Technical Report Analysis: A 24-storey high rise office building plan

Technical Report Analysis: A 24-storey high rise office building plan

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Executive Summary

This report provides a detailed preliminary analysis of a-24 storey high-rise office buildings in the city centre. The storey building, as per the plan, has a typical floor-to-floor height of 4.5m while the overall height of the building is 120m, which include the roof crown. The floor plan of the Office is showing the slab edge, perimeter columns, and the core. This report also provides an existing structural system of the building overview that includes information based on design concepts, assumptions, as well as required loadings. As such, various structural components, such as the ground foundation, loading system, floor framing, and wind load, are highlighted in this report. Wind load analysis has been carried out to analyse the lateral reinforcing system effectiveness. The analysis for all loads has been done as per BOCA National Building Code, ASCE7-98, as well as through the use of different construction documents.

Introduction

On any building, the imposed load is always the most significant applied, and it ever arises from the purpose or uses they are intended for during the structure or building’s service life. The imposed load includes forces that adjacent ground exerts but exclude wind and dead loads. Thus, the category of imposed load values of imposed loads is given as per particular structure or floor use, which are taken as the minimum values that a design can adopt. As such, in the event of anticipated high values of imposed loads, then appropriate knowledge of the intended floor purpose or use must be considered. However, the architect, designer, or engineer may consider reducing the distributed imposed loads based on the applicable clauses. Further, where positions are not indicated on the plan, then horizontal imposed loads and partitions allowance may be specified as per (ASCE7-02).

1. Proposed Minimum design imposed loads
2. Imposed loads for the Office

The proposed imposed load of an office in this high-rise building design would be calculated as follows:

Assuming that one of the offices of the high-rise building offices would be having a stack of 3m plasterboards of about 4.5m in height that is fixed in stud positions. Assuming boards are 12mm thick and weigh 6.5kg/m2 on an office floor (retail) of a high-rise building with a car park basement below the high-rise office with 24 floors above; then a proposed design imposed load for the Office would be calculated as;

Assuming that;

• The plasterboards are stacked to 3m
• Plasterboards are 0.012m thick and 3m in length and that there are 83 plasterboards stacked to a height of 0.0996m

Then,

1. Imposed loads for Staircase

Staircases are named under the direction of movement. For design purposes, staircases fall into two classes, namely longitudinally and transversely supported staircases.  Longitudinally supported stairs are those supported in the direction of movement spanning between the top and bottom support; thus, they are not supported at the sides. On the other hand, transversely supported stairs are simple, and cantilever supported steps from a beam or wall or a central spine beam. Since imposed loads include both live and dead loads, thus the imposed load for this design’s staircase is proposed based on the following computation:

Given that the 24-storey high-rise building is supported on reinforced concrete floor-to-floor height which are 4.5 m apart on either side. Assuming that the live load that would make over the staircase is 400Kg/m2 and that the goings and risers are 30cm and 16cm, respectively. Further, assuming that the goings will be fixed with marble finish that is 3cm thick while the risers will require a 2cm thick plaster on either side of the risers (top and bottom of the slab surfaces).

Given that the

Then the minimum imposed load would be given by live load plus dead load as follows

• Dead load

Step’s weight = (0.4)*(0.16/2)*(2.5) = 0.08t/m of stair

Slab’s weight = (0.44)*(0.075)*(2.5) = 0.0825t/m of stair

Marble finish weight = (2.6)*(0.4)*(0.03) = 0.0312t/m of stair

Plaster finish weight = (2.2)*(0.16+0.44)*(0.02) = 0.0264t/m of stair

Total dead load = 0.0264 + 0.0312 + 0.0825 + 0.08 = 0.2201t/m of stair

• Live load = (0.4)*(0.3) = 0.12t/m of step

Therefore, the proposed minimum imposed load would be dead load plus live load

Imposed load = dead load + live load

Imposed load = 0.2201 + 0.12

= 0.3401t/m of stair

• Imposed load for Mechanical plan room (AHU)

The proposed imposed load at the mechanical room should be a CMEP dead load of 8kpa with a live load of 300kpa. At the elevator machine room, a dead load of 8kpa and a live load of 150kpa. At the cooling tower, the load distribution on the 24-stories high-rise building structure should be relative to roof drift with a dead load of 62kpa and a superimposed dead load of 12kpa

1. Wind load

Wind loads for the high-rise building structure were derived per the methods outlined in ASCE7-02, chapters 6 and 9, as summarized in Fig. 1. Fig. 1 displays wind loads results done in the East-West direction, as shown below.

Fig. 1: Indicating the results of wind loads analysis

The design in question is a-24 storey building with a floor-to-floor height of 4.5m. The proposed high-rise is 120m tall. Therefore, there is a need for detailed design for internal spaces, structural elements, and representation of realistic behavior of structures. Thus, the wall-frame structure would be the primary structural form for this high-rise design while beams and columns would form the shear and frame walls around the service core and the lift. There is a hardcore lift system within the service, as well as the location of ducts and washrooms.

1. Advantage and Disadvantage of precast and composite steel/reinforced concrete

A precast is a concrete pile that is cast, cured, and then stored in a patch awaiting construction or installation.

Advantages

Save on time-most parts of the precast to be used are already created; thus, there is going to be no time to waste creating or forming, framing, curing, and pouring the precast. Once precast are on-site, a project would simply start.

Uniformity- precast concrete components are formed or molded using the same mix over and over, thus maintaining consistency among prefabricated concrete components is easy.

High-quality products-products made from precast concrete are monitored as well as quality checked before they are transported or used on-site construction. It is taken through a curing process for monitoring and to ensure the precast is durable and in good shape; this is done in a controlled environment. Precast products produced in this are more durable and last longer than other concrete types.

Cost saving-precast concrete is relatively cheap and less time consuming, thus saves on money spent on a project compared to many other construction materials.

Disadvantages

There is nothing good that lacks wrong. Although working with precast concretes come along with many advantages, using them also present some setbacks as discussed below:

Transportation-in construction, more often, precast concrete is formed, cured, and stored off-site, which makes transporting them to a jobsite a complicated task for various reasons. One is that precast concrete are relatively heavy they do damage trucks that are used to transport them. Besides, the shape and stability of precast are compromised during transportation, which means precast concretes need special methods of transportation like picker trucks.

Installation-mobile cranes are required in most cases during the installation of large precast concrete pieces. Therefore, it is essential to rent a crane when planning a project that involves the use of precast concrete materials.

Modification-modifying pieces of precast concrete is extremely difficult, especially if it fails to fit its placement. Usually, during the construction, especially when dealing with precast concrete components that do not perfectly fit, the project manager will be forced reorder and bring in proper materials that fit. As such, when there is no proper and careful planning, then this can be costly and time-consuming.

Composite/structural steel

Nearly all sectors of major economies today use structural steel from raw materials to finished products. From warehouses, high-rise buildings, bridges to buildings sections. Application of structural steel according to construction industry experts are preferred over other building materials due to their countless benefits. However, despite all the benefits application of structural steel in building structures have some shortcomings. In this section, focus is directed at the advantages and disadvantages of using structural steel.

Advantages

Composite/structural steel are highly tensile; that is, their strength per unit mass is high, which translate to high strength to weight ratio. Thus, the overall size of the structure does matter since the sections of steel will be lightweight and small compared to other building materials. Mass production or fabrication of structural/composite steel is easy and less expensive. Sections of structural/composite steel can be fabricated or produced off-site at the yard floors and assembly done on-site. This process increases the overall construction process efficiency and saves time. Composite/structural steel are highly flexible since it possible to mold it into any shape minus interfering with its properties.  Composite/structural last longer compared to other constructor concretes. They have the ability to tolerate external pressures like cyclones, thunderstorms, and earthquakes. For example, a properly built and well maintained composite steel can last for more than 30 years.

Disadvantages

Most composite steel alloys are made of iron, which make them highly prone to corrosion a condition that can be solved by applying anti-corrosion agents that is also costly. The cost involved of extensive fireproofing is high since steel is not fireproof as it lose its properties in high temperatures.  Another limitation of using composite steel structures is buckling; that is as you increase the length of the steel column there are possibilities of increasing buckling. Besides, composite steel structures are highly expansive at high temperatures, which can be so dangerous to the overall structure.

1. Proposed floor frame

This 24 high-rise building design will be a steel framed structure, with a fairly straightforward framing design system. This simple design encourages 24 out 24 stories to be designed using the same typical framing plan. Usually this system has beam sizes of W 18×40 that are typically spaced at 9’4” and span at 44’4”. Differences in this system of framing takes place at the extreme ends of the building (usually south and north) alongside the core of building because of the insertion of 20 elevator tower and mechanical system loads through the building’s height. Size of the exterior girders should be W21x44 with spans between 28’ and 12’. On the other hand, the primary size for interior girders should be shaped as W18 with and weigh between 26 and 86 pounds per foot. The connections of interior beam-to-girder and beam-to-column should be simple shear connections. The connections of beam-to-column in the moment resisting frames should be entirely either bolted end-plate moment resisting connections or welded moment connections. The design for all structural steel beams that span for more than 35’ with an upward camber must be specified in accordance with ASTM A992 grade 50 steel.

Fig. 2: Showing a proposed office floor framing plan

The lateral calculations for the proposed floor framing plan is as shown in Fig. 2 below.

Fig. 3: Showing calculations of four braced frames resisting shear in east-west direction.

1. Implication

Implication of increasing the clear height of the first storey by another 2.6m is that when wall thickness is more than 60% of the pilaster thickness then both the wall and the pilaster will bore a concentrated load. Thus, the designer may be compelled to design an element like a wall to support the concentrated load, which in turn will take advantage of the increased support area due to the projection of the pilaster. This process will render the wall’s thickness below 60% of the pilaster’s thickness, which implies that the pilaster’s design has to be like that of the column where there is minimum permissible stress due to addition of extra greater effective height in accordance to 4.5.2 of the Code.

1. Ground foundation

The 24-stories building foundation system should be made of normal weight reinforced concrete pile caps with depth ranges between 36” and 54”. The dimensional range for the pile caps will be approximately between 7’x7’ and 11’x10’. The pile caps should be anchored or supported by between four and eight 16” diameter auger–cast piles screwed at 13 ft average bearing depth below grade. Additionally, the piles should be made of concrete of normal weight with a-4,000psi compressive strength designed to a-100 tons capacity. The building’s core should be supported by a 4’3” RC mat or slab foundation besides some additional auger-cast piles. The whole high-rise building should be supported by 328 piles in total and this should be all connected by a RC grade for the pile caps including the foundation of the interior core mat.

At the lobby level, the slabs used should be made of a 5” concrete slab-on-grade having a one layer of 4×4 welded wire fabric reinforcement. The slab should sit on a loose granular fill that should also sit on a compacted sub-grade soil. A similar inner core slab-on-grade should be used; however, it should be cast at 8” thick with two welded wire fabric layers reinforcement. Similar to a parking garage, the lobby level should be designed to eliminate the space for HVAC equipment below the high-rise building, otherwise this would force roof placement. There is an additional dead load created by the mechanical equipment loading on the structure; also, it adds to the wind load calculations complexity.

Given that the ground level to 1m for the made ground, stiff clay 1m to 25m, safe bearing pressure 300kPa and below 25m if it is sandstone with a safe bearing pressure of 2500kPa. Now assuming that:

• the brick-work weighs 1920kg/cum
• accessible roof live load is 150 kg/cum
• normal floor live load is 200kg/cum
• lime terracing weight is 2,000kg/cum
• RCC weight is 2400kg/cum
• Plain cement concrete weight is 2300kg/cum
• Soil at foundation level safe bearing capacity 300kpa
• Repose angle of the soil 30^o
• Weigh of soil is 1800kg/cum

Therefore weight of the is given by;

Weight of the foundation

Concentrated base

Total concentration base

Reinforce concrete slab weight = 2,400 x 1 x .10 x1 = 240 kg/m²

Finish or fill weight = 2,300 x 1 x 0.025 = 57.5kg/m²

Weight of ceiling plaster = 2,300 x 1 x 0.006 x 1 = 13.8 kg/m²

Roof carrying lime terracing = 2,000 x 1 x 0.10 x 1 = 200 kg/m²

Total load (both roof and floors) = 150 + 200 + 200 + (13.8 x 2) + (1 x 57.5) + (2×240) = 1,115.1 kg/m²

Therefore, when the span is 3,000 mm or 3m, then the load on the wall from both roof and floors will be half the span, and this calculated as;

Based on the existing foundation, the total weight on ground is given as

Hence, the required foundation width

It can proposed that a ground foundation base with a width of 600 mm will just be safe.

However, if some floors are added with the same floor height of 4.5 m while maintaining the thickness of the sections, then the required foundation width will be:

Wall weight

Roof RCC, ceiling plaster, and floor finish dead load

Live load

Total load on foundation plus the added load

The proposed foundation width with additional height would be

This proposed foundation width exceed the 600mm existing base foundation, which will be unsafe. However, it would be safe with foundation width of 900 mm.

This implies that the load on the soil will have to be increased;

Hence, the total load will be;

The required foundation width will therefore be

Hence, the 839 mm would be safe for a base of 900 mm.

1. Safe construction Gantt chart