial and 20-person sweep, the ⅛-inch frame still develops about 13 lbf of peak chassis-seat uplift; the modeled thicker-wall cases keep those seats in compression for the sampled gravity cases. The original rail seats can resist tension in the solver. The added shores cannot, and automatically release if the roof rises away from them.
Rhythmic dancing, moving-bus loads, wind, other equipment, rails, stairs, shell loading, vehicle stability and axle loads remain outside this gravity study. A structural/vehicle engineer needs verified load paths and dynamic response before establishing an operating occupancy. The assumed landing stiffness supplies numerical stiffness; it does not establish landing strength.
Analysis method, reproducibility and limitations
The new solver starts from iteration 01’s beam extraction at model commit 031363d. It updates structural centerlines for the 38 ft roof and adds two unilateral vertical bearings. The recorded GLB and Blender source retain the original extraction geometry. The workshop preview and its download now apply the 38 ft roof revision: floor and rails extend forward, the joist and X-brace grid is redistributed, and the extended edge girders replace the front returns. Existing portal/post coordinates and all 206 panel meshes are preserved. Shore tubes and landings await measured geometry. Final shell, guardrail and access fit still require review.
Steel uses E = 200 GPa, ν = 0.30, density 7,850 kg/m³ and assumed yield 345 MPa. Plywood uses 19.05 mm at 600 kg/m³. The sharp-corner hollow sections, rigid internal joints, collapsed connection offsets and all original dead-load exclusions carry forward. Effective tube wall values are ⅛, 3/16 and ¼ in. The added support stiffness includes every compliant component down to the chassis.
For each wall, stiffness and integer headcount, the solver recomputes static response with compression-only bearings. Crowd options include the whole deck, the driver-side half, centered and aft 12 × 4 ft groups, and a moving patch. The patch sweep includes 12 ft length and 0.6 ft per person at 4 ft width, stepped by 1 ft plus exact centered and aftmost positions. “Worst sampled” reports an independent maximum for each result. Envelopes cover only the sampled static placements.
The static beam element is checked against axial and cantilever solutions. Both bearings are checked for compression, opening and the rigid limit. Every case checks total applied gravity and force/moment equilibrium. Baseline replay and mesh-refinement comparisons accompany the source. Results above assumed yield flag departure from elastic behavior.
Download iteration 02 solver and analytical geometry · Read dimensions and assumptions · Open the complete original report.
Support survey
Use one datum for all proposed support points, including the two speculative rear portal feet. Record fore-aft distance from the front axle centerline, offset from the bus centerline and height from a documented level reference. At each point, locate both the deck-to-post joint and the foot on the bus. Identify the supporting member, measure accessible dimensions and clearances, and photograph both sides with a scale.
| Proposed location | Driver side | Passenger side |
|---|---|---|
| Front portal shoe | Awaiting survey | Awaiting survey |
| Forward interior post | Awaiting survey | Awaiting survey |
| Aft interior post | Awaiting survey | Awaiting survey |
| Rear drop-frame / portal interface | Load transfer assumed; survey pending | Load transfer assumed; survey pending |
| New midspan shore landing | Body member, stiffness and strength await survey | Body member, stiffness and strength await survey |
Trace each front shoe’s load path to the chassis. At the rear, establish the actual main-rail, drop-frame, crossmember and engine-support geometry before locating any new foot. Measure the fully open engine-access envelope against the proposed rear portal, bearer, stairs and landing; record cooling, exhaust, and service clearances. The appearance of a rear foot in the model does not establish its load path or capacity.
Deliverables
| Deliverable | Status | Evidence to record |
|---|---|---|
| As-built survey | Open | Dimensioned sketch, ten labeled photo sets including the provisional rear feet, engine and stair clearances, rear deck overhang. |
| Design loads | Open | Iteration 02 covers 0–20 people, three tube walls and five assumed shoring stiffnesses. Confirm full dead load and parked and moving occupancy. The comparison brief assumes 20 dancers at 225 lb each, moving up to 5 mph. This is a design target. Record deck, rail, stair and equipment weights and locations. |
| Vehicle assessment | Pending inputs | Front and rear axle scale tickets with weighing configuration, axle ratings, and review of stability under uneven loading, braking and turning. |
| Checked framing and connections | 38 ft shoring study complete; survey and loads pending | Engineer-reviewed spans, sections, floor support, brace and splice details, welds, bolts, guardrail and stair connections, chassis interfaces, and operating limits. |
| Fabrication BOM and cut list | On hold | Record the released drawing revision and scope, then list stock grade, tube sizes and walls, lengths, cuts, plates, fasteners and quantities. |
Progress
· Owner approved a 38 × 8 ft floor to bring the front corner posts underneath it. Added two intermediate compression shores and explicitly assumed rear drop-frame load transfer. Preserved the 36 ft findings as iteration 01.
· Completed the original occupancy and wall-thickness gravity study. Open iteration 01.