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)
Figure 1 – Deformed shape of three storey unreinforced masonry building subjected to earthquake load in the Y-direction (NOT TO SCALE)
As a consulting structural engineer, The University of Newcastle has engaged your services to provide design and assessment for the following structures:
• An existing three (3) storey unreinforced masonry building
• A new reinforced concrete block propped cantilever retaining wall
Scale drawings are provided in addition to this brief.
The following design codes are to be used in your analysis:
• AS1170.4: 2007 – Structural Design Actions. Part 4: Earthquake actions in Australia • AS3700: 2018 – Masonry Structures
The existing three (3) storey unreinforced masonry building has the following design parameters:
• Importance Level 2
• Located in Sydney
• Subsoil Class Ce
• Wind loading has been found to be non-critical
• 10mm thick M3 mortar joints (full bedding)
• 230 mm × 110 mm × 76 mm clay units with compressive strength: f’uc = 25MPa
• All load bearing unreinforced masonry walls consist of two (2) skins of masonry (230mm thick total)
• Perimeter walls consist of cavity brick construction comprising a load bearing inner leaf (230mm thick) and a non-load bearing outer leaf (110mm thick). These leaves are separated by a 50 mm cavity and the outer leaf is supported vertically by a shelf angle at slab level and laterally by heavy duty ties connected to the inner leaf.
• No damp proof courses or slip joints between unreinforced masonry and concrete floors
• Control joints between plan intersections of all perpendicular walls
• All suspended slabs (level 1, level 2, level 3) 200 mm thick reinforced concrete
• Design loading is as follows for all suspended levels (including the trafficable roof):
o Superimposed dead load gSDL = 1.0 kPa o Live load q = 3.0 kPa (?c = 0.3)
Figure 2 – Isometric view of unreinforced masonry building (NOT TO SCALE)
Figure 3 – Isometric view of building with floors hidden (NOT TO SCALE)
Question 1 – Derivation of seismic actions on building (25%)
a) Determine the storey seismic weights (Wi) for each suspended level of the building, hence the seismic weight (Wt) of the entire building. When determining the storey seismic weights, include the slab selfweight, superimposed dead load, and the weight of walls, as well as the live load multiplied by the combination factor ?c (see §6.2.2 of AS1170.4: 2007 for guidance on determining Wt).
b) Calculate the base shear V for the structure.
c) Determine the storey seismic force distribution for the building. Present your results in a diagram like the example shown in Figure 4.
d) Determine the storey shear force distribution for the building. Present your results in a diagram like the example shown in Figure 4.
e) Determined the storey bending moment distribution for the building. Present your results in a diagram like the example shown in Figure 4.
Figure 4 – Suggested diagram layout for Question 1 parts c), d), e). NOTE
Question 2 – Compressive capacity of unreinforced masonry (15%)
a) Determine the ultimate (1.2G+1.5Q) axial force on wall W07 at all levels of the building. Draw an axial force diagram for this wall (use a similar format to the SFD drawn in Question 1 part d)).
b) Check the capacity of wall W07 using “Design by simple rules” in §7.3.3 of AS3700. Note that you only need to check the capacity of the wall at the critical location i.e. ground level.
c) Suppose that an alternate floor framing arrangement was used for the building with a 300mm wide reinforced concrete beam (so the bearing area is 300mm×230mm) transmitting an ultimate (already factored) load N* = 250 kN to W07 as shown in Figure 5. Check the capacity of wall W07 under this loading arrangement. It may be assumed that the restraint provided to the top of the wall is the same as that provided by the slab in part (b) above.
850mm 300mm 850mm
Figure 5 – Alternate loading arrangement for W07 (NOT TO SCALE)
Question 3 – In-plane analysis of unreinforced masonry (25%)
a) For wall W34, determine the dead load, live load for all levels of the building, hence determine 0.9Wt for W34 at the ground floor of the building (this is the load to be used for all shear and flexural checks of the wall in accordance with § and § of AS3700).
b) Calculate the relative proportion of seismic forces, hence shears and moments to each of the walls in the North-South direction (y-direction). You may assume the following:
• The centre of mass and centre of stiffness coincide
• No accidental eccentricity needs is to be considered
• Seismic loading is only applied in the y-direction (North-South direction)
• The floor slabs act as a rigid diaphragm in-plane, with no flexural coupling outof-plane, that is, the walls act as 12m tall cantilevers.
c) Using the results from parts a) and b), draw the axial force diagram (for load case 0.9Wt), shear force diagram, and bending moment diagram for wall W34. Note that although 0.9Wt is taken as the vertical load on the wall, the seismic loads are to be calculated using Wt (i.e. as previously determined in Question 1).
d) Check wall W34 for the following:
i. In-plane shear.
ii. Heel tension and toe compression.
e) FOR CIVL6120 STUDENTS ONLY: Repeat part d) ii. but instead of simply checking tension and compression stresses in the wall, determine the flexural capacity of the wall using rectangular stress-block theory (based on the assumptions outlined in Section 8.3 of AS3700). Compare this to the results obtained from d) ii) and comment on which method you think is the more appropriate method of checking the in-plane flexural capacity of an existing wall under earthquake loading? Justify your answer.
Question 4 – Out-of-plane analysis of unreinforced masonry (15%)
a) Check the out-of-plane capacity of W26 under seismic loading (see §8.3 of AS1170.4:2007) at the uppermost floor of the building. Assume that the inner (230 mm) skin of bricks is simply supported at the roof and second floor levels and one vertical edge is supported laterally by wall W36 (the other vertical edge is not supported). You may assume that the outer skin of bricks is connected to the inner skin using heavy duty ties but it is not necessary to check the wall ties. In performing the check assume that the inertia forces are shared between the two leaves of the wall (see §7.7 of
Question 5 – Reinforced masonry retaining wall (20%)
The retaining wall shown in Figure 6 is fixed to the footing at its base and laterally supported along its top edge by the attached pavement (that is, it is a propped cantilever). Assume there is no surcharge loading from the pavement.
a) Draw the bending moment diagram and shear force diagram for the retaining wall.
b) Using a block size of 290×190×390 units, design flexural reinforcement (and shear reinforcement if required) for the retaining wall. You may assume that M4 mortar is to be used and the units are face shell bedded, the grout compressive strength f’cg = 20MPa and block compressive strength: f’uc = 15MPa. Include the design of starter bars into the footing but do not design any other reinforcement in the footing.
c) Provide a neat sketch of your design.
Figure 6 – Propped cantilever retaining wall arrangement (NOT TO SCALE)