Nacimex hotel Nam Dinh | Structural Engineering for retrofitting

Highlights: Retrofit engineering, reinforced structure

 

Total floor area: 31,850m2. 20 floors, 1 basement.

 

Type: 5⭐️ Hotel

 

Designed and built in 2016. Completed and put into use.

Unlike developed countries, we mainly build new projects, so there are not many opportunities to hone construction skills. Renovation projects like this one. The old project had completed the rough construction 5 years before the renovation design. According to new business requirements, the investor decided to follow the new architectural concept, with the elegance of neoclassical language. One of the tasks assigned to the design engineering team is to strengthen the existing load-bearing structure according to the load requirements of the new function.

 

 

The first step is to establish a Survey Task for the existing project. This is a mandatory content in Vietnamese Standards. The design engineer is based on the new architectural idea, comparing it with the collected current data, including field trips. From there, establish a Task to serve as a basis for performing the work for the third party – the survey contractor. Main contents include:

 

🔶Survey procedure

 

➖ Preliminary survey

 

+ Collect design and completion documents of old works

 

+ Observe, take photos

 

➖ Detailed survey

 

+ Test to check the strength of concrete structures and compare with the design;

 

+ Test to check the quantity, distance of steel bars and thickness of protective concrete layer to steel bars in structures;

 

+ Assess damage: cracks, sagging, corrosion…

 

+ Test to check concrete corrosion.

 

🔶Survey method

 

➖ Observe the surface

 

➖ Test test:

 

+ concrete: rebound gun, core drilling to check concrete strength;

 

+ Reinforcement: ultrasonic, electromagnetic to determine the thickness of protective concrete, location, diameter of reinforcement

 

🔶Content to be surveyed

 

➖ Foundation survey:

 

+ Condition of reinforced concrete, signs of damage such as cracks, breaks, …

 

+ Monitoring subsidence, tilt

 

➖ Structural survey: main load-bearing structures: columns, beams, floors,.. Quantity, location according to standard requirements and judgment of design engineer at key locations.

 

 

Also in the Concept design phase, the design engineer establishes and compares options for Structural Reinforcement work. Factors to consider are: feasibility, difficulty of construction progress, cost.

 

 

3. Design Calulation method

 

After considering and presenting the options, the investor decided to choose from the design consultant: Carbon fiber reinforcement (CFRP) option for load-bearing structures: columns, beams, floors. Including reinforcement to repair structures that have been damaged.

 

Some columns require more load-bearing capacity by using reinforcement methods by expanding the size of the concrete. The reinforcement calculation method is summarized as follows:

 

🔶Expanding the size

 

The design engineer calculates the load-bearing capacity (axial force, bending moment in 2 directions) such as an eccentric column with an expanded cross-section. The calculated reinforcement is placed along the outer perimeter of the extension.

 

To ensure that the new concrete and the old concrete work together, it is necessary to ensure adhesion and transfer of shear force from the old concrete to the new concrete. In addition to the requirement to process the old concrete surface (chisel and remove cement mortar to expose gravel in the old concrete) before pouring new concrete. For safety, consider all the drilled steel reinforcement to transmit force to the new concrete through adhesion. The adhesion force between the new concrete and the drilled steel reinforcement can be considered equal to the tensile strength of the concrete.

 

🔶CFRP column

 

The design engineer calculates CFRP reinforcement according to the regulations of European standards. The carbon fiber material uses SIKAWRAP 231C with a tensile strength of 2800MPa (10 times higher than normal steel).

 

CFRP reinforcement for columns is often done by wrapping carbon fiber fabric around the perimeter of the column. The purpose is to create a horizontal tightening of the column, helping to increase the compressive strength of the column. The reason is because when subjected to compression, the column often tends to swell horizontally, leading to cracking of the concrete when damaged. The advantages of carbon fiber are high strength and large deformation capacity when subjected to tension, which is very suitable for the purpose of “casting”. The result is that the bearing capacity of the column increases as shown in the outer interaction curve as shown below:

🔶CFRP beams, floors

 

Beams and floors are bending members. The presence of CFRP fibers helps to increase the tensile strength due to the bending moment plus the tensile strength of the existing steel reinforcement.

 

When it is necessary to increase the shear strength of floor beams, CFRP is glued around the beam face along the height.

 

The adhesive acts as an adhesive to ensure the ability to work together between the existing concrete and CFRP fibers.

Unlike new construction projects, the author’s supervision of the renovation design engineer takes place more frequently due to the actual needs of the construction site. From drawings to actual measurements of the current status and the technical capabilities and construction organization of the contractor, the details need to be continuously updated.

 

Therefore, the form of renovation construction should be the General Contractor. With reputable contractors on the market today (Hoa Binh, CoteccCons, Delta …), they are capable of professionally creating fabrication drawings (Shop drawings) because the volume is very large: drawing each component, although similar, in different required positions due to the current status.

 

🎁 Attached are the construction design calculation tables of the project.