I wonder if the voters without a physical address can use an office building as their address?
In Texas, homeless are allowed to vote. You can even claim the freeway as your address, but you need to use another mailing address. You can use a church, shelter, PO Box, ..just about anywhere mail can be delivered..(You can't use a PO Box as your residential address, but can use it as a mailing address)
Can I register to vote if I do not have a home address or am experiencing homelessness?
Yes, people who are experiencing homelessness or living in a shelter can register to vote. In this situation, you can include the address where you sleep most often as the place you "reside." This can be a shelter address, a park address, or even a street corner.
If you cannot receive mail at the address you reside, you must also provide a valid mailing address when registering. You can list the address where you receive mail, including a P.O. Box. (Note: A P.O. Box can be only used as a mailing address, not a residential address.) Some shelters and local nonprofit organizations may also allow you to use their mailing address to receive mail.
https://texaslawhelp.org/article/voter-registration-basics