Presentation notes: Ibis Hotel Case study evening 27 May 2014 Andre Vreugdenburg, Principal Structural Engineer, PT Design INTRODUCTION SLIDE 1 IMAGE OF IBIS HOTEL Welcome everyone. Ibis is the brand name of an international chain of hotel with over 650 hotels, with Adelaide soon to be their newest with 311 rooms over 16 floors. In the absence of an Architect presenting this evening I d like to provide a quick overview about the architectural design and clients brief for this building. The design intent is best explained by reading some excerpts from Pruszinski Architect s Statement submitted with the planning documents. The streetscape around 122 Grenfell Street is defined by strong, low podiums and a procession of dominant canopies. The design of the hotel is an asymmetrical composition of two contrasting boxes and 2 connector elements. 1. The patterned textured heavy box (from levels 2 to 12). Its precast concrete panels provide a strong grounding element to the building. The precast concrete panels have an angular faceted faces which cast different shadows throughout the day. 2. The smaller, smooth, floating box, contracting with the heavy box. It is set floating on Level 4, directly above the podium screen and forms a focal point of the building.. 3. The Podium is the first connecting element, which connects the two contrasting boxes within the building design and also draws connections to the adjoining buildings within the streetscape.
4. The dark glass of the façade is the second connecting element as it provides a consistent backdrop to the other parts of the building, weaving within and around the contrasting boxes and the podium. SLIDE 2 CLOSE UP OF FACETED PANELS Next time you get the chance to pass by the building, please stop and have a look at the precast panels. I think they are some of the best examples of high quality off form panels in Adelaide with a creamy texture offset by the numerous facets which play with the light and shadows. SLIDE 3 GROUND FLOOR The hotel is a complex arrangement of functions, each requiring a high level of building services requirements. The ground floor has retail space, lobby and public space, hotel back of house facilities and driveway off Twin Street. SLIDE 4: LEVEL 1 Level 1 consists of hotel dining facilities, bar, balconies, kitchen, conference rooms,. SLIDE 5: LEVEL 2 Level 2 consists of plant rooms, hotel gymnasium and outdoor space, and the start of the guest hotel rooms. SLIDE 6: LEVEL 3 Level 3 and up are the 14 typical floors of guest rooms. 311 rooms as mentioned previously with only 2 different room types. The site is not large with the overall dimensions being roughly 30m x 30m. The requirement for natural light to the hotel rooms also meant that the total site area was not developed and the building is L shaped above L3. The site dimensions and required number of rooms led to the footprint you see before you.
CLIENT SLIDE 7: HINES SQUARE The Ibis Hotel is the latest project for the Hines Group. Originally from Adelaide and now based in Melbourne. The slide shows the many projects that we have done for the Hines group. In regards to the structures of these building they are: The Mantra is a serviced apartment building with flat plate, PT, Columns and LB external precast The Conservatory building is a mixed use building of retail, carpark, office and apartments: built using flat plat, PT, Columns and LB external precast Domain apartments, containing carpark, ground floor retail and upper floor apartments: also built using flat plate, PT, Columns and LB external precast Crown Plaza Hotel: flat plate, PT, Columns and LB external precast. Crown Apartment building, apartment building constructed using flat plate PT etc. There are many other significant projects around town that have been built using this formula. These systems were proven economical systems at the time and appropriate for the competitive trade market, when these projects were constructed. These projects typically went to tender on partial documentation and the successful builder eventually drove the choice of floor system as part of a GMP, D&C contract. Move forward to 2012 and the market trade profile is perceived to be very different with major projects such as Convention Centre, SAMHRI, the new Hospital and Adelaide Oval all soaking up key structural trades. So why did we depart from this proven system that we are so familiar with?
The Ibis solution is a direct response to the market conditions as well as the client brief, with every structural element assessed not only in terms of functional requirements but also constructability, and availability of trades to provide competitive pricing. With that in mind, and as we do with all our projects, we initially explore the use of various structural systems and test them against the architectural and client design requirements, trade availability and competitiveness. Steel framed buildings in Adelaide generally have not been successful due to a number of factors: 1. Increased floor to floor heights due to the size of structural members and the need to provide space for services under these beams. 2. The costs associated with the requirement to fire rate elements of the steel structure. So when these conditions are not required, such as carpark structures, we do see that steel framed buildings are commonly used, as they can be competitive against concrete framed alternatives. In the IBIS Hotel design we recognised that we could do a legitimate steel option that would incorporate steelwork into the structure and that did not introduce cost and height penalties to the project. Our contract on this project was to provide 80% documentation for a D&C tender. Having recognised the potential to do a steel frame for this project, the decision was made to document in steel and provide the opportunity for the tenderers to propose alternative systems. The design process started by looking at the typical floors SLIDE 8 (TYPICAL FLOOR) Here we see the typical room arrangement either side of the central corridor. Typical span across the building is..m, which would dictate a central column grid regardless of which way the slab spans.
Spanning the slab from the external loadbearing walls across a central support was discounted as this would be an inefficient system as each bay would be an end span, requiring slab thicknesses in the order of 200mm and steel beams running parallel to the corridor meant a problem for services entering the apartments. The most efficient slab system was to span across the rooms. Being only 140mm thick, this is a really good outcome for seismic mass considerations and footing loads. The party walls between units are the support lines, that could have been load bearing walls. Again for seismic weight considerations we opted to leave these as lightweight and provide steel beams to support the slab. So we have a solution for the apartment slabs, which leaves the slab over the corridors. Because the corridor zone is a services intense zone, the party wall beams could not extend over the corridor to maintain the support grid. Instead, inverted T beams cast into the slab were used to provide a clear soffit and support the corridor slab. SLIDE 9 (SERVICES IN CORRIDOR) SLIDE 10 (STEEL T BEAMS) SLIDE 11 (SIMPLE SLAB AND REO READY FOR POUR) Hence the steel solution that you see here is the equivalent of a flat plate slab from a services perspective. SLIDE 12 (FIRE RATED DETAIL) The requirement to fire rate the floor supporting structure is a real cost for any project. In this case the party walls had a fire rating requirement under the BCA and the support beams were able to be accommodated within the fire rated wall structure at no extra penalty to the project. (major win.) Only the exposed flanges of the small T beams needed to be protected at a relatively low cost.
SLIDE 13 (LEVEL 2) The column locations were defined by the ends of the floor beams. Being a relatively small grid meant that the columns could be efficient in size and also fit within the cavity of the fire rated party walls. This slide shows the transfer floor at level 2, where the steel corridor columns sit on a concrete transfer structure. The structure from level 2 to ground is dictated by a column grid that suits the functional requirements for these floors. The concrete transfer structure was an appropriate support platform for the steel frame to spring off. Regardless of the final structure above level 2 there would have been a requirement for a transfer structure and hence the importance of making the upper typical floors as light as possible. Internal columns were designed in 2 story lengths, and the wall thickness increased every 2 floors and when we ran out of capacity on the thickest section we then doubled up the columns. SLIDE 14: ELEVATION OF THE BUILDING & BUILDING PERSPECTION SIDE BY SIDE. On the external façade there are a number of support opportunities in the use steel columns or pre-cast panels as a loadbearing element. The faceted panels as a load bearing panel were considered an erection coordination risk by slowing down the steel frame erection, and so we elected to provide steel columns on the external façade line. This would facilitate the erection of the floors ahead of the external walls. In a few locations, temporary columns were installed to facilitate the erection of the frame. These columns were removed once the load had been transferred to the precast. Where possible the capacity of the façade panels as columns were still used, and that allowed a reduction in column size for the external columns.
SLIDE 15: EAST ELEVATION OF PANELS. The faceted precast panels do not stand out as being columns as the black rebate at floor level give the panels a floating look, but their inherent load carrying capacity has been used in the design. SLIDE 16: IMAGE OF STEEL STRUCTURE So in summarizing the design, We put to the market a design that was a hybrid of structural elements. An Insitu concrete core and a combination of insitu and precast shear walls that dealt with the lateral force requirements. Insitu concrete columns and floors for the lower 2 floors to deal with the transfer of loads from above on a structural column grid that suited the complex hotel functional requirements. A major component of the structure from level 2 to 16 as a steel framed alternative to a conventional flat plate concrete design. The normal disadvantages associated with the use of a steel frame were negated by the use of small steel sections, hidden within fire rated walls, and the use of T-beams cast within floor slabs to provide structure free zones for services, effectively equivalent to a flat plate design. The design also resulted in a minimum weight solution in regards to earthquake mass, with resulting savings in shear forces and foundation loads. The tender process did not preclude the use of alternative solutions and none were offered. The design of the steel frame system above level 2 was a catalyst for the builder Watpac, and fabricator SA Structural to add their creative input and
smarts to solving logistical issues and incorporate other cost effective measures into the project. The post tender and construction process from our perspective has been highly integrative and rewarding, and I ll leave it to Tim and Michael to elaborate on their respective roles and experiences. Thank you.