Recent Question/Assignment

As a consulting structural engineer, The University of Newcastle has engaged your services to provide part of the structural design for a new timber framed laboratory building. The building is to be designed to exactly duplicate the overall dimensions of an existing laboratory building (Refer drawing DR9155123 – Portal frame shed.pdf). However, the proposed new laboratory will use timber structural members, including bracing, to support steel purlins and girts and steel roof and wall sheeting. This is in contrast to the existing building which uses steel structural framing.
- The overall dimensions for the structure are shown on the attached drawing.
- The relevant Australian standards are to be used.
- Roof and wall dead load (sheeting, purlins/girts, bracing and services) = 0.3 kPa - Roof imposed load = 0.25 kPa
- Earthquake data – earthquake loading is assumed not to be critical for this structure
- Wind data:
o Roof suction (North-South and East-West winds) 1.0 kPa (ultimate),
0.55 kPa (serviceability) o Wall pressure (windward walls) 0.85 kPa (ultimate), 0.5 kPa
(serviceability) o Wall suction (leeward and side walls) 0.6 kPa (ultimate), 0.35 kPa (serviceability)
PORTAL FRAME: Design is required for the timber portal frame at Grid 2. The portal frame must resist gravity and wind loads. The following information is provided:
- The portal members (columns C1 and rafters R1) are to be constructed using “Mixed Australian Hardwoods” – seasoned. The timber available has a stress grade of F27.
- Assume that the timber is available in rectangular sections of width b = 120 mm and depths d starting at 190 mm and increasing in 50 mm increments (that is, 190, 240, 290, etc). Use the same cross section size for the columns and rafters.
- All design loadings for the structure (except column and rafter self weight) have been determined and are provided in the general data (note that these are forces per unit area acting on the roof and walls).
- Knee braces (member K1 in the attached drawing) are not to be used.
- The locations of purlins and girts are as shown on the attached drawings. Assume that purlins are attached to the top edge of the rafters and that girts are attached to the outside edge of the columns and that fly bracing will not be provided.
a. Analysis - Using Multiframe (or equivalent software) and the design loads provided, analyse the portal frame to determine critical design actions for the columns and rafter (strength limit state) and critical frame deflections
(serviceability limit state). (10 marks)
b. Member Design – Determine a suitable cross section size (b and d) for the columns (C1) and rafters (R1) of the portal frame to satisfy strength and serviceability requirements. Show all assumptions and calculations. (10 marks)
c. Connection Design - Design a nailed connection for the moment resisting connection between the column and rafter. (8 marks)
d. Provide a summary of your design with engineering sketches showing all relevant information. (2 marks) Hints:
1. Both the columns and rafter are subjected to combined bending and axial actions as well as shear.
2. The effects of duration of load must be considered for both the strength and serviceability limit states.
3. For the Serviceability Limit State ensure that
Rafter sag under permanent loading G does not exceed span/250 where the span in this case is the total span from Grid A to Grid C.
Lateral column deflection (sway) at eaves level due to G + Ws does not exceed column height/150
(neglect any deformations occurring in the joints)
a. Design a timber cross brace to run from the top of the column at A2 to the bottom of the column at A3. The cross brace must be designed only for the ultimate limit state (assume deflections are not critical). The cross brace is to be constructed from MGP15 (seasoned) timber. (4 marks)
b. Design and detail a 2 bolt connection between the timber cross brace and the column base at A3. (2 marks)