Staggered Truss Steel Framing: A Sweetheart Structural System for Hotels, Condominiums, Dormitories, Apartment Buildings

Staggered Truss Framing (STF): A Sweetheart Structural System for Hotels, Condominiums, Dormitories, Apartment Buildings. The beauty of this system is there is no interrior column and it has become architect's favourite in slab type towers with double loaded corriodors mostly applied to above type of buildings.  But before we delve into more detail sbout this sweetheart system, listed below are some of the marvelous structures around the world where this STF system has been adopted. Just Google to find out more information about them.

1. Planet Hollywood, Las Vegas, USA: 39 Storey, 119 m high, Completed 2000

Planet Hollywood, Las Vegas, USA

2. Aladdin Hotel, Las Vegas, USA: 38 Storey, 2600 rooms, floor to floor truss height 9 feet

Aladdin Hotel, Las Vegas, USA

3. Delta Bow Valley Hotel, Calgary, Canada

Delta Bow Valley Hotel, Calgary, Canada

4. Half Moon Harbour Apartments, North Bergen, Newzealand

Half Moon Harbour Apartmants, North Bergen

5. Hartford Marriot Hotel, Cunneticut, USA

Hartford Marriot Hotel, Connecticut, USA

6.  Mystic Marriot Hotel & Spa, Connecticut, USA 

Mystic Marriot Hotel & Spa, Conneticut, USA

7. Staybridge Suites, Chicago, USA 14 storey steel floors over 4 concrete parking level, floor to floor height 10 feet, total height 186 feet 

Staybridge Suites, Chicago, USA

8. Summer House, Stamford, Connecticut, USA 22 Storey, 226 units, 5 level of parking, ground level for retail and restaurants

Summer House, Stamford, Conneticut, USA

Few more names of famous buildings with STF system

• Hocking College Dormitories, Nelsonville, Ohio 2 nos 200 bed each of 45000 sqfeet
• Embassy Suites Hotel, New York City
• Lafayette Place Hotel, Boston
• Resort International Hotel, Atlantic City
• Trump TajMahal, Atlantic City: 42 storey, 131m high, 1971 rooms, completed 1990
• Renaissance Hotel, Nashville
• Towers on the Central Park, New York City, complex of 20 storey and 9 storey, 599 apartments
• Seascape Point Condominium, Osage Beach, Missouri
• Macallen Building Condominium, Boston market, 140 units, completed 2007
• 360 State Street, New Haven, Connecticut
• Clayton Park Apartments, White Plains, New York

What is a Staggered Truss Framing (STF)?

The staggered truss system is a type of structural steel framing used in high-rise buildings specifically where slab type towers are planned with double loaded corridors. The system consists of a series of story-high trusses spanning the total width between two rows of exterior columns (and no interrior columns - makes it architect's favourite) and arranged in a staggered pattern on adjacent column lines giving huge free areas for space planing.

There are so many technical advantages with few challenges hence it is call a sweetheart structural system. William LeMessurier,  has been credited in developing this award winning system as part of his research at the Massachusetts Institute of Technology and got it approved in 1960. See the figures below for better understanding the STF concept. Major benefits discovered were high resistance to wind loads and versatility of floor layout with large column-free areas and reduction in overall height of buildings.

Nowadays most of the trusses are designed with hollow structural sections (HSS) for vertical and diagonal members because they are more structurally efficient and easier to fabricate. The trusses are fabricated with camber to compensate for dead load and are transported to the site, stored and then erected—generally in one piece.

STF Arrangement

Benefits of STF

• The building has a column-free interior throughout the entire ground level providing greater architectural design flexibility. Ground floor may be used for retail,bnquate, restourant, fitness activities

• The lightweight steel and plank system can reduce foundation loads by 20% or more, resulting in a significant reduction in foundation construction and cost. In addition, the majority of interior foundations will be eliminated.

• The 8-in. thick floor system provides the lowest floor to floor heights achievable provides one of the lowest floor to floor heights achievablein the industry.

• The staggered arrangement of the trusses allows clear open spaces of 20 m or more by the full width (say 2x9m) of the building at every bay, on every floor thus 360 m2 of area is column free between walls.

• The floor and frame components are plant fabricated in a controlled environment, resulting in increased quality and reduced chance of errors.

• Field labor is kept to a minimum, with fewer pieces to erect, resulting in faster construction, and faster building completion.

Seismic Horizontal Bracings for STF

The most common floor system used with staggered truss is precast hollow-core slab, or plank, which is grouted in the seams but left untopped. The hollow-core slab is a very cost-effective, dry, all-weather system that provides semi-finished floors and ceilings. Although other floor systems are feasible, hollow-core slab is used most often to keep floor-to-floor heights to a minimum.

For a typical hotel/residential loading of 4kN/m2/2kN/m2 plus 1kN/m2 partition load and a span of 9 m, an 8 to 10in (200 to 250 mm) thick hollow-core slab (stresscore or similar) is usually adequate. The lightweight steel and hollow-core slab system results in reduced foundation loads which, in turn, results in reduced foundation costs.

During construction, erection of the frame proceeds rapidly due to the quick placement of trusses and slab. Time of completion is much earlier than convention steel or concrete buildings guarenteeing early Rate of Return on Investment.

Hollow Core Slab/Precast Plank Flooring

Advantages of Staggered Truss Framing

1. Large clear span open areas for ballrooms, or other wide concourse are possible at the ground floor level, because columns are located only on the exterior faces of the building and no interrior column inside. This allows for spaces as much as 20 m in each direction (400 m2 free area) with columns often only appearing on the perimeter of a structure. This also increases design flexibility especially for atrium placement and open space floor plans.
2. Floor spans may be short bay lengths, while providing two column bay spacing for room arrangements. This results in low floor-to-floor heights. Typically, an 8'-8" floor-to-floor height is achieved. Overall building height is drastically reduced giving economy in HVAC costs too.
3. Columns have minimum bending moments due to gravity and wind loads, because of the cantilever action of the double-planar system of framing. It is easy to have full height cantilevered balconies.
4. Columns are oriented with their strong axis resisting lateral forces in the longitudinal direction of the building. Plus horizontal bracings are suggested at floor levels for effective seismic load transfer.
5. Maximum live load reductions may be realized because tributary areas may be adjusted to suit local code requirements.
6. Foundations are on column lines only and may consist of two strip footings. Because the vertical loads are concentrated at a few column points, less foundation cost and formwork is envisaged.
7. Drift is small, because the total frame is acting as a stiff truss with direct axial loads only acting in most structural members. Secondary bending occurs only in the virendeel opening/chords of the trusses.
8. High strength low alloy steels may be used to advantage, because all truss members and columns are subjected, for all practical purposes, to axial loads only except for virendeel panel (corridor opening)
9. A lightweight steel structure is achieved by the use of high strength low alloy steels and an efficient framing system. Since this reduces the weight of the superstructure, there is a substantial cost savings in steelwork and foundation work.
10. Faster to erect than comparable concrete structures. Once two floors are erected, window installation can start and stay right behind the steel and floor erection. No time is lost in waiting for other trades, such as bricklayers, to start work. Except for foundations, topping slab, and grouting, all "wet" trades are eliminated. More environment-friendly, no waste, can work in crawled space. trusses are concealed within the vertical deviding walls.
11. Fire resistance; steel is localized to the trusses, which only occur at every 58-to-70-feet on a floor, so the fireproofing operation can be completed efficiently. Furthermore, the trusses are typically placed within demising walls and it is possible that the necessary fire rating can be entirely by enclosing the trusses with cement/gypsum wallboard partitions. Finally, if spray-on- protection (cement based vermiculite) is desired, the applied thickness can be kept to a minimum due to the compact nature of the truss elements.

Challenges with STF system

• Early involvement and coordination between architect, MEP, structural is mandetory
• To establish floor to floor height considering all parameters
• Early interactions between fabricators, erectors, qnd Plank suppliers is critical
• For planks and slabs, blockouts and openings for essential services to be precisely done beforehand with proper coordination.
• Headed Stud coordination - when are the put on and welded with steel members
• Precast cambers, Truss cambers, topping slabs as finished floor plus tiles
• Erection sequence of truss particularly when there is a space constraint
• Moment connection plates with precast supplier

References

1. AISC Design Guide 14: Staggered Truss System, 2003
2. High-Rise Housing in Steel, the Staggered Truss System, Massachusetts Institute of Technology, Departments of Architecture and Civil Engineering, January 1967.

Recommendation

The above writing is meant to inspire upcoming structural engineers who are fearless to explore the opportunities of using staggered truss framing system in buildings. Hope this information would serve as an aid to structural engineers and decision makers to explore feasibility of staggered truss system with more zeal and confidence. Excellent references are given for acquiring thorough knowledge on the design and erection of staggered truss framing. The author has no conflict of interest and the opinions are of his own.

Disclaimer

The contents of this article is for informational purpose only and proper professional guidence must be sought before implementation of design and construction of steel buildings with staggered truss framing system.